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TM 3-34.64 (FM 5-410/23 Dec 1992)/MCRP 3-17.7G
MILITARY SOILS ENGINEERING
September 2012
DISTRIBUTION RESTRICTION: Approved for public release; distribution is unlimited.
HEADQUARTERS, DEPARTMENT OF THE ARMY
This publication is available at Army Knowledge Online
(https//armypubs.us.army.mil/doctrine/index.html).
*TM 3-34.64/MCRP 3-17.7G
Technical Manual No 3-34.64/Marine Corps Reference Publication 3-17.7G
Headquarters Department of the Army
Washington, D.C., 25 September 2012
Military Soils Engineering
Contents
Page
PREFACE……………………………………………………………………………………… xiv
Chapter 1 ROCKS AND MINERALS…………………………………………………………………. 1-1
Section I – Minerals………………………………………………………………………… 1-1
Physical Properties………………………………………………………………………….. 1-1
Common Rock-Forming Minerals………………………………………………………… 1-5
Section II – Rocks…………………………………………………………………………… 1-7
Formation Processes……………………………………………………………………….. 1-7
Classification………………………………………………………………………………… 1-12
Identification…………………………………………………………………………………. 1-20
Chapter 2 STRUCTURAL GEOLOGY……………………………………………………………….. 2-1
Section I – Structural Features in Sedimentary Rocks………………………….. 2-1
Bedding Planes………………………………………………………………………………. 2-1
Folds……………………………………………………………………………………………. 2-3
Cleavage and Schistosity………………………………………………………………….. 2-6
Faults……………………………………………………………………………………………. 2-7
Joints……………………………………………………………………………………………. 2-9
Strike and Dip……………………………………………………………………………….. 2-10
Section II – Geologic Maps…………………………………………………………….. 2-13
Types………………………………………………………………………………………….. 2-13
Symbols………………………………………………………………………………………. 2-16
Outcrop Patterns……………………………………………………………………………. 2-19
Section III – Engineering Considerations………………………………………….. 2-25
Rock Distribution……………………………………………………………………………. 2-25
Rock Fragmentation……………………………………………………………………….. 2-26
Rock Slides and Slumps………………………………………………………………….. 2-27
Weak Rocks…………………………………………………………………………………. 2-28
Fault Zones………………………………………………………………………………….. 2-28
Groundwater…………………………………………………………………………………. 2-28
DISTRIBUTION RESTRICTION: Approved for public release; distribution is unlimited.
* This manual supersedes FM 5-410, 23 December 1992.
i
Contents
Road Cut Alignment……………………………………………………………………….. 2-28
Quarry Faces………………………………………………………………………………… 2-30
Rock Deformation………………………………………………………………………….. 2-30
Earthquakes (Fault Movements)……………………………………………………….. 2-31
Dams………………………………………………………………………………………….. 2-31
Tunnels……………………………………………………………………………………….. 2-31
Bridges………………………………………………………………………………………… 2-32
Buildings……………………………………………………………………………………… 2-32
Quarry Operations…………………………………………………………………………. 2-33
Section IV – Applied Military Geology………………………………………………. 2-33
Military Geographic Intelligence………………………………………………………… 2-34
Geology in Resolving Military Problems………………………………………………. 2-36
Remote Imagery……………………………………………………………………………. 2-36
Field Data Collection………………………………………………………………………. 2-37
Equipment for Field Data Collection……………………………………………………. 2-38
Geological Surveying……………………………………………………………………… 2-39
Chapter 3 SURFICIAL GEOLOGY……………………………………………………………………. 3-1
Fluvial Process……………………………………………………………………………….. 3-1
Glacial Process……………………………………………………………………………… 3-22
Eolian Process………………………………………………………………………………. 3-32
Sources of Construction Aggregate……………………………………………………. 3-39
Chapter 4 SOIL FORMATION AND CHARACTERISTICS……………………………………… 4-1
Section I – Soil Formation……………………………………………………………….. 4-1
Weathering…………………………………………………………………………………….. 4-1
Discontinuities and Weathering…………………………………………………………… 4-3
Effects on Climate……………………………………………………………………………. 4-3
Effects on Relief Features…………………………………………………………………. 4-3
Soil Formation Methods……………………………………………………………………. 4-3
Soil profiles……………………………………………………………………………………. 4-4
Section II – Soil Characteristics………………………………………………………… 4-5
Physical Properties………………………………………………………………………….. 4-5
Consistency (Atterberg) Limits………………………………………………………….. 4-20
Chapter 5 SOIL CLASSIFICATION…………………………………………………………………… 5-1
Section I – Unified Soil Classification System…………………………………….. 5-1
Soil Categories……………………………………………………………………………….. 5-1
Laboratory Testing…………………………………………………………………………… 5-5
Desirable Soil Properties for Road and Airfields……………………………………… 5-9
Desirable Soil Properties for Embankments and Foundations…………………… 5-12
Soil Graphics………………………………………………………………………………… 5-15
Field Identification………………………………………………………………………….. 5-15
Optimum Moisture Content (OMC)…………………………………………………….. 5-28
Section II – Other Soil Classification Systems…………………………………… 5-28
Commonly used systems…………………………………………………………………. 5-28
Typical Soil Classification………………………………………………………………… 5-34
ii TM 3-34.64/MCRP 3-17.7G/MCRP 3-17.7G 25 September 2012
Contents
Chapter 6 CONCEPTS OF SOIL ENGINEERING…………………………………………………. 6-1
Section I – Settlement……………………………………………………………………… 6-1
Factors………………………………………………………………………………………….. 6-1
Compressibility……………………………………………………………………………….. 6-1
Consolidation…………………………………………………………………………………. 6-2
Section II – Shearing Resistance………………………………………………………. 6-3
Importance…………………………………………………………………………………….. 6-3
Laboratory Tests……………………………………………………………………………… 6-3
California Bearing Ratio…………………………………………………………………….. 6-3
Airfield Index (AI)…………………………………………………………………………….. 6-4
Correlation Between CBR and AI………………………………………………………… 6-7
Section III – Bearing Capacity…………………………………………………………… 6-7
Importance…………………………………………………………………………………….. 6-7
Foundations…………………………………………………………………………………… 6-8
Section IV – Earth-Retaining Structures…………………………………………… 6-10
Purpose………………………………………………………………………………………. 6-10
Types………………………………………………………………………………………….. 6-11
Chapter 7 MOVEMENT OF WATER THROUGH SOILS………………………………………… 7-1
Section I – Water…………………………………………………………………………….. 7-1
Hydrologic Cycle……………………………………………………………………………… 7-1
Surface Water………………………………………………………………………………… 7-1
Springs and Seeps…………………………………………………………………………… 7-2
Groundwater………………………………………………………………………………….. 7-3
Permeability…………………………………………………………………………………… 7-9
Drainage Characteristics…………………………………………………………………. 7-10
Filter Design…………………………………………………………………………………. 7-10
Porosity and Permeability of Rocks……………………………………………………. 7-12
Water Table………………………………………………………………………………….. 7-13
Section II – Frost Action…………………………………………………………………. 7-14
Problems……………………………………………………………………………………… 7-14
Conditions……………………………………………………………………………………. 7-16
Effects…………………………………………………………………………………………. 7-17
Investigational Procedures……………………………………………………………….. 7-19
Control………………………………………………………………………………………… 7-20
Chapter 8 SOIL COMPACTION……………………………………………………………………….. 8-1
Section I – Soil Properties Affected by Compaction…………………………….. 8-1
Advantages of Soil Compaction………………………………………………………….. 8-1
Settlement……………………………………………………………………………………… 8-1
Shearing Resistance………………………………………………………………………… 8-2
Movement of Water………………………………………………………………………….. 8-2
Volume Change………………………………………………………………………………. 8-2
Section II – Design Considerations…………………………………………………… 8-2
Moisture-Density Relationships…………………………………………………………… 8-2
Compaction Specifications………………………………………………………………… 8-8
Section III – Construction Procedures……………………………………………… 8-13
25 September 2012 TM 3-34.64/MCRP 3-17.7G iii
Contents
General Considerations…………………………………………………………………… 8-13
Selection of Materials……………………………………………………………………… 8-13
Dumping and Spreading………………………………………………………………….. 8-13
Compaction of Embankments…………………………………………………………… 8-14
Density Determinations…………………………………………………………………… 8-16
Field Control of Compaction……………………………………………………………… 8-17
Compaction Equipment…………………………………………………………………… 8-19
Compactor Selection………………………………………………………………………. 8-22
Section IV – Quality Control……………………………………………………………. 8-24
Purpose………………………………………………………………………………………. 8-24
Quality-Control Plan……………………………………………………………………….. 8-24
Theater-of-Operations Quality Control………………………………………………… 8-25
Corrective Actions………………………………………………………………………….. 8-26
Chapter 9 SOIL STABILIZATION FOR ROADS AND AIRFIELDS…………………………… 9-1
Section I – Methods of Stabilization………………………………………………….. 9-1
Basic Considerations……………………………………………………………………….. 9-1
Mechanical Stabilization……………………………………………………………………. 9-2
Chemical Admixture Stabilization………………………………………………………. 9-10
Section II – Design Concepts………………………………………………………….. 9-28
Structural Categories………………………………………………………………………. 9-28
Stabilized Pavement Design Procedure………………………………………………. 9-30
Thickness Design Procedures…………………………………………………………… 9-31
Roads…………………………………………………………………………………………. 9-32
Airfields……………………………………………………………………………………….. 9-39
Examples of Design……………………………………………………………………….. 9-47
Theatre-of-Operations Airfield Considerations………………………………………. 9-51
Section III – Dust Control……………………………………………………………….. 9-57
Effects of Dust………………………………………………………………………………. 9-57
Dust Formation……………………………………………………………………………… 9-58
Dust Palliatives……………………………………………………………………………… 9-58
Dust-Control Methods…………………………………………………………………….. 9-60
Selection of Dust Palliatives……………………………………………………………… 9-69
Dust Control on Roads and Cantonment Areas…………………………………….. 9-76
Dust Control for Heliports…………………………………………………………………. 9-76
Control of Sand……………………………………………………………………………… 9-78
Section IV – Construction Procedures……………………………………………… 9-82
Mechanical Soil Stabilization…………………………………………………………….. 9-82
Chapter 10 SLOPE STABILIZATION………………………………………………………………… 10-1
Geologic Features………………………………………………………………………….. 10-1
Soil Mechanics………………………………………………………………………………. 10-2
Slope Failure………………………………………………………………………………… 10-8
Stable Slope-Construction in Bedded Sediments…………………………………. 10-15
Chapter 11 GEOTEXTILES……………………………………………………………………………… 11-1
Applications………………………………………………………………………………….. 11-1
Unpaved Aggregate Road Design……………………………………………………… 11-3
Selecting a Geotextile……………………………………………………………………. 11-10
iv TM 3-34.64/MCRP 3-17.7G/MCRP 3-17.7G 25 September 2012
Contents
Chapter 12 SPECIAL SOIL PROBLEMS……………………………………………………………. 12-1
Aggregate Behavior……………………………………………………………………….. 12-1
Soil-Aggregate Mixtures………………………………………………………………….. 12-1
LateRites and Lateritic Soils……………………………………………………………… 12-1
Coral…………………………………………………………………………………………… 12-5
Desert Soils………………………………………………………………………………….. 12-7
Artic and Subartic Soils……………………………………………………………………. 12-7
Appendix A CALIFORNIA BEARING RATIO DESIGN METHODOLOGY ……………………… A-1 Appendix B AVAILABILITY OF FLY ASH …………………………………………………………………. B-1
Appendix C HAZARDS OF CHEMICAL STABILIZERS ………………………………………………. C-1 GLOSSARY ……………………………………………………………………………… Glossary-1
REFERENCES……………………………………………………………………….References-1
INDEX…………………………………………………………………………………………….Index-1
Figures
Figure 1-1. Crystal forms……………………………………………………………………………… 1-3
Figure 1-2. Cleavage…………………………………………………………………………………… 1-3
Figure 1-3. Fractures…………………………………………………………………………………… 1-4
Figure 1-4. The rock cycle……………………………………………………………………………. 1-8
Figure 1-5. Intrusive and extrusive rock bodies…………………………………………………. 1-9
Figure 1-6. Cross section of igneous rock………………………………………………………… 1-9
Figure 1-7. Bedding planes…………………………………………………………………………. 1-10
Figure 1-8. Contact metamorphism zone……………………………………………………….. 1-11
Figure 1-9. Metamorphic foliation…………………………………………………………………. 1-12
Figure 1-10. Jointing in igneous rocks……………………………………………………………. 1-13
Figure 1-11. Cross bedding in sandstone……………………………………………………….. 1-17
Figure 1-12. Metamorphism of existing rocks………………………………………………….. 1-20
Figure 1-13. Crushed shape……………………………………………………………………….. 1-25
Figure 2-1. The major plates of the earth’s crust………………………………………………… 2-2
Figure 2-2. Major features of the plate tectonic theory………………………………………… 2-3
Figure 2-3. Location of rock outcrops……………………………………………………………… 2-3
Figure 2-4. Folding of sedimentary rock layers………………………………………………….. 2-4
Figure 2-5. Common types of folds…………………………………………………………………. 2-5
Figure 2-6. Topographic expression of plunging folds…………………………………………. 2-5
Figure 2-7. Fold symmetry……………………………………………………………………………. 2-6
Figure 2-8. Faulting…………………………………………………………………………………….. 2-7
Figure 2-9. Fault zone…………………………………………………………………………………. 2-7
Figure 2-10. Thrust fault with drag folds…………………………………………………………… 2-8
Figure 2-11. Fault terminology………………………………………………………………………. 2-8
Figure 2-12. Types of faults………………………………………………………………………….. 2-9
25 September 2012 TM 3-34.64/MCRP 3-17.7G v
Contents
Figure 2-13. Graben and horst faulting……………………………………………………………. 2-9
Figure 2-14. Jointing in sedimentary and igneous rock……………………………………… 2-10
Figure 2-15. Strike……………………………………………………………………………………. 2-11
Figure 2-16. Dip………………………………………………………………………………………. 2-11
Figure 2-17. Measuring strike and dip with a Brunton compass…………………………… 2-12
Figure 2-18. Strike and dip symbols……………………………………………………………… 2-12
Figure 2-19. Strike and dip symbols of sedimentary rocks…………………………………. 2-13
Figure 2-20. Symbolic patterns for rock types…………………………………………………. 2-16
Figure 2-21. Geologic map symbols…………………………………………………………….. 2-17
Figure 2-22. Placement of strike and dip symbols on a geologic map…………………… 2-18
Figure 2-23. Geologic map and cross section…………………………………………………. 2-18
Figure 2-24. Outcrop patterns of horizontal strata…………………………………………….. 2-19
Figure 2-25. Outcrop patterns of inclined strata……………………………………………….. 2-20
Figure 2-26. Outcrop patterns of an eroded dome……………………………………………. 2-20
Figure 2-27. Outcrop patterns of and eroded basin………………………………………….. 2-21
Figure 2-28. Outcrop patterns of plunging folds………………………………………………. 2-21
Figure 2-29. Outcrop patterns produced by faulting…………………………………………. 2-22
Figure 2-30. Outcrop patterns of intrusive rocks………………………………………………. 2-22
Figure 2-31. Outcrop patterns of surficial deposits…………………………………………… 2-23
Figure 2-32. Ripping in the direction of dip…………………………………………………….. 2-26
Figure 2-33. Rock drills……………………………………………………………………………… 2-26
Figure 2-34. Rock slide on inclined bedding plane……………………………………………. 2-27
Figure 2-35. Rules of thumb for inclined sedimentary rock cuts…………………………… 2-27
Figure 2-36. Road cut alignment………………………………………………………………….. 2-29
Figure 2-37. Quarry in the direction of the strike………………………………………………. 2-30
Figure 3-1. Typical drainage patterns……………………………………………………………… 3-3
Figure 3-2. Topographic expression of a braided stream…………………………………….. 3-4
Figure 3-3. Stream evolution and valley development…………………………………………. 3-5
Figure 3-4. A youthful stream valley……………………………………………………………….. 3-7
Figure 3-5. A mature stream valley………………………………………………………………… 3-8
Figure 3-6. An old age stream valley………………………………………………………………. 3-9
Figure 3-7. Meander erosion and deposition…………………………………………………… 3-10
Figure 3-8. Point bar deposits designated by gravel symbols……………………………… 3-11
Figure 3-9. Channel bar deposits, oxbow lakes, and backswamp/floodplain deposits . 3-13 Figure 3-10. Meander development and cutoff……………………………………………….. 3-14
Figure 3-11. Oxbow lake deposits, natural levees, and backswamp deposits…………. 3-14
Figure 3-12. Alluvial terraces………………………………………………………………………. 3-15
Figure 3-13. Topographic expression of alluvial terraces……………………………………. 3-16
Figure 3-14. Growth of a simple delta……………………………………………………………. 3-17
Figure 3-15. Arcuate, bird’s foot, and elongate deltas……………………………………….. 3-17
Figure 3-16. Alluvial fan and coalescing alluvial fans………………………………………… 3-18
vi TM 3-34.64/MCRP 3-17.7G/MCRP 3-17.7G 25 September 2012
Contents
Figure 3-17. Cedar Creek alluvial fan……………………………………………………………. 3-19
Figure 3-18. Coalescing alluvial fans…………………………………………………………….. 3-20
Figure 3-19. Major floodplain features…………………………………………………………… 3-21
Figure 3-20. World distribution of fluvial landforms…………………………………………… 3-22
Figure 3-21. Ice sheets of North America and Europe………………………………………. 3-22
Figure 3-22. Contintental glaciation………………………………………………………………. 3-23
Figure 3-23. Alpine glaciation……………………………………………………………………… 3-24
Figure 3-24. Moraine topographic expression with kettle lakes, swamps, and eskers . 3-26 Figure 3-25. Valley deposits from melting ice………………………………………………….. 3-27
Figure 3-26. Idealized cross section of a drumlin……………………………………………… 3-28
Figure 3-27. Topographic expression of a drumlin field……………………………………… 3-29
Figure 3-28. Distribution of major groups of glacial landforms across the United
States……………………………………………………………………………………. 3-30
Figure 3-29. World distribution of glacial landforms………………………………………….. 3-31
Figure 3-30. Three stages illustrating the development of desert armor…………………. 3-33
Figure 3-31. Cutting of a ventifact………………………………………………………………… 3-33
Figure 3-32. Eolian features………………………………………………………………………… 3-34
Figure 3-33. Sand dune types……………………………………………………………………… 3-35
Figure 3-34. Loess landforms……………………………………………………………………… 3-36
Figure 3-35. Topographic expression of dunes and desert pavement……………………. 3-37
Figure 3-36. Worldwide distribution of eolian landforms……………………………………. 3-38
Figure 4-1. Residual soil forming from the in-place weathering of igneous rock………… 4-4
Figure 4-2. Soil profile showing characteristic soil horizons………………………………….. 4-5
Figure 4-3. Dry sieve analysis……………………………………………………………………….. 4-7
Figure 4-4. Data sheet, example of dry sieve analysis………………………………………… 4-8
Figure 4-5. Grain-size distribution curve from sieve analysis………………………………… 4-9
Figure 4-6. Well-graded soil……………………………………………………………………….. 4-10
Figure 4-7. Uniformly graded soil…………………………………………………………………. 4-11
Figure 4-8. Gap-graded soil……………………………………………………………………….. 4-11
Figure 4-9. Typical grain-size distribution curves for well-graded and poorly graded
soils………………………………………………………………………………………. 4-11
Figure 4-10. Bulky grains……………………………………………………………………………. 4-12
Figure 4-11. Volume-weight relationships of a soil mass……………………………………. 4-13
Figure 4-12. Layer of adsorbed water surrounding a soil particle…………………………. 4-17
Figure 4-13. Capillary rise of water in small tubes……………………………………………. 4-18
Figure 4-14. U-shaped compaction curve………………………………………………………. 4-20
Figure 4-15. Liquid limit test………………………………………………………………………… 4-21
Figure 5-1. Sample plasticity chart…………………………………………………………………. 5-3
Figure 5-2. Graphical summary of grain-size distribution…………………………………… 5-18
Figure 5-3. Breaking or dry strength test………………………………………………………… 5-19
Figure 5-4. Roll or thread test……………………………………………………………………… 5-20
Figure 5-5. Ribbon test (highly plastic clay)…………………………………………………….. 5-21
25 September 2012 TM 3-34.64/MCRP 3-17.7G vii
Contents
Figure 5-6. Wet shaking test……………………………………………………………………….. 5-22
Figure 5-7. Suggested procedure for hasty field identification…………………………….. 5-25
Figure 5-8. Group index formula and charts, revised public roads system…………….. 5-31
Figure 5-9. Relationship between LL and PI for silt-clay groups, revised public roads system…………………………………………………………………………………… 5-32
Figure 5-10. US Department of Agriculture textural classification chart…………………. 5-33
Figure 6-1. Laboratory shear tests…………………………………………………………………. 6-4
Figure 6-2. Correlation of CBR and AI…………………………………………………………….. 6-7
Figure 6-3. Typical failure surfaces beneath shallow foundations………………………….. 6-8
Figure 6-4. Bearing piles……………………………………………………………………………. 6-10
Figure 6-5. Principal types of retaining walls…………………………………………………… 6-12
Figure 6-6. Common types of retaining wall drainage………………………………………… 6-14
Figure 6-7. Eliminating frost action behind retaining walls…………………………………… 6-15
Figure 6-8. Typical timber crib retaining wall……………………………………………………. 6-16
Figure 6-9. Other timber retaining walls…………………………………………………………. 6-17
Figure 6-10. Typical gabion………………………………………………………………………… 6-18
Figure 6-11. Bracing a narrow shallow excavation……………………………………………. 6-20
Figure 6-12. Bracing a wide shallow excavation………………………………………………. 6-20
Figure 7a. Hydrologic cycle………………………………………………………………………….. 7-2
Figure 7b. Artesian groundwater……………………………………………………………………. 7-3
Figure 7c. Groundwater zones………………………………………………………………………. 7-4
Figure 7-1. Capillary rise of moisture………………………………………………………………. 7-5
Figure 7-2. Base drains in an airfield pavement…………………………………………………. 7-7
Figure 7-3. Typical subgrade drainage installation……………………………………………… 7-8
Figure 7-4. Mechanical analysis curves for filter material…………………………………… 7-12
Figure 7-4a. Porosity in rocks……………………………………………………………………… 7-13
Figure 7-5. Determination of freezing index……………………………………………………. 7-14
Figure 7-6. Formation of ice crystals on frost line…………………………………………….. 7-15
Figure 7-7. Sources of water that feed growing ice lenses………………………………….. 7-20
Figure 8-1. Typical moisture-density relationship……………………………………………….. 8-3
Figure 8-2. Moisture-density relationships of seven soils…………………………………….. 8-5
Figure 8-3. Moisture-density relationships of two soils………………………………………… 8-7
Figure 8-4. Density, compaction, and moisture content………………………………………. 8-9
Figure 8-5. Density and moisture determination by CBR design method……………….. 8-10
Figure 8-6. Self-propelled, pneumatic-tired roller……………………………………………… 8-20
Figure 8-7. Compaction by a sheepsfoot roller………………………………………………… 8-21
Figure 8-8. Two axle, tandem steel-wheeled roller……………………………………………. 8-22
Figure 8-9. Self-propelled, smooth-drum vibratory roller……………………………………. 8-22
Figure 8-10. Use of test strip data to determine compactor efficiency…………………… 8-23
Figure 9-1. Graphical method of proportioning two soils to meet gradation requirements…………………………………………………………………………….. 9-6
Figure 9-2. Arithmetical method of proportioning soils to meet gradation requirements . 9-8
viii TM 3-34.64/MCRP 3-17.7G/MCRP 3-17.7G 25 September 2012
Contents
Figure 9-3. Graphical method of estimating plasticity characteristics of a combination
two soils………………………………………………………………………………….. 9-9
Figure 9-4. Gradation triangle for use in selecting a stabilizing additive…………………. 9-12
Figure 9-5. Group index for determining average cement requirements…………………. 9-18
Figure 9-6. Alternate method of determining initial design lime content…………………. 9-21
Figure 9-7. Classification of aggregate………………………………………………………….. 9-26
Figure 9-8. Approximate effective range of cationic and aniomic emulsion of various
types of asphalt……………………………………………………………………….. 9-27
Figure 9-9. Determination of asphalt grade for expedient construction………………….. 9-27
Figure 9-10. Selection of asphalt cement content…………………………………………….. 9-28
Figure 9-11. Typical sections for single-layer and multilayer design……………………… 9-29
Figure 9-12. Design curve for Class A and Class B single-layer roads using stabilized soils………………………………………………………………………………………. 9-33
Figure 9-13. Design curve for Class C single layer roads using stabilized soils……….. 9-33
Figure 9-14. Design curve for Class D single-layer roads using stabilized soils………. 9-34
Figure 9-15. Design curve for Class E single-layer roads using stabilized soils……….. 9-34
Figure 9-16. Design curve for Class A and Class B multilayer roads using stabilized
soils………………………………………………………………………………………. 9-35
Figure 9-17. Design curve for Class C multilayer roads using stabilized soils…………. 9-36
Figure 9-18. Design curve for Class D multilayer roads using stabilized soils…………. 9-36
Figure 9-19. Design curve for Class E multilayer roads using stabilize soils…………… 9-37
Figure 9-20. Equivalency factors for soils stabilized with cement, lime, or cement and
lime mixed with fly ash………………………………………………………………. 9-38
Figure 9-21. Design curves for single layer airfields using stabilized soils in close
battle areas…………………………………………………………………………….. 9-40
Figure 9-22. Design curves for single-layer airfields using stabilized soils in rear
areas……………………………………………………………………………………… 9-41
Figure 9-23. Design curves for single-layer airfields using stabilized soils in rear area 6,000′………………………………………………………………………………………………………………….. 9-42
Figure 9-24. Design curves for single-layer airfields using stabilized soils in tactical
rear area………………………………………………………………………………… 9-42
Figure 9-25. Design curves for single-layer airfields using stabilized soils in tactical COMMZ area…………………………………………………………………………… 9-43
Figure 9-26. Design curves for single-layer airfields using stabilized soils in liaison COMMZ airfields……………………………………………………………………… 9-44
Figure 9-27. Design curves for single-layer airfields using stabilized soils in COMMZ. 9-45 Figure 9-28. Design curves for single-layer airfields using stabilized soils in semi-
permanent COMMZ airfields………………………………………………………. 9-46
Figure 9-29. Thickness design procedure for subgrades that increase in strength with depth…………………………………………………………………………………….. 9-56
Figure 9-30. Thickness design procedure for subgrades that decrease in strength
with depth………………………………………………………………………………. 9-57
Figure 9-31. Polypropylene membrane layout for tangential sections……………………. 9-68
Figure 9-32. Polypropylene membrane layout for curved sections……………………….. 9-68
Figure 9-33. Dust control effort required for heliports……………………………………….. 9-77
25 September 2012 TM 3-34.64/MCRP 3-17.7G ix
Contents
Figure 9-34. Cross section of dune showing initial and subsequent fences…………….. 9-79
Figure 9-35. Three fences installed to control dune formation…………………………….. 9-79
Figure 9-36. Three types of solid fencing or paneling for control of dune formation….. 9-80
Figure 9-37. Schematic of dune destruction or stabilization by selective treatment…… 9-81
Figure 10-1. Slope of bedding planes……………………………………………………………. 10-2
Figure 10-2. Normal force………………………………………………………………………….. 10-3
Figure 10-3. Downslope or driving force………………………………………………………… 10-4
Figure 10-4. Frictional resistance to sliding…………………………………………………….. 10-5
Figure 10-5. Frictional resistance to sliding with uplift force of groundwater……………. 10-8
Figure 10-6. Frictional resistance to sliding with and without groundwater………………. 10-8
Figure 10-7. Debris avalanche……………………………………………………………………. 10-10
Figure 10-8. Backward rotation of a slump block……………………………………………. 10-11
Figure 10-9. Jackstrawed trees………………………………………………………………….. 10-11
Figure 10-10. Structural features of a slump…………………………………………………. 10-12
Figure 10-11. Road construction across short slopes……………………………………… 10-12
Figure 10-12. Increasing slope stability with surface drainage……………………………. 10-13
Figure 10-13. Using rock riprap to provide support for road cuts or fills……………….. 10-14
Figure 10-14. Installing interceptor drains along an existing road……………………….. 10-14
Figure 10-15. Building a road on a blanket……………………………………………………. 10-15
Figure 10-16. Block diagram of type I site…………………………………………………….. 10-16
Figure 10-17. Topographic map of type I site………………………………………………… 10-17
Figure 10-18. Pistol-butted trees………………………………………………………………… 10-18
Figure 10-19. Tipped trees………………………………………………………………………… 10-18
Figure 10-20. Tension cracks…………………………………………………………………….. 10-19
Figure 10-21. Safe disposal site…………………………………………………………………. 10-20
Figure 11-1. Comparison of aggregate depth requirements with and without a
geotextile……………………………………………………………………………….. 11-2
Figure 11-2. Effect of pumping action on a base course…………………………………….. 11-2
Figure 11-3. Separating a weak subgrade from a granular subbase with a geofabric.. 11-3
Figure 11-4. Determining the soil’s shear strength by converting CBR value or cone
index……………………………………………………………………………………… 11-4
Figure 11-5. Thickness design curve for single-wheel load on gravel-surfaced
pavements……………………………………………………………………………… 11-8
Figure 11-6. Thickness design curve for dual-wheel load on gravel-surfaced
pavements……………………………………………………………………………… 11-9
Figure 11-7. Thickness design curve for tandem-wheel load on gravel-surfaced pavements……………………………………………………………………………… 11-9
Figure 11-8. Construction sequence using geotextiles…………………………………….. 11-13
Figure 11-9. Constructing an earth retaining wall using geofabrics…………………….. 11-15
Figure 11-10. Sand grid……………………………………………………………………………. 11-16
Figure 12-1. Maximum depth to permafrost below a road after 5 years in a subarctic region……………………………………………………………………………………. 12-8
Figure 12-2. Thickness of base required to prevent thawing of subgrade………………. 12-9
x TM 3-34.64/MCRP 3-17.7G/MCRP 3-17.7G 25 September 2012
Contents
Figure 12-3. Distribution of mean air thawing indexes (°) — North America………….. 12-10
Figure 12-4. Distribution of mean air thawing indexes (°) — Northern Eurasia……….. 12-11
Figure 12-5. Distribution of mean air thawing index values for pavements in North America (°)……………………………………………………………………………. 12-12
Figure 12-6. Determining the depth factor of thaw beneath pavements with gravel bases…………………………………………………………………………………… 12-14
Figure 12-7. Determining the depth of freeze beneath pavements with gravel bases 12-15 Figure 12-8. Permafrost degradation under different surface treatments………………. 12-16
Figure A-1. CBR design flowchart ……………………………………………………………………….. A-2
Figure A-2. Grain size distribution of Rio Meta Plain soil ………………………………………… A-7
Figure A-3. Plasticity chart plotted with Rio Meta Plain soil data ……………………………… A-8
Figure A-4. Density moisture curve for Rio Meta Plain soil ……………………………………… A-9
Figure A-5. Swelling curve for Rio Meta Plain soil ……………………………………………….. A-10
Figure A-6. CBR family of curves for Rio Meta Plain soil ………………………………………. A-11 Figure A-7. Density-moisture curve for Rio Meta Plain soil with density and moisture
ranges plotted ……………………………………………………………………………….. A-12
Figure A-8. CBR Family of Curves for Rio Meta Plain soil with density range plotted.. A-13 Figure A-9. Design CBR from Rio Meta Plain soil from CBR family of curves………….. A-14
Tables
Table 1-1.The Mohs hardness scale……………………………………………………………….. 1-2
Table 1-2. Classification of igneous rocks………………………………………………………. 1-14
Table 1-3. Classification of sedimentary rocks…………………………………………………. 1-16
Table 1-4. Classification of metamorphic rocks………………………………………………… 1-19
Table 1-5. Identification of geologic materials………………………………………………….. 1-22
Table 1-6. Field-estimating rock hardness………………………………………………………. 1-24
Table 1-7. Field-estimating rock density…………………………………………………………. 1-26
Table 1-8. Engineering properties of rocks……………………………………………………… 1-27
Table 1-9. Aggregate suitability based on physical properties……………………………… 1-28
Table 1-10. Use of aggregates for military construction missions…………………………. 1-28
Table 2-1. Geologic time scale…………………………………………………………………….. 2-15
Table 2-2. Reports for geographic/terrain intelligence……………………………………….. 2-34
Table 2-3. Sources of remote imagery…………………………………………………………… 2-37
Table 3-1. Stream evolution process………………………………………………………………. 3-6
Table 3-2. Fluvial surficial features……………………………………………………………….. 3-20
Table 3-3. Glacial surficial features……………………………………………………………….. 3-32
Table 3-4. Aggregate types by feature…………………………………………………………… 3-39
Table 5-1. Unified soil classification (including identification and description)……………. 5-6
Table 5-2. Auxiliary laboratory identification procedure……………………………………….. 5-8
Table 5-3. Characteristics pertinent to roads and airfields………………………………….. 5-13
Table 5-4. Classifications pertinent to embankment and foundation construction…….. 5-14
25 September 2012 TM 3-34.64/MCRP 3-17.7G xi
Contents
Table 5-5. Comparison of the USCS, revised public roads system, and FAA system .. 5-28 Table 5-6. Revised public roads system of soil classification……………………………… 5-30
Table 5-7. Agricultural soil classification system……………………………………………… 5-36
Table 5-8. Classification of four inorganic soil types…………………………………………. 5-37
Table 5-9. Comparison of soils under three classification system………………………… 5-37
Table 7a. Hydrogeologic indicators for groundwater exploration…………………………… 7-8
Table 7-1. Frost-susceptible soil groups………………………………………………………… 7-17
Table 8-1. Compaction test comparisons………………………………………………………… 8-3
Table 8-2. Minimum compaction requirements………………………………………………….. 8-8
Table 8-3. Soil classification and compaction requirements (average)…………………… 8-15
Table 9-1. Numerical example of proportioning…………………………………………………. 9-6
Table 9-2. Stabilization methods most suitable for specific applications………………… 9-11
Table 9-3. Guide for selecting a stabilizing additive………………………………………….. 9-12
Table 9-4. Minimum unconfined compressive strengths for cement, lime, and
combined lime-cement-fly ash stabilized soils…………………………………. 9-14
Table 9-5. Durability requirements………………………………………………………………… 9-15
Table 9-6. Gradation requirements……………………………………………………………….. 9-16
Table 9-7. Estimated cement requirements for various soil types………………………… 9-16
Table 9-8. Average cement requirements for granular and sandy soils…………………. 9-17
Table 9-9. Average cement requirements for silty and clayey soils………………………. 9-17
Table 9-10. Average cement requirements of miscellaneous materials…………………. 9-19
Table 9-12. Recommended gradations for bituminous-stabilized base and subbase materials………………………………………………………………………………… 9-24
Table 9-13. Bituminous materials for use with soils of different gradations…………….. 9-25
Table 9-14. Emulsified asphalt requirements…………………………………………………… 9-26
Table 9-15. Thickness design procedures by airfield category……………………………. 9-30
Table 9-16. Design determinations……………………………………………………………….. 9-30
Table 9-17. Estimated time required for test procedures……………………………………. 9-31
Table 9-18. Road classifications………………………………………………………………….. 9-32
Table 9-19. Recommended minimum thickness of pavement and base coarse for
roads in the theater-of-operations………………………………………………… 9-37
Table 9-20. Reduced thickness criteria for permanent and nonexpedient road and
airfield design………………………………………………………………………….. 9-38
Table 9-22. Thickness reduction factors for Navy design…………………………………… 9-47
Table 9-23. Equivalency factors for Air Force bases and Army airfields………………… 9-47
Table 9-24. Recommended minimum thickness of pavements and bases for airfields 9-48 Table 9-25. Stabilization functions pertinent to theater-of-operations airfields…………. 9-51
Table 9-26. Basic airfield expedient surfacing requirements……………………………….. 9-53
Table 9-27. Design requirements for strength improvement……………………………….. 9-55
Table 9-28. Recommended aggregate gradation for dust control on airfields and
heliports…………………………………………………………………………………. 9-66
Table 9-29. Dust palliative numbers for dust control in nontraffic areas…………………. 9-70
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Contents
Table 9-30. Dust palliative numbers for dust control in occasional-traffic areas……….. 9-71
Table 9-31. Dust palliative numbers for dust control in traffic areas………………………. 9-72
Table 9-32. Dust palliative electives………………………………………………………………. 9-73
Table 9-33. Roads and cantonment area treatments…………………………………………. 9-76
Table 9-34. Helipad/helicopter maintenance area treatments……………………………… 9-78
Table 11-1. Vehicle input parameters……………………………………………………………. 11-5
Table 11-2. Boussinesq theory coefficients…………………………………………………….. 11-6
Table 11-3. Compacted strength properties of common structural materials…………… 11-7
Table 11-4. Criteria and properties for geotextile evaluation……………………………… 11-11
Table 11-5. Geotextile survivability for cover material and construction equipment…. 11-11
Table 11-6. Minimum properties for geotextile survivability……………………………….. 11-12
Table 11-7. Recommended minimum overlap requirements……………………………… 11-14
Table 12-1. Gradation requirements for laterite and Iaterite gravels……………………… 12-3
Table 12-2. Criteria for laterite base course materials……………………………………….. 12-4
Table 12-3. Criteria for laterite subbase materials…………………………………………….. 12-4
Table 12-4. Measured depth of thaw below various surfaces in the subarctic after 5 years. (Fairbanks, Alaska, mean annual temperature 26 degrees
Fahrenheit)……………………………………………………………………………… 12-8
Table A-1. Recommended maximum permissible values of gradation and Atterberg
limit requirements in subbases and select materials …………………………….. A-5
Table A-2. Desirable gradation for crushed rock, gravel, or slag and uncrushed sand
and gravel aggregates for base courses …………………………………………….. A-5
Table B-1. Percentage of hard and brown coal reserves in major coal-producing countries…………………………………………………………………………………………. B-1
25 September 2012 TM 3-34.64/MCRP 3-17.7G xiii
SCOPE
Preface
Construction in the theater of operations is normally limited to roads, airfields, and structures necessary for military operations. This manual emphasizes the soils engineering aspects of road and airfield construction. The references give detailed information on other soils engineering topics that are discussed in general terms. This manual provides a discussion of the formation and characteristics of soil and the system used by the United States (US) Army to classify soils. It also gives an overview of classification systems used by other agencies. It describes the compaction of soils and quality control, settlement and shearing resistance of soils, the movement of water through soils, frost action, and the bearing capacity of soils that serve as foundations, slopes, embankments, dikes, dams, and earth-retaining structures. This manual also describes the geologic factors that affect the properties and occurrences of natural mineral/soil construction materials used to build dams, tunnels, roads, airfields, and bridges. Theater-of-operations construction methods are emphasized throughout the manual.
PURPOSE
This manual supplies engineer officers and noncommissioned officers with doctrinal tenets and technical facts concerning the use and management of soils during military construction. It also provides guidance in evaluating soil conditions, predicting soil behavior under varying conditions, and solving soil problems related to military operations. Military commanders should incorporate geologic information with other pertinent data when planning military operations, to include standing operating procedures.
The proponent of this publication is the US Army Engineer School. Submit changes for improving this publication on DA Form 2028 and forward it to: Commandant, US Army Engineer School, ATTN: ATSE-TD-D, Fort Leonard Wood, MO 65473-6650.
Unless otherwise stated, masculine nouns and pronouns do not refer exclusively to men.
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Chapter 1
Rocks and Minerals
The crust of the earth is made up of rock; rock, in turn, is composed of minerals. The geologist classifies rocks by determining their modes of formation and their mineral content in addition to examining certain chemical and physical properties. Military engineers use a simpler diagnostic method that is discussed below. Rock classification is necessary because particular rock types have been recognized as having certain properties or as behaving in somewhat predictable ways. The rock type implies information on many properties that serves as a guide in determining the geological and engineering characteristics of a site. This implied information includes—
This chapter describes procedures for field identification and classification of rocks and minerals. It also explains some of the processes by which rocks are formed. The primary objective of identifying rock materials and evaluating their physical properties is to be able to recommend the most appropriate aggregate type for a given military construction mission.
SECTION I – MINERALS
PHYSICAL PROPERTIES
1-1. Rocks are aggregates of minerals. To understand the physical properties of rocks, it is necessary to understand what minerals are. A mineral is a naturally occurring, inorganically formed substance having an ordered internal arrangement of atoms. It is a compound and can be expressed by a chemical formula. If the mineral’s internal framework of atoms is expressed externally, it forms a crystal. A mineral’s characteristic physical properties are controlled by its composition and atomic structure, and these properties are valuable aids in rapid field identification. Properties that can be determined by simple field tests are introduced here to aid in the identification of minerals and indirectly in the identification of rocks. These properties are—
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Chapter 1
HARDNESS
1-2. The hardness of a mineral is a measure of its ability to resist abrasion or scratching by other minerals or by an object of known hardness. A simple scale based on empirical tests has been developed and is called the Mohs Hardness Scale. The scale consists of 10 minerals arranged in increasing hardness with 1 being the softest. The 10 minerals selected to form the scale of comparison are listed in table 1-1. Hardness kits containing most of the reference minerals are available, but equivalent objects can be substituted for expediency. Objects with higher values on Mohs’ scale are capable of scratching objects with lower values. For example, a rock specimen that can be scratched by a copper coin but not by the fingernail is said to have a hardness of about 3. Military engineers describe a rock as either hard or soft. A rock specimen with a hardness of 5 or more is considered hard. The hardness test should be performed on a fresh (unweathered) surface of the specimen.
Table 1-1.The Mohs hardness scale
|
Mineral |
Relative Hardness |
Equivalent Objects |
|
Diamond |
10 |
|
|
Corundum |
9 |
|
|
Topaz |
8 |
|
|
Quartz |
7 |
Porcelain (7) |
|
Feldspar |
6 |
Steel file (6.5) |
|
Apatite |
5 |
Window glass (5.5) Knife blade or nail (5) |
|
Fluorite |
4 |
|
|
Calcite |
3 |
Copper coin (3.5) |
|
Gypsum |
2 |
Fingernail (2) |
|
Talc |
1 |
|
CRYSTAL FORM
1-3. Most, but not all, minerals form crystals. The form, or habit, of the crystals can be diagnostic of the mineral and can help to identify it. The minerals galena (a lead ore) and halite (rock salt) commonly crystallize as cubes. Crystals of garnet (a silicate mineral) commonly have 12 or 24 equidimensional faces. Some minerals typically display long needle-like crystals. Minerals showing no crystal form are said to be amorphous. Figure 1-1 illustrates two of the many crystal forms.
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Rocks and Minerals
CLEAVAGE
Figure 1-1. Crystal forms
1-4. Cleavage is the tendency of a mineral to split or separate along preferred planes when broken. It is fairly consistent from sample to sample for a given mineral and is a valuable aid in the mineral’s identification. Cleavage is described by noting the direction, the degree of perfection, and (for two or more cleavage directions) the angle of intersection of cleavage planes. Some minerals have one cleavage direction; others have two or more directions with varying degrees of perfection. Figure 1-2 illustrates a mineral with one cleavage direction (mica) and one with three directions (calcite). Some minerals, such as quartz, form crystals but do not cleave.
FRACTURE
Figure 1-2. Cleavage
1-5. Fracture is the way in which a mineral breaks when it does not cleave along cleavage planes. It can be helpful in field identification. Figure 1-3, page 1-4, illustrates the common kinds of fracture. They are—
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Chapter 1
LUSTER AND COLOR
Figure 1-3. Fractures
1-6. The appearance of a mineral specimen in reflected light is called its luster. Luster is either metallic or nonmetallic. Common non-metallic lusters are—
1-7. For some minerals, especially the metallic minerals, color is diagnostic. Galena (lead sulphide) is steel gray, pyrite (iron sulphide) is brass yellow, and magnetite (an iron ore) is black. However, many nonmetallic minerals display a variety of colors. The use of color in mineral identification must be made cautiously since it is a subjective determination.
STREAK
1-8. The color of a powdered or a crushed mineral is called the streak. The streak is obtained by rubbing the mineral on a piece of unglazed porcelain, called a streak plate. The streak is much more consistent in a mineral than the color of the intact specimen. For example, an intact specimen of the mineral hematite (an iron ore) may appear black, brown, or red, but the streak will always be dark red. The streak is most useful
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Rocks and Minerals
for the identification of dark-colored minerals such as metallic sulfides and oxides. Minerals with hardness
6.5 will not exhibit a streak, because they are harder than a piece of unglazed porcelain.
SPECIFIC GRAVITY
1-9. The specific gravity of a substance is the ratio of its weight (or mass) to the weight (or mass) of an equal volume of water. In field identification of minerals, the heft, or apparent weight, of the specimen is an aid to its identification. Specific gravity and heft are controlled by the kinds of atoms making up the mineral and the packing density of the atoms. For example, ores of lead always have relatively high specific gravity and feel heavy.
COMMON ROCK-FORMING MINERALS
1-10. There are approximately 2,000 known varieties of minerals. Only about 200 are common enough to be of geologic and economic importance. Some of the more important minerals to military engineers are—
QUARTZ
1-11. Quartz (silicon dioxide) is an extremely hard, transparent to translucent mineral with a glassy or waxy luster. Colorless to white or smoky-gray varieties are most common, but impurities may produce many other colors. Like man-made glass, quartz has a conchoidal (shell-like) fracture, often imperfectly developed. It forms pointed, six-sided prismatic crystals on occasion but occurs most often as irregular grains intergrown with other minerals in igneous and metamorphic rocks; as rounded or angular grains in sedimentary rocks (particularly sandstones); and as a microcrystalline sedimentary rock or cementing agent. Veins of milky white quartz, often quite large, fill cracks in many igneous and metamorphic rocks. Unlike nearly all other minerals, quartz is virtually unaffected by chemical weathering.
FELDSPARS
1-12. Feldspars form very hard, blocky, opaque crystals with a pearly or porcelainlike luster and a nearly rectangular cross section. Crystals tend to cleave in two directions along flat, shiny, nearly perpendicular surfaces. Plagioclase varieties often have fine parallel grooves (striations) on one cleavage surface. Orthoclase varieties are usually pink, reddish, ivory, or pale gray. Where more than one variety is present, color differences are normally distinct. Crystalline feldspars are major components of most igneous rocks, gneisses, and schists. In the presence of air and water, the feldspars weather to clay minerals, soluble salts, and colloidal silica.
MICAS
1-13. Micas form soft, extremely thin, transparent to translucent, elastic sheets and flakes with a bright glassy or pearly luster. “Books” of easily separated sheets frequently occur. The biotite variety is usually
25 September 2012 TM 3-34.64/MCRP 3-17.7G 1-5
Chapter 1
brown or black, while muscovite is yellowish, white, or silvery gray. Micas are very common in granitic rocks, gneisses, and schists. Micas weather slowly to clay minerals.
AMPHIBOLES
1-14. Amphiboles (chiefly hornblende) are hard, dense, glassy to silky minerals found chiefly in intermediate to dark igneous rocks and gneisses and schists. They generally occur as short to long prismatic crystals with a nearly diamond-shaped cross section. Dark green to black varieties are most common, although light gray or greenish types occur in some marbles and schists. Amphiboles weather rapidly to form chlorite and, ultimately, clay minerals, iron oxides, and soluble carbonates.
PYROXENES
1-15. Pyroxenes (chiefly augite) are hard, dense, glassy to resinous minerals found chiefly in dark igneous rocks and, less often, in dark gneisses and schists. They usually occur as well-formed, short, stout, columnar crystals that appear almost square in cross section. Granular crystals are common in some very dark gabbroic rocks. Masses of nearly pure pyroxene form a rock called pyroxenite. Colors of green to black or brown are most common, but pale green or gray varieties sometimes occur in marbles or schists. Pyroxenes weather much like the amphiboles.
OLIVINE
1-16. Olivine is a very hard, dense mineral that forms yellowish-green to dark olive-green or brown, glassy grains or granular masses in very dark, iron-rich rocks, particularly gabbro and basalt. Masses of almost pure olivine form a rare rock called peridotite. Olivine weathers rapidly to iron oxides and soluble silica.
CHLORITE
1-17. Chlorite is a very soft, grayish-green to dark green mineral with a pearly luster. It occurs most often as crusts, masses, or thin sheets or flakes in metamorphic rocks, particularly schists and greenstone. Chlorite forms from amphiboles and pyroxenes by weathering or metamorphism and, in turn, weathers to clay minerals and iron oxides.
CALCITE
1-18. Calcite is a soft, usually colorless to white mineral distinguished by a rapid bubbling or fizzing reaction when it comes in contact with dilute hydrochloric acid (HC1). Calcite is the major component of sea shells and coral skeletons and often occurs as well-formed, glassy to dull, blocky crystals. As a rock- forming mineral, it usually occurs as fine to coarse crystals in marble, loose to compacted granules in ordinary limestone, and as a cementing agent in many sedimentary rocks.
1-19. Calcite veins, or crack fillings, are common in igneous and other rocks. Calcite weathers chiefly by solution in acidic waters or water containing dissolved carbon dioxide.
DOLOMITE
1-20. Dolomite is similar to calcite in appearance and occurrence but is slightly harder and more resistant to solutioning. It is distinguished by a slow bubbling or fizzing reaction when it comes in contact with dilute HCl. Usually the reaction can be observed only if the mineral is first powdered (as by scraping it with a knife). Coarse dolomite crystals often have curved sides and a pinkish color. Calcite and dolomite frequently occur together, often in intimate mixtures.
LIMONITE
1-21. Limonite occurs most often as soft, yellowish-brown to reddish-brown, fine-grained, earthy masses or compact lumps or pellets. It is a common and durable cementing agent in sedimentary rocks and the
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Rocks and Minerals
major component of laterite. Most weathered rocks contain some limonite as a result of the decomposition of iron-bearing minerals.
CLAY
1-22. Clay minerals form soft microscopic flakes that are usually mixed with impurities of various types (particularly quartz, limonite, and calcite). When barely moistened, as by the breath or tongue, clays give off a characteristic somewhat musty “clay” odor. Clays form a major part of most soils and of such rocks as shale and slate. They are a common impurity in all types of sedimentary rocks.
SECTION II – ROCKS
FORMATION PROCESSES
1-23. A rock may be made of many kinds of minerals (for example, granite contains quartz, mica, feldspar, and usually hornblende) or may consist essentially of one mineral (such as a limestone, which is composed of the mineral calcite). To the engineer, rock is a firm, hardened substance that, in contrast to soil, cannot be excavated by standard earthmoving equipment. In reality, there is a transitional zone separating rock and soil so that not all “rock” deposits require blasting. Some “rock” can be broken using powerful and properly designed ripping equipment. The geologist places less restriction on the definition of rock.
1-24. Rocks can be grouped into three broad classes, depending on their origin. They are—
1-25. Igneous rocks are solidified products of molten material from within the earth’s mantle. The term igneous is from a word meaning “formed by fire.” Igneous rocks underlie all other types of rock in the earth’s crust and may be said to form the basement of the continents on which sedimentary rocks are laid down. Most sedimentary rocks are formed by the deposition of particles of older rocks that have been broken down and transported from their original positions by the agents of wind, water, ice, or gravity. Metamorphic rocks are rocks that have been altered in appearance and physical properties by heat, pressure, or permeation by gases or fluids. All classes of rocks (sedimentary, igneous, and metamorphic) can be metamorphosed. Igneous, sedimentary, and metamorphic rocks often occur in close association in mountainous areas, in areas once occupied by mountains but which have since eroded, and in broad, flat continental regions known as shields. Flat-lying sedimentary rocks form much of the plains of the continents and may occupy broad valleys overlain by recent or active deposits of sediments. The sediments being deposited in today’s oceans, lakes, streams, floodplains, and deserts will be the sedimentary rocks of tomorrow. The rock-forming processes continually interact in a scheme called the rock cycle, illustrated in figure 1-4.
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Chapter 1
Figure 1-4. The rock cycle
IGNEOUS
1-26. Igneous rocks are solidified from hot molten rock material that originated deep within the earth. This occurred either from magma in the subsurface or from lava extruded onto the earth’s surface during volcanic eruptions. Igneous rocks owe their variations in physical and chemical characteristics to differences in chemical composition of the original magma and to the physical conditions under which the lava solidified.
1-27. The groups forming the subdivisions from which all igneous rocks are classified are—
1-28. Figure 1-5 is a block diagram illustrating the major kinds of intrusive and extrusive rock bodies formed from the crystallization of igneous rocks. Dikes and sills are tabular igneous intrusions that are thin relative to their lengths and widths. Dikes are discordant; they cut across the bedding of the strata penetrated. Sills are concordant; they intrude parallel to and usually along bedding planes or contacts of the surrounding strata. Dikes and sills may be of any geologic age and may intrude young and old sediments. Batholiths are large, irregular masses of intrusive igneous rock of at least 40 square miles in area. A stock is similar to a batholith but covers less than 40 square miles in outcrop. Stocks and batholiths generally increase in volume (spread out) with depth and originate so deep that their base usually cannot be detected. Magma that reaches the earth’s surface while still molten is ejected onto the ground or into the air (or sea) to form extrusive igneous rock. The molten rock may be ejected as a viscous liquid that flows out of a volcanic vent or from fissures along the flanks of the volcano. The flowing viscous mass is called lava and the lava flow may extend many miles from the crater vent. Lava that is charged with gases and ejected violently into the air forms pyroclastic debris consisting of broken and pulverized rock and molten material. The pyroclastics solidify and settle to the ground where they form deposits of ash and larger-sized material that harden into layered rock (tuff). Igneous rocks are usually durable and resistant and form ridges, caps, hills, and mountains while surrounding rock material is worn away.
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Rocks and Minerals
Figure 1-5. Intrusive and extrusive rock bodies
1-29. The chemical composition and thus the mineral content of intrusive and extrusive igneous rocks can be similar. The differences in appearance between intrusive and extrusive rocks are largely due to the size and arrangement of the mineral grains or crystals. As molten material cools, minerals crystallize and separate from solution. Silica-rich magma or lava solidifies into rocks high in silicon dioxide (quartz) and forms the generally light-colored igneous rocks. Molten material rich in ferromagnesian (iron-magnesium) compounds form the darker-colored igneous rocks, which are deficient in the mineral quartz. If the magma cools slowly, large crystals have time to grow. If the magma (or lava) cools quickly, large crystals do not have the chance to develop. Intrusive rocks are normally coarse-grained and extrusive igneous rocks are fine-grained for this reason. If lava cools too quickly for crystals to grow at all, then natural glass forms. Figure 1-6 illustrates the difference between intrusive and extrusive igneous rock crystals.
Figure 1-6. Cross section of igneous rock
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Chapter 1
SEDIMENTARY
1-30. Sedimentary rocks, also called stratified rocks, are composed of chemical precipitates, biological accumulations, or clastic particles. Chemical precipitates are derived from the decomposition of existing igneous, sedimentary, and metamorphic rock masses. Dissolved salts are then transported from the original position and eventually become insoluble, forming “precipitates”; or, through evaporation of the water medium, they become deposits of “evaporites.” A relatively small proportion of the sedimentary rock mass is organic sediment contributed by the activities of plants and animals. Clastic sediments are derived from the disintegration of existing rock masses. The disintegrated rock is transported from its original position as solid particles. Rock particles dropped from suspension in air, water, or ice produce deposits of “clastic” sediments. Volcanically ejected material that is transported by wind or water and then deposited forms another class of layered rocks called “pyroclastics.” Most pyroclastic deposits occur in the vicinity of a volcanic region, but fine particles can be transported by the wind and deposited thousands of miles from the source. Inorganic clastic sediments constitute about three-fourths of the sedimentary rocks of the earth’s crust. Loose sediments are converted to rock by several processes collectively known as lithification. These are—
1-31. The weight of overlying sediments that have accumulated over a long time produces great pressure in the underlying sediments. The pressure expels the water in the sediments by the process of compaction and forces the rock particles closer together. Compaction by the weight of overlying sediments is most effective in fine-grained sediments like clay and silt and in organic sediments like peat. Cementation occurs when precipitates of mineral-rich waters, circulating through the pores of sediments, fill the pores and bind the grains together. The most common cementing materials are quartz, calcite (calcium carbonate), and iron oxides (limonite and hematite). Recrystallization and crystal growth of calcium carbonate dissolved in saturated lime sediments develop rocks (crystalline limestone and dolomite) with interlocking, crystalline fabrics.
1-32. Sedimentary rocks are normally deposited in distinct parallel layers separated by abrupt, fairly even contact surfaces called bedding planes. Each layer represents a successive deposit of material. Bedding planes are of great significance as they are planes of structural weakness. Masses of sedimentary rock can move along bedding planes during rock slides. Figure 1-7 represents the layer-cake appearance of sedimentary rock beds. Sedimentary rocks cover about 75 percent of the earth’s surface. Over 95 percent of the total volume of sediments consists of a variety of shales, sandstones, and limestones.
Figure 1-7. Bedding planes
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METAMORPHIC
1-33. The alteration of existing rocks to metamorphic rocks may involve the formation within the rock of new structures, textures, and minerals. The major agents in metamorphism are—
1-34. Heat increases the solvent action of fluids and helps to dissociate and alter chemical compounds. Temperatures high enough to alter rocks commonly result from the intrusion of magma into the parent rock in the form of dikes, sills, and stocks. The zone of altered rock formed near the intrusion is called the contact metamorphism zone (see figure 1-8). The alteration zone may be inches to miles in width or length and may grade laterally from the unaltered parent rock to the highly metamorphosed derivative rock. Pressures accompanying the compressive forces responsible for mountain building in the upper earth’s crust produce regional metamorphic rocks characterized by flattened, elongated, and aligned grains or crystals that give the rock a distinctive texture or appearance called foliation (see figure 1-9). Hot fluids, especially water and gases, are powerful metamorphic agents. Water under heat and pressure acts as a solvent, promotes recrystallization, and enters into the chemical composition of some of the altered minerals.
Figure 1-8. Contact metamorphism zone
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Chapter 1
Figure 1-9. Metamorphic foliation
CLASSIFICATION
1-35. Igneous, metamorphic, and sedimentary rocks require different identification and classification procedures. The fabric, texture, and bonding strength imparted to a rock by its formation process determine the procedures that must be used to classify it.
IGNEOUS
1-36. Igneous rocks are classified primarily on the basis of—
1-37. Texture is the relative size and arrangement of the mineral grains making up the rock. It is influenced by the rate of cooling of the molten material as it solidifies into rock. Intruded magmas cool relatively slowly and form large crystals if the intrusion is deep and smaller crystals if the intrusion is shallow. Extrusive lava is exposed abruptly to the air or to water and cools quickly, forming small crystals or no crystals at all. Therefore, referring to table 1-2, igneous rocks may have textures that are coarse-grained (mineral grains and crystals that can be differentiated by the unaided eye), very fine-grained (mineral crystals too small to be differentiated by the unaided eye), or of contrasting grain sizes (large crystals, or “phenocrysts,” in a fine-grained “ground mass”). The intrusive igneous rocks generally have a distinctive texture of coarse interlocking crystals of different minerals. Under certain conditions, deep-seated intrusions form “pegmatites” (rocks with very large crystals). The extrusive (volcanic) igneous rocks, however, show great variation in texture. Very fine-grained rocks may be classified as having stony, glassy, scoriaceous, or fragmental texture. A rock with a stony texture consists of granular particles. Fine-grained rocks with a shiny smooth texture showing a conchoidal fracture are said to be “glassy.” An example is obsidian, a black volcanic glass. Gases trapped in the extrusive lava may escape upon cooling, forming bubble cavities, or vesicles, in the rock. The result is a rock with scoriaceous texture. Fragmental rocks are those composed of lithified, pyroclastic material. The pyroclastic (volcanoclastic) deposits are made up of volcanic rock particles of various sizes that have drifted and accumulated by the action of wind and water after ejection from the volcanic vent. Fine-grained (smaller than 32 millimeters (mm)) ejecta (called lapilli, or ash) form deposits that become volcanic tuff. Lava flowing out of the volcanic vent or fissure forms a flow with a ropelike texture if the lava is very fluid. More viscous lava forms a blocky flow. Upon cooling,
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basaltic lava flows, sills, and volcanic necks sometimes crack. They often acquire columnar jointing characterized by near-vertical columns with hexagonal cross sections (see figure 1-10).
Figure 1-10. Jointing in igneous rocks
1-38. Igneous rocks are further grouped by their overall color, which is generally a result of their mineral content (see table 1-2). The light-colored igneous rocks are silica-rich, and the dark-colored igneous rocks are silica-poor, with high ferromagnesian content. The intermediate rocks show gradations from light to dark, reflecting their mixed or gradational mineral content. An example illustrating the use of table 1-2 is as follows: lava and other ejecta charged with gases may form scoria, a dark-colored, highly vesicular basaltic lava or pumice, a frothy, light-colored felsite lava so porous that it floats on water.
1-39. The common igneous rocks are—
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Chapter 1
Table 1-2. Classification of igneous rocks
Granite
1-40. This is a coarsely crystalline, hard, massive, light-colored rock composed mainly of potassium feldspar and quartz, usually with mica and/or hornblende. Common colors include white, gray, and shades of pink to brownish red. Granite makes up most of the large intrusive masses of igneous rock and is frequently associated with (and may grade into) gneisses and schists. In general, it is a reasonably hard, tough, and durable rock that provides good foundations, building stones, and aggregates for all types of construction. Relatively fine-grained varieties are normally much tougher and more durable than coarse- grained types, many of which disintegrate rather rapidly under temperature extremes or frost action. Very coarse-grained and quartz-rich granites often bond poorly with cementing materials, particularly asphaltic cements. Antistripping agents should be employed when granite is used in bituminous pavements.
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Felsite
1-41. This is a very fine-grained, usually extrusive equivalent of granite. Colors commonly range from light or medium gray to pink, red, buff, purplish, or light brownish gray. Felsites often contain scattered large crystals of quartz or feldspar. Isolated gas bubbles and streaklike flow structures are common in felsitic lavas. As a rule, felsites are about as hard and dense as granites, but they are generally tougher and tend to splinter and flake when crushed. Most felsites contain a form of silica, which produces alkali- aggregate reactions with portland cements. Barring these considerations, felsites can provide good general- purpose aggregates for construction.
Gabbro and Diorite
1-42. They form a series of dense, coarsely crystalline, hard, dark-colored intrusive rocks composed mainly of one or more dark minerals along with plagioclase feldspar. Since gabbro and diorite have similar properties and may be difficult to distinguish in the field, they are often grouped under the name gabbro- diorite. They are gray, green, brown, or black. Gabbro-diorites are common in smaller intrusive masses, particularly dikes and sills. As a group, they make strong foundations and excellent aggregates for all types of construction. However, their great toughness and high density make excavation and crushing costs very high, particularly in finer-grained varieties.
Basalt
1-43. This is a very fine-grained, hard, dense, dark-colored extrusive rock that occurs widely in lava flows around the world. Colors are usually dark gray to black, greenish black, or brown. Scattered coarse crystals of olivine, augite, or plagioclase are common, as are gas bubbles that may or may not be mineral-filled. With increasing grain size, basalt often grades into diabase, an extremely tough variety of gabbro. Both basalt and diabase make aggregates of the highest quality despite a tendency to crush into chips or flakes in sizes smaller than 2 to 4 centimeters.
Obsidian
1-44. This is a hard, shiny, usually black, brown, or reddish volcanic glass that may contain scattered gas bubbles or visible crystals. Like man-made glass, it breaks readily into sharp-edged flakes. Obsidian is chemically unstable, weak, and valueless as a construction material of any type.
Pumice
1-45. This is a very frothy or foamy, light-colored rock that forms over glassy or felsitic lava flows and in blocks blown from erupting volcanoes. Innumerable closely spaced gas bubbles make pumice light enough to float on water and also impart good insulating properties. Although highly abrasive, pumice is very weak and can usually be excavated with ordinary hand tools. It is used in the manufacture of low-strength, lightweight concrete and concrete blocks. Most varieties are chemically unstable and require the use of low-alkali portland cements.
Scoria
1-46. It looks very much like a coarse, somewhat cindery slag. In addition to its frothy texture, scoria may also exhibit stony or glassy textures or a combination of both. The color of scoria ranges from reddish brown to dark gray or black. Scoria is somewhat denser and tougher than pumice, and the gas bubbles that give it its spongy or frothy appearance are generally larger and more widely spaced than those in pumice. Scoria is very common in volcanic regions and generally forms over basaltic lava flows. It is widely used as a lightweight aggregate in concrete and concrete blocks. Like pumice, it may require the use of special low-alkali cements.
SEDIMENTARY
1-47. Sedimentary rocks are classified primarily by—
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Chapter 1
1-48. They can be described as either clastic or nonclastic (see table 1-3). The clastic rocks are composed of discrete particles, or grains. The nonclastic rocks are composed of interlocking crystals or are in earthy masses. Clastic sedimentary rocks are further classified as coarse-grained or fine-grained. The coarse- grained rocks have individual grains visible to the naked eye and include sandstones, conglomerates (rounded grains), and breccias (angular grains). These are the rock equivalents of sands and gravels. The fine-grained rocks have individual grains that can only be seen with the aid of a hand lens or microscope and include siltstones, shales, clay stones, and mudstones.
Table 1-3. Classification of sedimentary rocks
|
Group |
Dominant Composition |
Rock Type |
||
|
Clastic |
Coarse-grained |
Rock fragments larger than 2mm |
Rounded |
Conglomerate |
|
Angular |
Breccia |
|||
|
Mineral grains (chiefly quartz) 1/16 mm to 2 mm) |
Sandstone |
|||
|
Fine grained |
Clay and silt-sized particles (smaller than 1/16 mm) |
Shale |
||
|
Nonclastic |
Inorganic |
Dolomite Microcrystalline silica |
Dolomite |
|
|
Chert |
||||
|
Organic/inorganic |
Calcite |
Limestone |
||
|
Organic |
Carbonaceous plant debris |
Coal |
||
1-49. Shales, claystones, and mudstones are composed of similar minerals and may be similar in overall appearance; however, a shale is visibly laminated (composed of thin tabular layers) and often exhibits “fissility,” that is, it can be split easily into thin sheets. Clay stone and mudstone are not fissile. Mudstone is primarily a field term used to temporarily identify fine-grained sedimentary rocks of unknown mineral content.
1-50. The nonclastic sedimentary rocks can be further described as inorganic (or chemical) or organic. Dolomite is an inorganic calcium-magnesium carbonate. Chert, a widespread, hard, durable sedimentary rock, composed of microcrystalline quartz, precipitates from silica-rich waters and is often found in or with limestones. Limestone is a calcium carbonate that can be precipitated both organically and inorganically. A diagnostic feature of limestone is its effervescence in dilute HCl. Coal is an accumulation and conversion of the organic remains of plants and animals under certain environments.
1-51. Important features of the exposed or sampled portion of a deposit include stratification, the thickness of strata, the uniformity or nonuniformity of strata laterally, and the attitude (strike and dip) of the bedding planes. Special sedimentary bedding features are—
1-52. Some typical sedimentary rocks are—
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Rocks and Minerals
Figure 1-11. Cross bedding in sandstone
Conglomerate and Breccia
1-53. They resemble man-made concrete in that they consist of gravel-sized or larger rock fragments in a finer-grained matrix. Different varieties are generally distinguished by the size or composition of the rock fragments (such as limestone breccia, boulder conglomerate, or quartz pebble conglomerate). Wide variations in composition, degree of cementation, and degree of weathering of component particles make these rocks highly unpredictable, even within the same deposit. Generally, they exhibit poor engineering properties and are avoided in construction. Some very weakly cemented types may be crushed for use as fill or subbase material in road or airfield construction.
SANDSTONE
1-54. This is a medium- to coarse-grained, hard, gritty, clastic rock composed mainly of sand-sized (1/16 mm to 2 mm) quartz grains, often with feldspar, calcite, or clay. Sandstone varies widely in properties according to composition and cementation. Clean, compact, quartz-rich varieties, well-cemented by silica or iron oxides, generally provide good material for construction of all types. Low-density, poorly cemented, and clayey varieties lack toughness and durability and should be avoided as sources of construction material; however, some clay-free types may be finely crushed to provide sand.
Shale
1-55. This is a soft to moderately hard sedimentary rock composed of very fine-grained silt and clay-sized particles as well as clay minerals. Silica, iron oxide, or calcite cements may be present, but many shales lack cement and readily disintegrate or slake when soaked in water. Characteristically, shales form in very thin layers, break into thin platy pieces or flakes, and give off a musty odor when barely moistened. Occasionally, massive shales (called mudstones) occur, which break into bulky fragments. Shales are frequently interbedded with sandstones and limestones and, with increasing amounts of sand or calcite, may grade into these rock types. Most shales can be excavated without the need for blasting. Because of their weakness and lack of durability, shales make very poor construction material.
Tuff
1-56. This is a low-density, soft to moderately hard pyroclastic rock composed mainly of fine-grained volcanic ash. Colors range from white through yellow, gray, pink, and light brown to a rather dark grayish
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Chapter 1
brown. When barely moistened, some tuffs give off a weak “clay” odor. Very compact varieties often resemble felsite but can usually be distinguished by their softness and the presence of glass or pumice fragments. Loose, chalky types usually feel rough and produce a gritty dust, unlike the smooth particles of a true chalk or clay. Tuff is a weak, easily excavated rock of low durability. When finely ground, it has weak cementing properties. It is often used as an “extender” for portland cement and as a pozzolan to improve workability and neutralize alkali-aggregate reactions. It can also be used as a fill and base course material.
Limestone
1-57. This is a soft to moderately hard rock composed mainly of calcite in the form of shells, crystals, grains, or cementing material. All varieties are distinguished by a rapid bubbling or fizzing reaction when they come in contact with dilute HCl. Colors normally range from white through various shades of gray to black; other colors may result from impurities. Ordinary limestone is a compact, moderately tough, very fine-grained or coarsely crystalline rock that makes a quality material for all construction needs. Hardness, toughness, and durability will normally increase with greater amounts of silica cement. However, more than about 30 percent silica may produce bonding problems or alkali-aggregate reactions. Clayey varieties usually lack durability and toughness and should be avoided. Weak, low-density limestones (including limerock and coral) are weakly recemented when crushed, wetted, and compacted. They are widely used as fill and base course material. In mild climates, some may prove suitable for use in low-strength portland cement concrete.
Dolomite
1-58. It is similar to limestone except that the mineral dolomite occurs in lieu of calcite. It is distinguished by a slow bubbling or fizzing reaction when it comes into contact with dilute HCl. Often the reaction cannot be seen unless the rock is first powdered (as by scraping with a knife). Limestone and dolomite exhibit similar properties, and often one grades into the other within a single deposit.
Chert
1-59. This is a very hard, very fine-grained rock composed of microcrystalline silica precipitated from seawater or groundwater. It occurs mainly as irregular layers or nodules in limestones and dolomites and as pebbles in gravel deposits or conglomerates. Most cherts are white to shades of gray. Very dark, often black, cherts are called flint, while reddish-brown varieties are called jasper. Pure, unweathered cherts break along smooth, conchoidal (shell-like) surfaces with a waxy luster; weathered or impure forms may seem dull and chalky-looking. Although cherts are very hard and tough, they vary widely in chemical stability and durability. Many produce alkali-aggregate reactions with portland cement, and most require the use of antistripping agents with bituminous cements. Low-density cherts may swell slightly when soaked and break up readily under frost action. Despite these problems, cherts are used in road construction in many areas where better materials are not available.
METAMORPHIC
1-60. Metamorphic rocks are classified primarily by—
1-61. They are readily divided into two descriptive groups (see table 1-4, page 1-18) known as—
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Table 1-4. Classification of metamorphic rocks
|
Structure |
Characteristics |
Rock Type |
|
Foliated |
Very fine-grained; cleaves readily into thin sheets or plates |
slate |
|
Fine- to coarse-grained; thin semiparallel layers of platy minerals; splits into flakes between layers |
Schist |
|
|
Fine- to coarse-grained; streaks or bands of differing mineralogic composition; breaks into bulky pieces |
Gneiss |
|
|
Nonfoliated |
Mostly fused quartz grains |
Quartzite |
|
Mostly calcite or dolomite |
Marble |
1-62. Foliated metamorphic rocks display a pronounced banded structure as a result of the deformational pressures to which they have been subjected. The nonfoliated, or massive, metamorphic rocks exhibit no directional structural features.
1-63. The common foliated or banded metamorphic rocks include—
Slate
1-64. This is a very fine-grained, compact metamorphic rock that forms from shale. Unlike shale, slates have no “clay” odor. They split into thin, parallel, sharp-edged sheets (or plates) usually at some angle to any observable bedding. Colors are normally dark red, green, purple, or gray to black. Slates are widely used as tiles and flagstones, but their poor crushed shape and low resistance to splitting makes them unsuitable for aggregates or building stones.
Schist
1-65. This is a fine- to coarse-grained, well-foliated rock composed of discontinuous, thin layers of parallel mica, chlorite, hornblende, or other crystals. Schists split readily along the structural layers into thin slabs or flakes. This characteristic makes schists undesirable for construction use and hazardous to excavate. However, varieties intermediate to gneiss may be suitable for fills, base courses, or portland cement concrete.
Gneiss
1-66. (Pronounced “nice”) This is a roughly foliated, medium- to coarse-grained rock that consists of alternating streaks or bands. The banding is caused by segregation of light-colored layers of quartz and feldspar alternating with dark layers of ferromagnesian minerals. These streaks may be straight, wavy, or crumpled and of uniform or variable thickness. Gneisses normally break into irregular, bulky pieces and resemble the granitic rocks in properties and uses. With increasing amounts of mica or more perfect layering, gneisses grade into schists.
1-67. The common nonfoliated metamorphic rocks include—
Quartzite
1-68. This is an extremely hard, fine- to coarse-grained, massive rock that forms from sandstone. It is distinguished from sandstone by differences in fracture. Quartzite fractures through its component grains rather than around them as in sandstone, because the cement and sand grains of quartzite have been fused or welded together during metamorphism. Therefore, broken surfaces are not gritty and often have a
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Chapter 1
splintery or sugary appearance like that of a broken sugar cube or hard candy. Quartzite is one of the hardest, toughest, and most durable rocks known. It makes excellent construction material, but excavation and crushing costs are usually very high. Because of its high quartz content, antistripping agents are normally required with bituminous cements. Even so, bonds may be poor with very fine-grained types.
Marble
1-69. This is a soft, fine- to coarsely crystalline, massive metamorphic rock that forms from limestone or dolomite. It is distinguished by its softness, acid reaction, lack of fossils, and sugary appearance on freshly broken surfaces. Marble is similar to ordinary compact or crystalline limestones in its engineering properties and uses. However, because of its softness, it is usually avoided as an aggregate for pavements on highways and airfields. White calcite or pinkish dolomite veins and subtle swirls or blotches of trace impurities give marble its typical veined or “marbled” appearance.
1-70. Metamorphic rocks have been derived from existing sedimentary, igneous, or metamorphic rocks as depicted in figure 1-4, page 1-7, and figure 1-12.
Figure 1-12. Metamorphism of existing rocks
IDENTIFICATION
1-71. Military engineers must frequently select the best rock for use in different types of construction and evaluate foundation or excavation conditions.
FIELD IDENTIFICATION
1-72. A simple method of identification of rock types that can be applied in the field will assist in identifying most rocks likely to be encountered during military construction. This method is presented in simple terms for the benefit of the field engineer who is not familiar with expressions normally used in technical rock descriptions.
1-73. The identification method is based on a combination of simple physical and chemical determinations. In some cases, the grains composing a rock may be seen, and the rock may be identified from a knowledge
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of its components. In other cases, the rock may be so fine-grained that the identification must be based on its general appearance and the results of a few easy tests.
1-74. The equipment required consists only of a good steel knife blade or a nail and a bottle of dilute (10 percent solution) HCl, preferably with a dropper. A small 6- to 10-power magnifying glass may also be helpful. HCl (muriatic acid) is available at most hardware stores and through the military supply system. Military hospitals are a typical source for HCl.
1-75. Samples for identification should be clean, freshly broken, and large enough to clearly show the structure of the rock. In a small sample, key characteristics (such as any alignment of the minerals composing the rock) may not be observed as readily as in a larger one. Pieces about 3 inches by 4 inches by 2 inches thick are usually suitable.
GENERAL CATEGORIES
1-76. The identification system of geologic materials is given in flow chart form (see table 1-5). In this method, all considerations are based on the appearance or character of a clean, freshly broken, unweathered rock surface. Weathered surfaces may exhibit properties that may not be true indicators of the actual rock type.
1-77. For most rocks the identification process is direct and uncomplicated. If a positive identification cannot be made, the more detailed rock descriptions (in the preceding paragraphs) should be consulted after first using the flow chart to eliminate all clearly inappropriate rock types. By the use of descriptive adjectives, a basic rock classification can be modified to build up a “word picture” of the rock (for example, “a pale brown, fine-grained, thin-bedded, compact, clayey, silica-cemented sandstone”).
1-78. To use the flow chart, the sample must be placed into one of three generalized groups based on physical appearance. These groups are—
Foliated
1-79. Foliated rocks are those metamorphic rocks that exhibit planar orientation of their mineral components. They may exhibit slaty cleavage (like slate) expressed by closely spaced fractures that cause the sample to split along thin plates. If the sample exhibits a parallel arrangement of platy minerals in thin layers and has a silky or metallic reflection, the sample has schistosity and is called a schist. If the sample exhibits alternating streaks or bands of light and dark minerals of differing composition, then the sample has gneissic layering and is called agneiss.
Very Fine-Grained
1-80. If the sample appears pitted or spongy, it is called frothy. The pits are called vesicles and are the result of hot gases escaping from magma at the top of a lava pool. If the sample is light enough to float on water and is light-colored, it is called pumice. If the frothy rock sample is dark-colored and appears cindery, it is called scoria.
1-81. If the sample has the appearance of broken glass, it is called either obsidian or quartz. Obsidian is a dark-colored natural volcanic glass that cooled too fast for any crystals to develop. Quartz is not a glass but is identified on the flow chart as light-colored and glassy. Quartz is a mineral and not one of the aggregates classifed for use in military construction. It is often found in its crystalline form with six-sided crystals. It is common as veins in both igneous and metamorphic rock bodies.
1-82. A hardness test is conducted to determine whether a stony sample is hard or soft. If the sample can be scratched with a knife or nail, then it is said to be soft. If it cannot be scratched, then it is said to be hard.
1-83. Fine-grained rocks may be glassy, frothy, or stony. The term “stony” is used to differentiate them from “glassy” and “frothy” rocks.
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Chapter 1
Table 1-5. Identification of geologic materials
|
Foliated |
Very fine-grained; splits along thin planes Metallic reflection; splits into slabs and flakes or thin semitransparent sheets Contains streaks or bands of light and dark minerals; breaks to bulky, angular fragments |
Slate Schist
Gneiss |
|||
|
Very Fine Grained |
Frothy |
Light Colored; lightweight; easily crushed
Dark colored; cindery |
Pumice
Scoria |
||
|
Glassy |
Light colored; massive; extremely hard Dark colored; may have some gas bubbles |
Quartz Obsidian |
|||
|
Stony |
Soft |
No acid reaction |
Earthy; clay odor; platy May have small pieces of glass; low density |
Shale Tuff |
|
|
Acid reaction |
Sugary appearance Dull and massive |
Marble Limestone |
|||
|
Hard |
Waxy; very hard; weathers to soft white Dull; may contain some gas bubbles or visible crystals
Sandy; mostly one mineral (quartz) |
Light colored Dark colored
Gritty sandpaper feel Sugary; not gritty |
Chert
Felsite Basalt
Sandstone Quartzite |
||
|
Coarse-Grained |
Hard |
Sandy; mostly one mineral (quartz) Mixed minerals; salt-and-pepper appearance
Fragmental; appearance of broken concrete |
Gritty sandpaper feel Sugary; not gritty Light Colored Dark colored |
Sandstone Quartzite Granite Gabbro-diorite
Conglomerate |
|
|
Soft |
Fragmental; may contain small pieces of glass |
Low density |
Tuff |
||
|
Acid reaction |
Sugary appearance Shell fragments |
Marble Limestone |
|||
1-84. If the sample is soft, a chemical determination must be made using dilute HCl, HCl tests determine the presence or absence of calcite (calcium carbonate) which comprises limestone/dolomite and marble. Very fine-grained rocks that are soft and stony in appearance and have an acid reaction are composed of calcium carbonate. If the sample is dull and massive, it is called a limestone. Dolomites are also dull and massive and are not separated from limestones on this chart, but they do not readily react to acid. Their surface must be powdered first by scratching the sample with a nail. The dolomite powder then readily reacts to the acid. If the sample exhibits a sugary appearance, then it is the metamorphic rock, marble. The sugary appearance is due to the partial melting or fusing of calcite crystals by heat and pressure. It is
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similar to the appearance of the sugar coating on a breakfast cereal. These rocks are susceptible to chemical attack by acidic solutions that form when carbon dioxide (CO2) is absorbed in groundwater. This produces a mild carbonic acid that dissolves the calcite in the rock. This is the process that produces caves and sinkholes. If there is not an acid reaction (no effervescence) and the sample has a platy structure, it is a shale. A shale may be any color. It is normally dull and separates into soft, thin plates. When freshly broken, it may have a musty odor similar to clay. Shales are derived from lithification of clay particles and fine muds. Shale is a sedimentary rock and should not be confused with its metamorphic equivalent, slate. Shales often occur interbedded within layers of limestone and dolomite. Because they are often found with carbonate rocks, they may exhibit an acid reaction due to contamination by calcium carbonate.
1-85. If the sample has no acid reaction and has a relatively low density, with small pieces of glass in its fine-grained matrix, the rock is called a tuff. Tuff is a volcanic sedimentary rock comprised mostly of ash particles that have been solidified. By the flow chart, it is characterized as soft; however, it may be hard if it has been welded by hot gases during the eruption. If it is hard, tuff may exhibit physical properties that make it suitable for some construction applications.
1-86. Fine-grained, stony samples that are hard may be waxy, dull, or sandy. Waxy samples resemble candle wax and have a conchoidal fracture and sharp edges. These rocks are called chert. They often weather into soft, white materials. Dull samples are either light- or dark-colored igneous extrusive rocks. If the sample is light-colored, it is a felsite. Felsites are normally massive but may appear banded or layered. Unlike gneiss, the layers are not made up of alternating light and dark minerals. Felsites may also have cavities filled with other lighter-colored minerals. They represent a variety of lava rocks that are high in silica. Because of their great variety, they may be hard to identify properly. The dark counterpart to the felsites is basalt. Basalt is normally black and very tough. It is also a lava rock and often exhibits columnar jointing. It often occurs as dikes or sills in other rock bodies.
1-87. Hard, sandy, very fine-grained samples composed mostly of quartz may be either sandstone or quartzite. If the sample feels gritty like sandpaper, it is called sandstone. Sandstones are sedimentary rocks composed of sand grains that have been cemented together. If the sample appears sugary and does not feel like sandpaper, the rock is called quartzite. Quartzite is the metamorphic equivalent of sandstone. Like marble, it has a sugary appearance, which is due to the partial melting and fusing of the crystal grains.
Coarse-Grained
1-88. Coarse-grained rocks refer to those that have either crystals or cemented particles that are large enough to be readily seen with the unaided eye. Samples may be hard or soft. Hard samples may appear sandy, mixed, or fragmental. A sandy sample would be a sandstone or a quartzite. Because coarse sands grade into fine sands, there are coarse sandstones and fine sandstones. The sugary-appearing metamorphic version of a coarse sandstone is called a quartzite.
1-89. Hard, coarse-grained rocks that are comprised of mixed interlocking crystals have a salt-and-pepper appearance due to their light and dark minerals. If the rock is predominantly light in color, it is a granite. If it is predominantly dark in color, it is called a gabbro-diorite. Both of these rocks are igneous intrusive rocks that cooled slowly, allowing the growth of large interlocking crystals. If the sample appears to be made of round, cemented rock fragments similar to the appearance of concrete, it is called a conglomerate. If the rock fragments are angular instead of round, the rock is called a breccia.
1-90. If a coarse-grained sample is soft, it may either be fragmental or have an acid reaction. If it appears fragmental and has a low density and small pieces of glass, it is a tuff. Tuff has already been described as a very fine-grained, stony, soft rock material. This entry on the chart is for the coarse-grained version of the same material. The coarse-grained rock samples that are soft and have an acid reaction are coarse-grained limestones and marbles, as already described.
1-91. The system of identification of rock types used by military engineers serves to identify most common rock types. This method requires the user to approach rock identification in a consistent and systematic manner; otherwise, important rock characteristics may go unnoticed. Many important rock features may not appear in small hand specimens. To enable better identification and evaluation, personnel who are in charge of geologic exploration should maintain careful notes on such rock features as bedding, foliation, gradational changes in composition or properties, and on the associations of rocks in the field. During
25 September 2012 TM 3-34.64/MCRP 3-17.7G 1-23
Chapter 1
preliminary reconnaissance work, geologic maps or map substitutes should be used to make preliminary engineering estimates based on the typical rock properties.
ENGINEERING PROPERTIES
1-92. The following engineering properties and tests are provided to help make engineering judgments concerning the use of rock materials as construction aggregate:
1-93. Preliminary engineering estimates can be made based on the typical rock properties ascertained from these tests. If a rock sample cannot be identified using the flow chart, a decision can be made as to its suitability for use as a construction material by these tests.
Toughness
1-94. This is mechanical strength, or resistance to crushing or breaking. In the field, this property may be estimated by attempting to break the rock with a hammer or by measuring its resistance to penetration by impact drills.
Hardness
1-95. This is resistance to scratching or abrasion. In the field, this may be estimated by attempting to scratch the rock with a steel knife blade. Soft materials are readily scratched with a knife, while hard materials are difficult or impossible to scratch (see table 1-6).
Table 1-6. Field-estimating rock hardness
|
Hardness |
Characteristics |
|
Very hard |
Not scratched by a steel file |
|
Hard |
Scratched by a steel file but difficult or impossible to scratch with a steel knife blade (or nail) |
|
Moderately hard |
Scratched by a knife but not by a copper coin |
|
Soft |
Scratched by a copper coin |
|
Very soft |
Scratched by a fingernail |
Durability
1-96. This is resistance to slaking or disintegration due to alternating cycles of wetting and drying or freezing and thawing. Generally, this may be estimated in the field by observing the effects of weathering on natural exposures of the rock.
Crushed Shape
1-97. Rocks that break into irregular, bulky fragments provide the best aggregates for construction because the particles compact well, interlock to resist displacement and distribute loads, and are of nearly equal strength in all directions. Rocks that break into elongated pieces or thin slabs, sheets, or flakes are weak in their thin dimensions and do not compact, interlock, or distribute loads as effectively (see figure 1-13, page 1-24).
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Rocks and Minerals
Figure 1-13. Crushed shape
Chemical Stability
1-98. This is resistance to reaction with alkali materials in portland cements. Several rock types contain impure forms of silica that react with alkalies in cement to form a gel. The gel absorbs water and expands to crack or disintegrate the hardened concrete. This potential alkali-aggregate reaction may be estimated in the field only by identifying the rock and comparing it to known reactive types or by investigating structures in which the aggregate has previously been used.
Surface Character
1-99. This refers to the bonding characteristics of the broken rock surface. Excessively smooth, slick, nonabsorbent aggregate surfaces bond poorly with cementing materials and shift readily under loads. Excessively rough, jagged, or absorbent surfaces are likewise undisturbed because they resist compaction or placement and require excessive amounts of cementing material.
Density
1-100. This is weight per unit volume. In the field, this may be estimated by “hefting” a rock sample (see table 1-7). Density reflects on excavation and hauling costs and may influence the selection of rocks for special requirements (such as riprap, jetty stone, or lightweight aggregate). Among rocks of the same type, density is often a good indicator of the toughness and durability to be expected. Table 1-8, page 1-26, lists the general ratings of rock properties for each of the 18 typical military construction aggregates. These ratings serve only as a guide; each individual rock body must be sampled and evaluated separately. Table 1-9, page 1-27, provides general guidance to determine the suitability of an unidentified rock sample for general military construction missions based on the evaluation of its physical properties.
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Chapter 1
Table 1-7. Field-estimating rock density
|
Description |
Density (g/cm3) |
|
Very dense |
over 3.0 |
|
Dense |
2.8 to 3.0 |
|
Moderately dense |
2.6 to 2.8 |
|
Low density |
2.4 to 2.6 |
|
Low density |
2.4 to 2.6 |
|
Very low density |
Below 2.4 |
1-101. Table 1-10, page 1-27, provides a rating for selected geologic materials concerning their suitability as an aggregate for concrete or asphalt or for their use as a base course material. Rock materials that typically exhibit chemical instability in concrete are marked with an asterisk. These materials may cause a concrete-alkali reaction due to their high silica content. In general, felsites, chert, and obsidian should not be used as concrete aggregate. The end result of these reactions is the weakening, and in extreme cases the failure, of the concrete design. Apparently, silica is drawn out of the aggregate to make a gel that creates weaknesses within the mix. The gel may expand with temperature changes and prevent the proper bonding of the cement and aggregate.
1-102. Rock types with two asterisks may not bond readily to bituminous materials. They require special antistripping agents to ensure that they do not “strip,” or separate, from the pavement mix. Stripping severely reduces pavement performance.
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Rocks and Minerals
Table 1-8. Engineering properties of rocks
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Chapter 1
Table 1-9. Aggregate suitability based on physical properties
|
Aggregate Used In |
Toughness |
Hardness |
Durability |
ChemicalStability |
Crushed Shape |
Surface Character |
|
Portland Cement |
Good |
Fair |
Good |
Good |
Good* |
Good |
|
Bituminous Surfaces |
Good |
Good |
Good |
Any/All |
Good |
Good |
|
Base Coarse |
Good |
Good |
Good |
Any/All |
Good |
Good |
|
* The preferred shape for portland cement aggregate is an irregular, bulky shape. |
||||||
Table 1-10. Use of aggregates for military construction missions
|
|
Rock type |
Use as Aggregates |
Use as a Base Course or Subbase |
|
|
Concrete |
Asphalt |
|||
|
Igneous |
Granite Gabbro-Diorite Basalt Felsite |
Fair-good Excellent Excellent Poor* |
Fair-good** Excellent Excellent Fair |
Good Excellent Excellent Fair-good |
|
Sedimentary |
Congolmerate Sandstone Shale Limestone Dolomite Chert |
Poor Poor-fair Poor Fair-good Good Poor* |
Poor Poor-fair Poor Good
Poor** |
Poor Fair-good Poor Good
Poor-fair |
|
Metamorphic |
Gneiss Schist Slate Quartzite Marble |
Good Poor-fair Poor Good Fair |
Good Poor-fair Poor Fair-good** Fair |
Good Poor-fair Poor Fair-good Fair |
|
* Reacts (alkali-aggregate). ** Antistripping agents should be used. |
||||
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Chapter 2
Structural Geology
Structural geology describes the form, pattern, origin, and internal structure of rock and soil masses. Tectonics, a closely related field, deals with structural features on a larger regional, continental, or global scale. Figure 2-1, page 2-2, shows the major plates of the earth’s crust. These plates continually undergo movement as shown by the arrows. Figure 2-2, page 2-3, is a more detailed representation of plate tectonic theory. Molten material rises to the earth’s surface at midoceanic ridges, forcing the oceanic plates to diverge. These plates, in turn, collide with adjacent plates, which may or may not be of similar density. If the two colliding plates are of approximately equal density, the plates will crumple, forming a mountain range along the convergent zone. If, on the other hand, one of the plates is more dense than the other, it will be subducted, or forced below, the lighter plate, creating an oceanic trench along the convergent zone. Active volcanism and seismic activity can be expected in the vicinity of plate boundaries. In addition, military engineers must also deal with geologic features that exist on a smaller scale than that of plate tectonics but which are directly related to the deformational processes resulting from the force and movements of plate tectonics.
The determination of geologic structure is often made by careful study of the stratigraphy and sedimentation characteristics of layered rocks. The primary structure or original form and arrangement of rock bodies in the earth‘s crust is often altered by secondary structural features. These secondary features include folds, faults, joints, and schistosity. These features can be identified and mapped in the field through site investigation and from remote imagery.
SECTION I – STRUCTURAL FEATURES IN SEDIMENTARY ROCKS
BEDDING PLANES
2-1. Structural features are most readily recognized in the sedimentary rocks. They are normally deposited in more or less regular horizontal layers that accumulate on top of each other in an orderly sequence. Individual deposits within the sequence are separated by planar contact surfaces called bedding planes (see figure 1-7, page 1-10). Bedding planes are of great importance to military engineers. They are planes of structural weakness in sedimentary rocks, and masses of rock can move along them causing rock slides. Since over 75 percent of the earth’s surface is made up of sedimentary rocks, military engineers can expect to frequently encounter these rocks during construction.
2-2. Undisturbed sedimentary rocks may be relatively uniform, continuous, and predictable across a site. These types of rocks offer certain advantages to military engineers in completing horizontal and vertical construction missions. They are relatively stable rock bodies that allow for ease of rock excavation, as they will normally support steep rock faces. Sedimentary rocks are frequently oriented at angles to the earth’s “horizontal” surface; therefore, movements in the earth’s crust may tilt, fold, or break sedimentary layers. Structurally deformed rocks add complexity to the site geology and may adversely affect military construction projects by contributing to rock excavation and slope stability problems.
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Chapter 2
Figure 2-1. The major plates of the earth’s crust
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Structural Geology
Figure 2-2. Major features of the plate tectonic theory
2-3. Vegetation and overlying soil conceal most rock bodies and their structural features. Outcrops are the part of a rock formation exposed at the earth’s surface. Such exposures, or outcrops, commonly occur along hilltops, steep slopes, streams, and existing road cuts where ground cover has been excavated or eroded away (see figure 2-3). Expensive delays and/or failures may result when military engineers do not determine the subsurface conditions before committing resources to construction projects. Therefore, where outcrops are scarce, deliberate excavations may be required to determine the type and structure of subsurface materials. To determine the type of rock at an outcrop, the procedures discussed in chapter 1 must be followed. To interpret the structure of the bedrock, the military engineer must measure and define the trend of the rock on the earth’s surface.
Figure 2-3. Location of rock outcrops
FOLDS
2-4. Rock strata react to vertical and horizontal forces by bending and crumpling. Folds are undulating expressions of these forces. They are the most common type of deformation. Folds are most noticeable in layered rocks but rarely occur on a scale small enough to be observed in a single exposure. Their size varies considerably. Some folds are miles across, while others may be less than an inch. Folds are of significant importance to military engineers due to the change in attitude, or position, of bedding planes within the rock bodies (see figure 2-4, page 2-4). These can lead to rock excavation problems and slope instability. Folds are common in sedimentary rocks in mountainous areas where their occurrence may be inferred from
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Chapter 2
ridges of durable rock strata that are tilted at opposite angles in nearby rock outcrops. They may also be recognized by topographic and geologic map patterns and from aerial photographs. The presence of tilted rock layers within a region is usually evidence of folding.
Figure 2-4. Folding of sedimentary rock layers
TYPES
There are several basic types of folds. They are—
2-5. A rock body that dips uniformly in one direction (at least locally) is called a homocline (see figure 2-5a). A rock body that exhibits local steplike slopes in otherwise flat or gently inclined rock layers is called a monocline (see figure 2-5, b). Monoclines are common in plateau areas where beds may locally assume dips up to 90 degrees. The elevation of the beds on opposite sides of the fold may differ by hundreds or thousands of feet. Anticlines are upfolds, and synclines are downfolds (see figure 2-5, c and d, respectively). They are the most common of all fold types and are typically found together in a series of fold undulations. Differential weathering of the rocks composing synclines and anticlines tends to produce linear valleys and ridges. Folds that dip back into the ground at one or both ends are said to be plunging (see figure 2-6). Plunging anticline and plunging syncline folds are common. Upfolds that plunge in all directions are called domes. Folds that are bowed toward their centers are called basins. Domes and basins normally exhibit roughly circular outcrop patterns on geologic maps.
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Structural Geology
Figure 2-5. Common types of folds
Figure 2-6. Topographic expression of plunging folds
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Chapter 2
SYMMETRY
2-6. Folds are further classified by their symmetry. Examples are—
2-7. The axial plane of a fold is the plane that bisects the fold as symmetrically as possible. The sides of the fold as divided by the axial plane are called the limbs. In some folds, the plane is vertical or near vertical, and the fold is said to be symmetrical. In others, the axial plane is inclined, indicating an asymmetrical fold. If the axial plane is greatly inclined so that the opposite limbs dip in the same direction, the fold is overturned. A recumbent fold has an axial plane that has been inclined to the point that it is horizontal. Figure 2-7 shows the components of an idealized fold. An axial line or fold axis is the intersection of the axial plane and a particular bed. The crest of a fold is the axis line along the highest point on an anticline. The trough denotes the line along the lowest part of the fold. It is a term associated with synclines.
Figure 2-7. Fold symmetry
CLEAVAGE AND SCHISTOSITY
2-8. Foliation is the general term describing the tendency of rocks to break along parallel surfaces. Cleavage and schistosity are foliation terms applied to metamorphic rocks. Metamorphic rocks have been altered by heat and/or pressure due to mountain building or other crustal movements. They may have a pronounced cleavage, such as the metamorphic rock slate that was at one time the sedimentary rock shale. Certain igneous rocks may be deformed into schists or igneisses with alignment of minerals to produce schistosity or gneissic foliation. The attitudes of planes of cleavages and schistosity can be mapped to help determine the structure of a rock mass. Their attitudes can complicate rock excavation and, if unfavorable, lead to slope stability problems.
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FAULTS
Structural Geology
2-9. Faults are fractures along which there is displacement of the rock parallel to the fracture plane; once- continuous rock bodies have been displaced by movement in the earth’s crust (see figure 2-8). The magnitude of the displacement may be inches, feet, or even miles along the fault plane. Overall fault displacement often occurs along a series of small faults. A zone of crushed and broken rock may be produced as the walls are dragged past each other. This zone is called a “fault zone” (see figure 2-9). It often contains crushed and altered rock, or “gouge,” and angular fragments of broken rock called “breccia.” Fault zones may consist of materials that have been altered (reduced in strength) by both fault movement and accelerated weathering by water introduced along the fault surface. Alteration of fault gouge to clay lowers the resistance of the faulted rock mass to sliding. Recognition of faults is extremely important to military engineers, as they represent potential weakness in the rock mass. Faults that cut very young sediments may be active and create seismic (earthquake) damage.
Figure 2-8. Faulting
Figure 2-9. Fault zone
RECOGNITION
2-10. Faults are commonly recognized on rock outcrop surfaces by the relative displacement of strata on opposite sides of the fault plane and the presence of gouge or breccia. Slickensides, which are polished and striated surfaces that result from movement along the fault plane, may develop on the broken rock faces in a direction parallel to the direction of movement. Faulting may cause a discontinuity of structure that may be observed at rock outcrops where one rock layer suddenly ends against a completely different layer. This is often observed in road cuts, cliff faces, and streambeds. Although discontinuity of rock beds often
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Chapter 2
indicates faulting, it may also be caused by igneous intrusions and unconformities in deposition. Faults that are not visibly identifiable can be inferred by sudden changes in the characteristics of rock strata in an outcrop or borehole, by missing or repeated strata in a stratigraphic sequence, or (on a larger scale) by the presence of long straight mountain fronts thrust up along the fault. Rock strata may show evidence of dragging along the fault. Drag is the folding of rock beds adjacent to the fault (see figure 2-8, page 2-7 and figure 2-10). Faults are identifiable on aerial photographs by long linear traces (lineations) on the ground surface and by the offset of linear features such as strata, streams, fences, and roads. Straight fault traces often indicate near-vertical fault planes since traces are not distorted by topographic contours.
Figure 2-10. Thrust fault with drag folds
TERMINOLOGY
2-11. The strike and dip of a fault plane is measured in the same manner as it is for a layer of rock. (This procedure will be described later.) The fault plane intersection with the surface is called the fault line. The fault line is drawn on geologic maps. The block above the fault plane is called the hanging wall; the block below the fault plane is called the footwall. In the case of a vertical fault, there would be neither a hanging wall nor a footwall. The vertical displacement along a fault is called the throw. The horizontal displacement is the heave. The slope on the surface produced by movement along a fault is called the fault scarp. It may vary in height from a few feet to thousands of feet or may be eroded away (see figure 2-11).
Figure 2-11. Fault terminology
TYPES
2-12. Faults are classified by the relative direction of movement of the rock on opposite sides of the fault. The major type of movement determines their name. These types are—
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Structural Geology
2-13. Normal faults are faults along which the hanging wall has been displaced downward relative to the footwall (see figure 2-12, a). They are common where the earth’s surface is under tensional stress so that the rock bodies are pulled apart. Normal faults are also called gravity faults and usually are characterized by high-angle (near-vertical) fault planes. In a reverse fault, the hanging wall has been displaced upward relative to the footwall (see figure 2-12, b). Reverse faults are frequently associated with compressional forces that accompany folding. Low-angle (near-horizontal) reverse faults are called overthrust faults. Thrust faulting is common in many mountainous regions, and overthrusting rock sheets may be displaced many kilometers over the underlying rocks (see figure 2-10). Strike-slip faults are characterized by one block being displaced laterally with respect to the other; there is little or no vertical displacement (see figure 2-12, c). Many faults exhibit both vertical and lateral displacement. Some faults show rotational movement, with one block rotated in the fault plane relative to the opposite block. A block that is downthrown between two faults to form a depression is called a “graben” (see figure 2-13, a). An upthrown block between two faults produces a “horst” (see figure 2-13, b). Horsts and grabens are common in the Basin and Range Province located in the western continental United States. The grabens comprise the valleys or basins between horst mountains.
Figure 2-12. Types of faults
Figure 2-13. Graben and horst faulting
JOINTS
2-14. Rock masses that fracture in such a way that there is little or no displacement parallel to the fractured surface are said to be jointed, and the fractures are called joints (see figure 2-14, page 2-10). Joints influence the way the rock mass behaves when subjected to the stresses of construction. Joints characteristically form planar surfaces. They may have any attitude; some are vertical, others are horizontal, and many are inclined at various angles. Strike and dip are used to measure the attitude of joints. Some joints may occur as curved surfaces. Joints vary greatly in magnitude, from a few feet to thousands of
25 September 2012 TM 3-34.64/MCRP 3-17.7G 2-9
Chapter 2
feet long. They commonly occur in more or less parallel fractures called joint sets. Joint systems are two or more related joint sets or any group of joints with a characteristic pattern, such as a radiating or concentric pattern.
Figure 2-14. Jointing in sedimentary and igneous rock
FORMATION
2-15. Joints in rock masses may result from a number of processes, including deformation, expansion, and contraction. In sedimentary rocks, deformation during lithification or folding may cause the formation of joints. Igneous rocks may contain joints formed as lava cooled and contracted. In dense, extrusive igneous rocks, like basalt, a form of prismatic fracturing known as columnar jointing often develops as the rock cools rapidly and shrinks. Jointing may also occur when overlying rock is removed by erosion, causing a rock mass to expand. This is known as exfoliation. The outer layers of the rock peel, similar to the way that an onion does.
SIGNIFICANCE
2-16. Because of their almost universal presence, joints are of considerable engineering importance, especially in excavation operations. It is desirable for joints to be spaced close enough to minimize secondary plugging and blasting requirements without impairing the stability of excavation slopes or increasing the overbreakage in tunnels. The spacing of the joints can control the size of the material removed and can also affect drilling and blasting. The ideal condition is seldom encountered. In quarry operations, jointing can lead to several problems. Joints oriented approximately at right angles to the working face present the most unfavorable condition. Joints oriented approximately parallel to the working face greatly facilitate blasting operations and ensure a fairly even and smooth break, parallel to the face (see figure 2-14). Joints offer channels for groundwater circulation. In excavations below the groundwater table, they may greatly increase water problems. They also may exert an important influence on weathering.
STRIKE AND DIP
2-17. The orientation of planar features is determined by the attitude of the rock. The attitude is described in terms of the strike and dip of the planar feature. The most common planar feature encountered is a sedimentary bed. Strike is defined as the trend of the line of intersection formed between a horizontal plane and the bedding plane being measured (see figure 2-15). The strike line direction is given as a compass bearing that is always in reference to true north. Typical strikes would thereby fall between north 0 to 90 degrees east or north 0 to 90 degrees west. They are never expressed as being to the southeast or southwest.
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Structural Geology
Azimuths may be readily converted to bearings (for example, an azimuth of 350 degrees would be converted to a bearing of north 10 degrees west).
Figure 2-15. Strike
2-18. The dip is the inclination of the bedding plane. It is the acute angle between the bedding plane and a horizontal plane (see figure 2-16). It is a vertical angle measured at right angles from the strike line. The dip direction is defined as the quadrant of the compass the bed is dipping into (northeast, northwest, southeast, or southwest). By convention, the dip angle is given in degrees followed by the dip direction quadrant (for example, 30 degrees northeast).
Figure 2-16. Dip
2-19. The strike and dip measurements are taken in the field on rock outcrops with a standard Brunton compass. The Brunton compass is graduated in degrees and has a bull’s-eye level for determining the horizontal plane when measuring the strike direction. The strike is determined by aligning the compass along the strike direction and reading the value directly from the compass. Included with the Brunton compass is a clinometer to measure the dip angle. This angle is measured by placing the edge of the compass on the dipping surface at right angles to the strike direction and reading the acute angle indicated by the clinometer (see figure 2-17, page 2-12).
25 September 2012 TM 3-34.64/MCRP 3-17.7G 2-11
Chapter 2
Figure 2-17. Measuring strike and dip with a Brunton compass
2-20. Strike and dip symbols are used on geologic maps and overlays to convey structural orientation. Basic symbols include those for inclined, vertical, and horizontal beds (see figure 2-18). For inclined beds, the direction of strike is designated as a long line that is oriented in reference to the map grid lines in exactly the same compass direction as it was measured. The direction of the dip is represented by a short line that is always drawn perpendicular to the strike line and in the direction of the dip. The angle of the dip is written next to the symbol (see figure 2-18, a). For vertical beds, the direction of the strike is designated as it is for inclined beds. The direction of the dip is a short line crossing the strike line at a right angle extending on both sides of the strike line (see figure 2-18, b). For horizontal beds, the direction of strike is represented by crossed lines which indicate that the rock strikes in every direction. The dip is represented by a circle encompassing the crossed lines. The circle implies that there is no dip direction and the dip angle is zero (see figure 2-18, c). These basic symbols are commonly used to convey attitudes of sedimentary rocks (see figure 2-19). Similar symbols are used to convey attitudes of other types of planar features, such as folds, faults, foliation, and jointing in other rock bodies.
Figure 2-18. Strike and dip symbols
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Structural Geology
Figure 2-19. Strike and dip symbols of sedimentary rocks
SECTION II – GEOLOGIC MAPS
TYPES
2-21. Geologic maps show the distribution of geologic features and materials at the earth’s surface. Most are prepared over topographic base maps using aerial photography and field survey data. From a knowledge of geologic processes, the user of a geologic map can draw many inferences as to the geologic relationships beneath the surface and also much of the geologic history of an area. In engineering practice, geologic maps are important guides to the location of construction materials and the evaluation of foundation, excavation, and groundwater conditions.
2-22. The following geologic maps are used for military planning and operations:
BEDROCK OR AERIAL MAPS
2-23. These maps show the distribution of rock units as they would appear at the earth’s surface if all unconsolidated materials were removed. Symbols on such maps usually show the age of the rock unit as well as major structural details, such as faults, fold axes, and the attitudes of planar rock units or features. Thick deposits of alluvium (material deposited by running water) may also be shown.
SURFICIAL MAPS
2-24. These maps show the distribution of unconsolidated surface materials and exposed bedrock. Surface materials are usually differentiated according to their physical and/or chemical characteristics. To increase their usefulness as an engineering tool, most surficial maps show the distribution of materials at some shallow mapping depth (often one meter) so that minor residual soils and deposits do not mask the essential features of engineering concern.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 2-13
Chapter 2
SPECIAL PURPOSE MAPS
2-25. These maps show selected aspects of the geology of a region to more effectively present information of special geologic, military, or engineering interest. Special purpose maps are often prepared to show the distribution of—
2-26. Very detailed, large-scale geologic maps may show individual rock bodies, but the smallest unit normally mapped is the formation. A formation is a reasonably extensive, distinctive series of rocks deposited during a particular portion of geologic time (see table 2-1). A formation may consist of a single rock type or a continuous series of related rocks. Generally, formations are named after the locality where they were first defined. Formations may be grouped by age, structure, or lithology for mapping purposes.
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Table 2-1. Geologic time scale
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Chapter 2
SYMBOLS
2-27. Symbols are used to identify various features on a geologic map. Some of those features are—
FORMATIONS
2-28. Letters, colors, or symbolic patterns are used to distinguish formations or rock units on a geologic map. These designators should be defined in a legend on the map. Letter symbols usually consist of a capital letter indicating the period of deposition of the formation with subsequent letters (usually lower case) that stand for the formal name of the unit (see table 2-1, page 2-15). Maps prepared by the US Geological Survey and many other agencies use tints of yellow and orange for Cenozoic rocks, tints of green for Mesozoic rocks, tints of blue and purple for Paleozoic rocks, and tints of red for Precambrian rocks. Symbolic patterns for various rock types are given in figure 2-20.
Figure 2-20. Symbolic patterns for rock types
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Structural Geology
Figure 2-21. Geologic map symbols
CONTACTS
2-29. A thin, solid line shows contacts or boundaries between rock units if the boundaries are accurately located. A dashed line is used for an approximate location and a dotted line if the contact is covered or concealed. Questionable or gradational contacts are shown by a dashed or dotted line with question marks.
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Chapter 2
ATTITUDES
2-30. Strike and dip symbols describe planes of stratification, faulting, and jointing. These symbols consist of a strike line long enough so that its bearing can be determined from the map, a dip mark to indicate the dip direction of the plane being represented, and a number to show the value (in degrees) of the dip angle. The number is omitted on representations of both horizontal and vertical beds, because the values of the dips are automatically acknowledged to be 0 and 90 degrees, respectively. Figure 2-22 shows the placement of strike and dip symbols on a geologic map with respect to the location and orientation of a sedimentary rockbed.
Figure 2-22. Placement of strike and dip symbols on a geologic map
Figure 2-23. Geologic map and cross section
FAULT LINES AND FOLD AXES
2-31. Heavy black lines, which may be solid, dashed, or dotted (as described for contacts), show fault lines and fold axes. The direction of movement along faults is shown by arrows or by the use of symbols to indicate up thrown and down thrown sides. The arrows accompanying fold axes indicate the dip direction of the limbs and/or the plunge direction of the fold.
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CROSS SECTIONS
2-32. Cross sections show the distribution of geologic features and materials in a vertical plane along a line on a map. Cross sections are prepared in much the same way as topographic profiles using map, field, and borehole data. Geologic sections accompany many geologic maps to clarify subsurface relationships. Like geologic maps, geologic sections are often highly interpretive, especially where data is limited and structures are complex or concealed by overburden. Maps and sections use similar symbols and conventions. Because of the wealth of data that can be shown, geologic maps and sections are the two most important means of recording and communicating geologic information.
OUTCROP PATTERNS
2-33. An outcrop is that part of a rock formation that is exposed at the earth’s surface. Outcrops are located where there is no existing soil cover or where the soil has been removed, leaving the rock beneath it exposed. Outcrops may indicate both the type and the structure of the local bedrock. Major types of structural features can be easily recognized on geologic maps because of the distinctive patterns they produce. Figure 2-24 and figures 2-25 through 2-31, pages 2-20 through 2-23, show basic examples of common structural patterns.
Figure 2-24. Outcrop patterns of horizontal strata
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Chapter 2
Figure 2-25. Outcrop patterns of inclined strata
Figure 2-26. Outcrop patterns of an eroded dome
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Structural Geology
Figure 2-27. Outcrop patterns of and eroded basin
Figure 2-28. Outcrop patterns of plunging folds
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Chapter 2
Figure 2-29. Outcrop patterns produced by faulting
Figure 2-30. Outcrop patterns of intrusive rocks
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Structural Geology
Figure 2-31. Outcrop patterns of surficial deposits
2-34. Each illustration contains a block diagram showing a particular structural feature along with its topographic expression. The outcrop pattern of each rock unit shown on the block diagram is projected to a horizontal plane, resulting in the production of a geologic map that is also shown. This allows the reader to readily relate the structure shown on the block diagram to the map pattern. Structural details can be added to basic maps using the symbols in figure 2-21, page 2-17. The illustrations include some of the following structural features:
HORIZONTAL STRATA
2-35. Dendritic (branching or treelike) drainage patterns typically develop on horizontal strata and cut canyons or valleys in which progressively older rock units are exposed at depth (see figure 2-24, page 2-19). The result is that the map patterns of horizontal strata parallel stream valleys, producing a dendritic pattern on the geologic map. Although all maps do not show topographic contour lines, the contacts of horizontal rock units parallel the contours. Escarpments and gentle slopes generally develop on resistant and nonresistant beds, respectively, producing variations in the width of the map outcrop pattern. The upper and lower contacts are close together on steep cliffs; on gentle slopes of the same formation, the contacts are further apart. The map width of the outcrops of horizontal beds does not indicate the thickness of the strata. Gently dipping beds develop the same basic outcrop pattern as horizontal beds. However, the contacts of gently dipping strata, if traced far enough up a valley, cross topographic contours and form a large V-shaped pattern that points in the direction the beds dip, assuming that the beds do not dip in the direction of the stream gradient, but at a smaller angle.
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Chapter 2
INCLINED STRATA
2-36. When a sequence of rocks is tilted and cut off by erosion, the outcrop pattern appears as bands that, on a regional basis, are roughly parallel. Where dipping strata cross a valley, they produce a V-shaped outcrop pattern that points in the direction of dip, except in cases where the beds dip in the direction of the stream gradient at smaller angles than the gradient. The size of the V is inversely proportional to the degree of dip.
2-37. Other relationships that are basic to the interpretation of geologic maps are also shown in figure 2-25. For example, they show that older beds dip toward younger beds unless the sequence has been overturned (as by folding or faulting). Maps also show that outcrop width depends on the thickness of the beds, the dip of the beds (low dip, maximum width), and the slope of the topography (steep slope, minimum width).
DOMES
2-38. Eroded dome-shaped structures form a roughly circular outcrop pattern with beds dipping away from a central area in which the oldest rocks outcrop (see figure 2-26, page 2-20). These structures range from small features only a few meters across to great upwarps covering areas of hundreds or thousands of square kilometers.
2-39. Drainage patterns are helpful in interpreting a domal structure. Radial drainage patterns tend to form on domes. Streams cutting across the resistant beds permit one to apply the “rule of Vs” as explained above to interpret the direction of dip.
BASINS
2-40. Eroded structural basins form an outcrop pattern very similar to that of an eroded dome (see figure 2-27, page 2-21). However, two major features serve to distinguish them: younger rocks outcrop in the center of a basin and, if the structure has been dissected by stream erosion, the outcrop Vs normally point toward the center of a basin, whereas they usually point away from the center of a dome.
PLUNGING FOLDS
2-41. Folding is found in complex mountain ranges and sometimes in lowlands and plateaus. When folds erode, the oldest rocks outcrop in the center of the anticlines (or upfolds) and the youngest rocks outcrop in the center of the synclines (or downfolds). The axes of folded beds are horizontal in some folds, but they are usually inclined. In this case, the fold is said to plunge. Plunging folds form a characteristic zigzag outcrop pattern when eroded (see figure 2-28, page 2-21). A plunging anticline forms a V-shaped pattern with the apex (or nose) of the V pointing in the direction of the plunge. Plunging synclines form a similar pattern, but the limbs of the fold open in the direction of the plunge.
FAULTS
2-42. Fault patterns on geologic maps are distinc- tive in that they abruptly offset structures and terminate contacts (see figure 2-29, page 2-22). They are expressed on the geologic map by heavy lines in order to be readily distinguished. Some common types are—
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Normal and Reverse
2-43. (See A and B, respectively, in figure 2-29). Both normal and reverse fault planes generally dip at a high angle, so outcrop patterns are relatively straight. Older rocks are usually exposed on the upthrown block. It is thus possible to determine the relative movement on most high-angle faults from map relations alone. Linear streams, offsets, linear scarps, straight valleys, linear-trending springs or ponds, and omitted or repeated strata are common indications of faulting (see paragraph on recognition of faults, page 2-7).
Thrust
2-44. (See C in figure 2-29). Thrust faults are reverse faults that dip at low angles (less than 15 degrees) and have stratigraphic displacements, commonly measured in kilometers (see figure 2-10, page 2-8). The trace of the thrust commonly forms Vs where it intersects the valleys. The Vs point in the direction of the fault plane dip, except in cases where the fault plane dips in the direction of the stream gradient, but a smaller angle. Erosion may form windows (fensters) through the thrust sheet so that underlying rocks are exposed or produce isolation remnants (klippen) above the underlying rocks. Hachure symbols are used to designate the overthrust block that usually contains the oldest rocks.
INTRUSIONS
2-45. Larger igneous intrusions, such as batholiths and stocks, are typically discordant and appear on geologic maps as elliptical or roughly circular areas that cut across the contacts of surrounding formations (see figure 2-30, page 2-22). Smaller discordant intrusions, such as dikes, are usually tabular and appear on geologic maps as straight, usually short, bands. However, some dikes are lenticular and appear as such on the map. Concordant intrusions, such as sills and laccoliths, have contacts that parallel those of the surrounding formations (see figure 1-5, page 1-8).
2-46. The relative age of igneous bodies can be recognized from crosscutting relationships. The younger intrusions cut the older ones. With this in mind, it is clear from the relationships in figure 2-30, that the elliptical stock is the oldest intrusion, the northeast trending dike the next oldest, and the northwest trending dike the youngest. The age of the small discontinuous dikes near the western part of the map is younger than that of the stock, but the age relation with the other dikes is not indicated.
SURFICIAL DEPOSITS
2-47. Surficial deposits are recent accumulations of various types of sediment or volcanic debris on the surface of the landscape (see figure 2-31, page 2-23). The primary types are—
SECTION III – ENGINEERING CONSIDERATIONS
ROCK DISTRIBUTION
2-48. Geologic structure controls the distribution of rock bodies and features along and beneath the earth’s surface. The presence and orientation of such features as bedding, folding, faulting, and unconformities must be determined before construction begins. Otherwise, foundation, excavation, and groundwater conditions cannot be properly evaluated.
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Chapter 2
ROCK FRAGMENTATION
2-49. Rocks tend to fracture along existing zones of weakness. The presence and spacing of bedding, foliation, and joint planes can control the size and shape of rock fragments produced in quarries and other excavations. Operational and production costs may be prohibitive if rock fragments are too large, too small, too slabby, or too irregular for aggregate requirements. Advantageous joint or bedding spacings can significantly reduce excavation and aggregate production costs.
2-50. Many weak, thinly bedded, or highly fractured rocks can be excavated without blasting by using ripping devices drawn by heavy crawler tractors. When ripping is used to break up and loosen rock for removal, the work should proceed in the direction of the dip. This prevents the ripping devices from riding up the dip surfaces and out of the rock mass (see figure 2-32).
2-51. Most rock must be drilled and blasted for removal. Where joints or bedding planes incline across the axis of the drill hole, drill bits tend to follow these planes, causing the holes to be misaligned; or, more often, the bits to bind, stick, or break off in the holes (see figure 2-33). Open fractures and layers of weak rock greatly reduce blasting effectiveness by allowing the force of the blast to escape before the surrounding rock has been properly fragmented. Such situations require special drilling and blasting techniques that generally lower the efficiency of quarrying operations.
Figure 2-32. Ripping in the direction of dip
Figure 2-33. Rock drills
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ROCK SLIDES AND SLUMPS
Structural Geology
2-52. Massive rock slides may occur where unconfined rock masses overlie inclined bedding, foliation, fault, or joint surfaces (see figure 2-34). The risk of such slides is generally greatest over smooth, continuous, water- or clay-lubricated surfaces that dip steeply toward natural or man-made excavations. The following general observations may assist in evaluating hazards (see figure 2-35):
Figure 2-34. Rock slide on inclined bedding plane
Figure 2-35. Rules of thumb for inclined sedimentary rock cuts
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Chapter 2
WEAK ROCKS
2-53. Weak rocks, such as shales, may shear or crush under the weight of overlying rock and allow excavation sidewalls to slump or cave in. Such failures can be prevented by installing artificial supports or by using flattened or terraced side slopes to reduce the load on the potential failure zone.
FAULT ZONES
2-54. Fault zones are often filled with crushed and broken rock material. When these materials are water- soaked, they may weaken and cause the fault zone to become unstable. Such zones are extremely hazardous when encountered in tunneling and deep excavations because they frequently slump or cave in. Artificial supports are usually required to stabilize such materials.
GROUNDWATER
2-55. Water entering the ground percolates downward, through open fractures and permeable rocks, until it reaches a subsurface zone below which all void spaces are filled with water. Where such groundwater is intersected in an excavation, such as a road cut or tunnel, drainage problems may occur; rock slides triggered by the weakening and/or lubricating of the rock mass may result. In addition, water trapped under hydrostatic pressure in fault zones, joints, and permeable rock bodies can cause sudden flooding problems when released during excavation. Permeable rock zones may also permit water to escape from canals and reservoirs. However, if properly evaluated, the structural conditions that produce groundwater problems can also provide potential supplies of groundwater or subsurface drainage for engineering projects.
ROAD CUT ALIGNMENT
2-56. The most advantageous alignment for road cuts is generally at right angles (perpendicular) to the strike of the major planes of weakness in the rock (usually the bedding). This allows the rock surfaces to dip along the cut rather than into it (see figure 2-36, a and 2-36, b). Where roads must be aligned parallel to the strike of the major planes of weakness, the most stable alignment is one in which the major planes of weakness dip away from the excavation; however, some overhang should be expected (see figure 2-36, c).
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Structural Geology
Figure 2-36. Road cut alignment
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Chapter 2
QUARRY FACES
2-57. Quarries should normally be developed in the direction of strike so that the quarry face itself is perpendicular to strike (see figure 2-37, a). This particular orientation is especially important where rocks are steeply inclined, because it allows for the optimization of drilling and blasting efforts by creating a vertical or near-vertical rock face after each blast. If necessary, quarries may be worked perpendicular to the strike direction in instances where the rocks are not steeply inclined, but drilling and blasting will prove to be more difficult. In addition, if the rocks dip away from the excavation, overhang and oversized rocks can be expected (See figure 2-37, b). If the rocks dip toward the excavation, problems with slope instability and toeing may result (See figure 2-37, c). In massive igneous rock bodies and horizontal sedimentary rock layers, the direction of quarrying should be chosen based on the most prominent joint set or other discontinuity.
Figure 2-37. Quarry in the direction of the strike
ROCK DEFORMATION
2-58. Rocks may behave as elastic, plastic, or viscous solids under stress. Heavy loads, such as dams, massive fills, tall buildings, or bridge piers, may cause underlying rocks to compress, shear, or squeeze laterally. Particular problems exist where rocks of different strength underlie a site. For example, where weak shale and stronger limestone support different parts of the same structure, the structure may tile or crack due to uneven settlement.
2-59. The removal of confining stresses during excavation may cause rocks to expand or squeeze into the excavated area. Such problems seldom cause more than an increase in excavation or maintenance costs for roads, airfields, and railroads; however, they may cause serious damage to dams, buildings, canals, and tunnels where deformation cannot be tolerated. Weak clays and shales (especially compaction shales) are
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Structural Geology
the most common cause of such problems. Other rocks can also cause trouble if they are weathered or if they have been under great stress. To neutralize the effects of rock flow or rebound, the following may be required:
EARTHQUAKES (FAULT MOVEMENTS)
2-60. Movement along active faults produces powerful ground vibrations and rock displacements that can seriously disrupt engineering works. Unless proven otherwise by geological or historical evidence, all faults that disrupt recent geologic deposits should be considered active. Many areas suffer earthquakes as a result of deep-seated faults that do not appear at the earth’s surface. Consequently, seismic hazards must be thoroughly investigated before any major structure is undertaken. Power lines, dams, canals, tunnels, bridges, and pipelines across active faults must be designed to accommodate earth movements without failure. Buildings and airfields should be located away from known active fault zones. All vertical structures must be designed to accommodate the lateral movements and vibrations associated with earthquakes where seismic hazards exist. Expect increased seismic risk in marginal areas of continental plates (see figure 2-1, page 2-2).
DAMS
2-61. Dam sites are selected on the basis of topography, followed by a thorough geologic investigation. A geologic investigation should include as a minimum—
2-62. Generally, igneous rocks make the most satisfactory material for a dam foundation. Most igneous rocks are as strong as or stronger than concrete. However, many tuffs and agglomerates are weak. Solution cavities do not occur in igneous rocks because they are relatively insoluble; however, leakage will take place along joints, shear zones, faults, and other fissures. These can usually be sealed with cement grout.
2-63. Most metamorphic rocks have foundation characteristics similar to igneous rocks. Many schists are soft, so they are unsuitable as foundations for large, concrete dams. marble is soluble and sometimes contains large solution cavities. As a rule, metamorphic rocks can be treated with cement grout.
2-64. Sandstones allow seepage through pores, joints, and other fissures. The high porosity and low permeability of many sandstones make them difficult to treat with cement grout. Limestone’s solubility creates large underground cavities. Generally, the strength of shale compares favorably with that of concrete; however, its elasticity is greater. Shale is normally watertight.
TUNNELS
2-65. After determining the general location and basic dimensions of a tunnel, consider geological problems before designing and constructing it. Civil engineers dealing with tunnel construction understand
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Chapter 2
the need for geological data in this filed, so failure due to the lack of geological information seldom occurs. However, many failures do occur because engineers improperly interpret the available geological facts.
FOLDED STRATA
2-66. Extensive fracturing often exists along the axis of folded rock. This presents difficulties in tunneling operations. In an anticline, such fractures diverge upward; in a syncline, they diverge downward. If a tunnel is placed along the crest of a fold, the engineer can expect trouble from shattered rock. In such a case, the tunnel may have to be lined its entire length. In a syncline, an engineer could face additional trouble, even with moderate fracturing. The blocks bounded by fracture planes are like inverted keystones and are very likely to drop. When constructing tunnels in areas of folded rocks, the engineer should carefully consider the geologic structure. If a tunnel passes through horizontal beds, the engineer should encounter the same type of rock throughout the entire operation. In folded strata, many series of rock types can be encountered. therefore, carefully mapping geological structures in the construction area is important.
FAULTED STRATA
2-67. As with folded rocks, the importance of having firm, solid rock cannot be stressed enough, not only for safety and convenience in working but also for tunnel maintenance after completion. If rock is shattered by faulting, the tunnel must be lines, at least in the crushed-rock area. Also, if the fault fissure extends to the surface, it may serve as a channel for rainwater and groundwater.
GROUNDWATER PROBLEMS
2-68. Groundwater presence is often the main trouble source in tunnel construction. If tunnel grades cannot facilitate groundwater drainage, it may be necessary to pump throughout the tunnel. Therefore, it is necessary to have accurate information before beginning tunnel construction. Apply grout or cement, when possible, to solve water problems.
BRIDGES
2-69. Geological principles also apply in bridge construction. The weight of the bridge and the loads that it supports must be carried by the underlying foundation bed. In most cases, bridges are constructed for convenience and economy, so they must be located in specified areas. therefore, engineers cannot always choose the best site for piers and abutments. Once construction begins, the bridge location cannot be easily changed and should only be done so under exceptional circumstances.
2-70. As a rule, bridges are constructed to cross over rivers and valleys. An older riverbed or other depression (caused by glaciations or river deposits) could be completely hidden below the existing riverbed. Problems could arise if such a buried valley is not discovered before bridge construction begins. For example, riverbed contain many types of deposits, including large boulders. If preliminary work is not carefully done and correlated with geological principles, existing boulder deposits could be mistaken for solid bedrock.
BUILDINGS
2-71. Ground conditions at a building site may be one of three general types:
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2-72. If solid rock is present, its strength and physical properties must be determined. When the foundation consists of loose, unconsolidated sedimentary material, proper steps must be taken to solve the problem of subsidence. Structures that are supported on bedrock, directly or through piles or piers, will settle by extremely small amounts. If a foundation has been supported in unconsolidated strata, appreciable settlement can be expected.
QUARRY OPERATIONS
2-73. Natural sand and gravel are not always available, so it is sometimes necessary to produce aggregate by quarrying and processing rock. Quarrying is normally done only where other materials of adequate quality and size cannot be obtained economically and efficiently.
2-74. Many rock types are suitable for construction, and they exist throughout the world. Therefore, the quality and durability of the rock selected depend on local conditions. The following rock types are usually easy to quarry. They are also durable and resistant to weathering.
2-75. Factors that enhance the easy removal of rock often diminish its suitability for construction. Strong, durable, unweathered rock usually serves best for embankment and fill, base and surface course material, concrete aggregate, and riprap. However, these rocks are the most difficult to quarry or excavate.
2-76. Soils continuously change. Weathering, chemical alteration, dissolution, and precipitation of components all occur as soils accumulate and adjust to their environment. Particle coatings and natural cements are added and removed. Soluble components wash downward (leach) or accumulate in surface layers by evaporation. Plants take root and grow as soil profiles develop. These changes tend to become more pronounced with increasing age of the soil deposit. These possible alterations to the sediment may affect its utility in construction. The soils of natural geological deposits are commonly used as construction materials.
SECTION IV – APPLIED MILITARY GEOLOGY
2-77. The science and applied art of geology is an important component of military planning and operations. Today, computers, satellites, geographic information systems, geographic positioning systems, and similar technology have catapulted the art and science of geology as an everyday tool for military commanders and engineers.
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Chapter 2
MILITARY GEOGRAPHIC INTELLIGENCE
2-78. The purpose of geographic intelligence is to obtain data about terrain and climate. Commanders use the information to make sound decisions and soldiers use it to execute their missions. In planning an operation, commanders and their staff analyze the effects that terrain and climatic conditions will have on the activities of friendly and enemy forces. Knowledge of how geology controls and influences terrain is helpful for classifying and analyzing terrain and terrain effects. When provided with adequate geographic and geologic intelligence, commanders are able to exploit the advantages of the terrain and avoid or minimize its unfavorable aspects. Data on soil movement, the presence of hard rock, and the kind and distribution of vegetation is needed when considering concealment and cover, cross-country travel, and field fortifications. Strategic intelligence studies prepared by Department of Defense agencies provide detailed geographic and terrain information (table 2-2) useful for compiling and analyzing geographic intelligence.
Table 2-2. Reports for geographic/terrain intelligence
|
Report Title |
Report Application |
|
Imagery interpretation |
Planning combat and support operations Planning recon activities Supporting requests for terrain intelligence Analyzing areas of operations Planning terrain studies |
|
Terrain study on soils |
Supporting communications planning Executing movement, maneuver operations Planning combat operations (construction of landing strips, maintenance of culverts) Selecting avenues of approach |
|
Terrain study on rocks |
Planning movement, maneuver operations Planning combat operations (construction, maintenance, and destruction of roads, bridges, culverts, and defensive installations) Selecting avenues of approach |
|
Terrain study on water resources |
Selecting locations of and routes to water points Planning combat operations (street crossing, bridging) Supporting logistics planning |
|
Terrain study on drainage |
Supporting communications planning Planning combat operations (constructing roads, fortifications, and fjords) |
|
Supporting river crossings and cross-country movement |
|
|
Terrain study on surface configuration |
Supporting communications planning Planning observation posts and recon activity Planning tactical operations and executing tactical objectives Planning barrier and denial operations Planning artillery support |
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Structural Geology
Table 2-2. Reports for geographic/terrain intelligence
|
Report Title |
Report Application |
|
Terrain study on the state of the ground |
Planning movement, maneuver operations Planning ADM activity Planning combat operations (constructing, maintaining, and repairing roads, fjords, landing strips, and fortifications) Planning logistics support |
|
Terrain study on construction suitability |
Planning combat operations (constructing fortifications, landing strips, camouflage, obstacles, and a CP’s supply installations) Selecting construction supply-point locations |
|
Terrain study on coasts and landing beaches |
Planning amphibious operations (preparing and removing obstacles and fortifications) Planning recon activity Planning port construction |
|
Terrain study on cross- country movement |
Planning and executing maneuver, movement operations Planning logistics support Planning barrier and denial operations |
|
Planning engineer combat operations |
|
|
Terrain study on airborne landing areas |
Planning area-clearing support Planning recon activity Planning combat operations (constructing and repairing landing strips) Selecting helicopter landing zones |
MAPS AND TERRAIN MODELS
2-79. Maps are a basic source of terrain information. Geologic maps show the distribution, age, and characteristics of geologic units. They may also contain cross sections that show the subsurface occurrence of rock and soil. Maps may have text explaining physical properties; engineering properties; and other information on natural construction materials, rock formations, and groundwater resources. Soil maps are commonly presented under an agricultural classification but may be used for engineering purposes after they have been converted to engineering nomenclature.
2-80. A terrain model is a three-dimensional graphic representation of an area showing the conformation of the ground to scale. Computer-generated images have recently been developed that display two- dimensional form images in three-dimensional form for viewing from any angle.
REMOTE IMAGERY
2-81. Aerial and ground photography and remote imagery furnish information that is not readily available or immediately apparent by direct ground or aerial observation. Examples are infrared photography and side-looking radar. Imagery permanently preserves information so that it is available for later study. Photographs normally depict more recent terrain features than available maps.
TERRAIN CLASSIFICATION
2-82. Land forms are the physical expression of the land surface. For terrain intelligence purposes, major land forms are arbitrarily described on the basis of local relief (the difference in elevation between land forms in a given area).
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Chapter 2
GEOLOGY IN RESOLVING MILITARY PROBLEMS
2-83. To apply geology in solving military problems, personnel must first consider geologic techniques and uses and then determine how to acquire the needed information. Commanders can use geology in three ways-geologic and topographic map interpretation, photo interpretation, and ground reconnaissance.
MAPS
2-84. Combination topographic and geologic maps can tell commanders and engineers what the ground looks like. Map information should be made available early in operation planning. The success of many military operations depends on the speed of required advance construction. Speed, in turn, depends on completely understanding the needs and adequately planning to meet those needs in a particular area. Topographic maps are important sources of information on slope and land forms. One problem in using a topographic map is that some relief or roughness may be hidden between the contour lines. Generally, the larger the interval between contour lines and the smaller the scale of the map, the more the relief is hidden. Conversely, the smaller the contour intervals and the larger the scale of the map, the less the relief is hidden.
PHOTOGRAPHIC INTERPRETATION
2-85. Many principles that make a geologic map useful for estimating the terrain situation also apply to the usefulness of aerial photographs. The interpreter’s skill is very important. Without a professional photo interpretation and the knowledge of geologic process and forces, a great deal of information may be overlooked. In preparing terrain intelligence, aerial photographs alone will not provide enough information. Geologic maps must also be available. Aerial photographs are completely reliable for preparing tactical ter- rain intelligence. However, the usefulness of aerial photographs varies with the scale of the report being prepared.
RECONNAISSANCE
2-86. Interpretation of the land for military purposes can be accomplished by studying maps and can be even more accurate by adding photographs. Using maps or photographs is highly effective, but it cannot compare in accuracy to actual ground observation. In a tactical situation, such as when attacking a defended position, the knowledge of the terrain behind the position is vital for planning the next move. Terrain may be different on the side that cannot be seen. Generally, the difference is expressed in the geology of the slope that can be observed. By combining ground observation and reconnaissance by a trained observer (geologist) with aerial and map interpretation, the commander can plan ahead. Reconnaissance of secure territory can be used to an even greater advantage when developing the area of occupation or advancement.
REMOTE IMAGERY
2-87. The major kinds of remotely sensed imagery are photography, radar, and multispectral or digital scanner imagery. Since the 1930s, various worldwide agencies have acquired a large amount of remote imagery. The quality, scale, and nature of the coverage vary considerably because new techniques and equipment are being developed rapidly. Remote imagery can be—
2-88. High-altitude aircraft and spacecraft imagery are desirable for regional geologic mapping and delineating major structural features. Stereo coverage of low-, medium-, and high-altitude photography is used for detailed geologic mapping of rock units, structure, soil type, groundwater sources, and geologic hazards such as slope failure, sinkholes, fracturing zones, and flooding.
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Structural Geology
FIELD DATA COLLECTION
2-89. Field data collection is necessary to—
2-90. Mission constraints, such as time, resources, weather, and political climate, limit the amount of field investigation that can be conducted. The following methods and procedures, which were designed primarily for civilian peacetime projects, serve as a guide for the kinds of geologic information that can be obtained in field investigation.
Table 2-3. Sources of remote imagery
|
Procurement Platform |
Imagery Format |
Scale |
Coverage |
Source Agency |
|
Low-, medium-, and high-altitude aircraft |
Black and white or color stereo pairs of aerial photos of high resolution |
1:12,000 1:125,000 |
Limited worldwide low altitude, 1942 (to present; high altitude, 1965 to present) |
USGS, EROS data center, Sioux Falls, SD |
|
Low-, medium-, and high-altitude aircraft |
Black and white, color, color IR, black and white IR, thermal IR, SLAR, multiband imagery (reproductions of imagery are available to all US military organizations and US government agencies) |
1:1,000 1:100,000 |
Partial to full coverage of most foreign countries from late 1930s to present |
DIA, ATTN: DC- 6C2, Washington, DC 20301 |
|
Low-, medium-, and high-altitude aircraft |
Black and white, color IR |
Main scales: 1:20,000 1:40,000 Black and white: 20,000 to 1:120,000 |
Black and white: 80% of US Color IR: Partial US |
ASCS, Aerial Photo Field Office, PO Box 30010, Salt Lake City, UT 84130 |
|
Unmanned satel- lites ERTS LAND-SAT 1-5 MSS— Band 4: green (0.5-0.6 m) Band 5: red (0.7-0.8 m) Band 6: near IR (0.7-0.8 m) Band 7: near IR (0.8-1.0 m) |
Black and white, color composite (IR), color composition generation, 7 and 9—track computer— compatible tapes (800 and 1,600 bpi) |
1:250,000 1:3,369,000 |
Worldwide coverage, complete earth’s surface coverage every 18 days, 1972 to present; every 9 days, 1975 to present |
EROS data center |
25 September 2012 TM 3-34.64/MCRP 3-17.7G 2-37
Chapter 2
Table 2-3. Sources of remote imagery
|
Procurement Platform |
Imagery Format |
Scale |
Coverage |
Source Agency |
|
Manned spacecraft Skylab with S-190A multispectral cam- era and S-190B system, single lens (earth terrain camera) |
S-190A: black and white, black and white IR, color IR, high resolution color S-190B: black and white, color, stereo pairs |
S-190A: 1:1,250,000 1:2,850,000 S-190B: 1:125,000 1:950,000 |
Limited worldwide coverage |
EROS data center |
2-91. All available geologic information (literature, geologic and topographic maps, remote sensing imagery, boring logs, and seismic data relative to the general area of the project) should be collected, identified, and incorporated into the preliminary study for the project. This preliminary study should be completed before field investigations begin. A preliminary study allows those assigned to the project to become familiar with some of the engineering problems that they may encounter. Geologic information available in published and unpublished sources must be supplemented by data that is gathered in field investigations. Some of the most reliable methods available for field investigation are boring, exploration excavation, and geophysical exploration.
BORING
2-92. Borings are required to characterize the basic geologic materials at a project. They are broadly classified as disturbed, undisturbed, and core. Borings are occasionally made for purposes that do not require the recovery of samples, and they are frequently used for more than one purpose. Therefore, it is important to have a complete log of every boring, even if there is not an immediate use for some of the information. Initial exploration phases should concentrate on providing overall information about the site.
EXPLORATION EXCAVATION
2-93. Test pits and trenches can be constructed quickly and economically using bulldozers, backhoes, draglines, or ditching machines. Depths are normally less than 30 feet. Side excavations may require shoring, if personnel must work in the excavated areas. Exploratory tunnels allow for detailed examination of the composition and the geometry of rock structures such as joints, fractures, faults, and shear zones. Tunnels are helpful in defining the extent of the marginal strength of rock or adverse rock structures that surface mapping and boring information provide.
GEOPHYSICAL EXPLORATION
2-94. Geophysical exploration consists of making indirect measurements from the earth’s surface or in boreholes to obtain subsurface information. Boreholes or other subsurface explorations are needed for reference and control when using geophysical methods. Geophysical explorations are appropriate for rapidly locating and correlating geologic features such as stratigraphy, lithology, discontinuities, structure, and groundwater.
EQUIPMENT FOR FIELD DATA COLLECTION
2-95. The type of equipment needed on a field trip depends on the type of survey being conducted. The following items are always required:
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Structural Geology
GEOLOGICAL SURVEYING
2-96. The instruments and methods of geological surveying are numerous and varied. Instruments used on a particular project depend on the scale, time, detail, and accuracy required.
PACE-AND-COMPASS METHOD
2-97. The pace-and-compass method is probably the least-accurate procedure used. The survey is conducted by pacing the distance to be measured and determining the angle of direction with the compass. The field geologist records elevations on a topographic map, when it is available. When a topographic map or a good equivalent is unavailable, an aneroid barometer or some type of accurate altimeter is used to record elevations.
PLANE-TABLE-AND-ALIDADE METHOD
2-98. When accurate horizontal and vertical measurements are required, the plane-table-and-alidade method is used. The equipment consists of a stadia rod, a tripod, a plane table, and an alidade. Sheets of heavy paper are placed on the plane table to record station readings. After recording the stations, the geologist places formation contacts, faults, and other map symbols on the paper. This information can later be transferred to a finished map. The alidade is a precision instrument consisting of a flat base that can easily be moved on the plane table. The straight edge, along the side of the flat base and parallel to the line of sight, is used to plot directions on the base map. The alidade also consists of a telescopic portion with a lens that contains one vertical hair and three horizontal hairs (stadia hairs). The vertical hair is used to align the stadia rod with the alidade. The horizontal hairs are used to read the distance to the rod. Vertical elevations are determined from the stadia distance and the vertical angle of the points in question.
BRUNTON-COMPASS-AND-AERIAL-PHOTO METHOD
2-99. A Brunton compass and a vertical aerial photo provide a rapid, accurate method for geological surveys. The aerial photo is used in place of a base map. Contact, dips and strikes, faults, and other features may be plotted directly on the aerial photo or on a clear acetate overlay. Detailed notes of the geological features must be kept in a separate field notebook. If topographic maps are available, they will supplement the aerial photo and eliminate the need for an aneroid barometer. A topographic map may be used for the base map and to plot control for horizontal displacements.
PLANE-TABLE-,MOSAIC-, AND ALIDADE METHOD
2-100. A mosaic of aerial photos may be used with a plane table and an alidade, thus eliminating the base map. This procedure is used to eliminate the rod holder and to accelerate the mapping. By using a plane table and an alidade as a base, horizontal distance does not have to be measured; and the surveyor can compute vertical elevation. The surveyor—
25 September 2012 TM 3-34.64/MCRP 3-17.7G 2-39
Chapter 2
2-101. This method is faster than the normal plane-table method, but it is not as accurate.
PHOTOGEOLOGY METHOD
2-102. Photogeology is the fastest and least-expensive method of geological surveying. However, a certain amount of accuracy and detail is sacrificed. This type of survey is normally used for reconnaissance of large areas. When a geologic map is constructed from aerial-photo analysis alone, it shows only major structural trends. After a preliminary office investigation, a geologist may go to the field and determine detailed geology of particular areas.
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Chapter 3
Surficial Geology
An integral part of the military engineer’s mission is the location and processing of materials for construction use. Most construction materials are derived from rocks and soils that occur naturally on or near the surface of the earth. These materials may be obtained by developing a quarry or a borrow pit.
Quarries are sites where open excavations are made into rock masses by drilling, cutting, or blasting for the purpose of producing construction aggregate. These operations require extensive time, manpower, and machinery. Borrow pits are sites where unconsolidated material has been deposited and can be removed easily by common earth-moving machinery, generally without blasting.
This chapter covers the processes that form surficial features which are suitable for potential borrow pit operations and the types of construction materials found in these features.
FLUVIAL PROCESS
3-1. The main process responsible for the erosion and subsequent deposition of weathered material suitable for the development of borrow pits is that of moving water. When water moves very quickly, as over a steep gradient, it picks up weathered material and carries it away. When the stream slows down (for example, when the gradient is reduced), the capacity of the stream to carry the weathered material decreases; then it deposits the material in a variety of possible surficial features.
3-2. Stream deposits are characteristically stratified (layered) and composed of particles within a limited size range. Fluvial deposits are sorted by size based on the velocity of the water. When the velocity of the stream falls below the minimum necessary to carry the load, deposition occurs beginning with the heaviest material. In this way, rivers build gravel and sandbars on the inside of meander loops and dump fine silts and muds outside their levees during floods. This creates deposits of reasonably well-sorted, natural construction materials.
DRAINAGE PATTERNS
3-3. Without the benefit of geologic maps, it is difficult to determine the type and structure of the underlying rocks. However, by studying the drainage patterns as they appear on a topographic map, both the rock structure and composition may be inferred.
3-4. Many drainage patterns exist; however, the more common patterns are—
25 September 2012 TM 3-34.64/MCRP 3-17.7G 3-1
Chapter 3
Rectangular
3-5. This pattern is characterized by abrupt, nearly 90-degree changes in stream directions. It is caused by faulting or jointing of the underlying bedrock. Rectangular drainage patterns are generally associated with massive igneous and metamorphic rocks, although they may be found in any rock type. Rectangular drainage is a specific type of angular drainage and is usually a minor pattern associated with a major type, such as dendritic (see figure 3-1, a). Angular drainage is characterized by distinct angles of stream juncture.
Parallel
3-6. This drainage is characterized by major streams trending in the same direction. Parallel streams are indicative of gently dipping beds or uniformly sloping topography. Extensive, uniformly sloping basalt flows and young coastal plains exhibit this type of drainage pattern. On a smaller scale, the slopes of linear ridges may also reflect this pattern (see figure 3-1, b).
Dendritic
3-7. This is a treelike pattern, composed of branching tributaries to a main stream. It is characteristic of essentially flat- lying and/or relatively homogeneous rocks (see figure 3-1, c).
Trellis
3-8. This is a modified version of the dendritic pattern. Tributaries generally flow perpendicular to the main streams and join them at right angles. This pattern is found in areas where sedimentary or metamorphic rocks have been folded and the main streams now follow the strike of the rock (see figure 3-1, d).
Radial
3-9. This pattern, in which streams flow outward from a high central area, is found on domes, volcanic cones, or round hills (see figure 3-1, e).
Annular
3-10. This pattern is usually associated with radial drainage where sedimentary rocks are upturned by a dome structure. In this case, streams circle around a high central area (see figure 3-1, f).
Braided
3-11. A braided stream pattern commonly forms in arid areas during flash flooding or from the meltwater of a glacier. The stream attempts to carry more material than it is capable of handling. Much of the gravels and sands are deposited as bars and islands in the stream bed (see figure 3-1, g and figure 3-2, page 3-4). Figure 3-2 shows the vicinity of Valdez, Alaska. Both the Copper and Tonsina Rivers are braided streams.
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Surficial Geology
Figure 3-1. Typical drainage patterns
25 September 2012 TM 3-34.64/MCRP 3-17.7G 3-3
Chapter 3
Figure 3-2. Topographic expression of a braided stream
DENSITY
3-12. The nature and density of the drainage pattern in an area provides a strong indicator as to the particle size of the soils that have developed. Sands and gravels are usually both porous and permeable. This means that during periods of precipitation, water percolates down through the sediment. The density of the drainage and the surface runoff are minimal due to this good internal drainage.
3-13. Clays and silts are normally porous but not permeable. Most precipitated water is forced to run off, creating a fine network of stream erosion.
3-14. Sandstone and shale may exhibit the same type of drainage pattern. Sandstone, due to its porosity and permeability, has good internal drainage while shale does not. Therefore, the texture or density of the drainage pattern which develops on the sandstone is coarse while that on shale is fine.
STREAM EVOLUTION
3-15. The likelihood of finding construction materials in a particular stream valley can be characterized by the evolution of that valley. The evolutionary stages are described as—
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Surficial Geology
Youth
3-16. Youthful stream valleys, which are located in highland areas, are typified by steep gradients, high water velocities with rapids and waterfalls present, downcutting in stream bottoms resulting in the creation of V-shaped valleys, and the filling of the entire valley floor by the stream (see figure 3-3, a). Although there is considerable erosion taking place, there is very little deposition.
Maturity
3-17. A mature system has a developed floodplain and, while the stream no longer fills the entire valley floor, it meanders to both edges of the valley. The stream gradient is medium to low, deposition of materials can be found, and (when compared with the youthful stream) there is less downcutting and more lateral erosion that contributes to widening the valley (see figure 3-3, b).
Old Age
3-18. In an old-age system, the stream gradient is very gentle, and the water velocity is low. The river exhibits little downcutting, and lateral meandering produces an extensive floodplain. Because of the low water velocity, there is a great amount of deposition. The river only occupies a small portion of the floodplain (see figure 3-3, c).
Figure 3-3. Stream evolution and valley development
3-19. Recognition of the stream evolution stage of a particular river system is required to develop sources of construction aggregate. Rivers in maturity or old age provide the greatest quantities of aggregate. In youthful rivers, sources of aggregate are often scarce or unobtainable due to the steep gradients and high velocity. Table 3-1 summarizes the characteristics of each stage of stream evolution. Figure 3-4 shows an example of the topographic expression of a youthful stream valley in the vicinity of Portage, Montana.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 3-5
Chapter 3
Figure 3-5, page 3-8, shows a mature stream valley in the vicinity of Fort Leavenworth, Kansas. Figure 3-6, page 3-9, shows an old age stream valley in the vicinity of Philipp, Mississippi.
Table 3-1. Stream evolution process
|
Characteristic |
Youth |
Maturity |
Old Age |
|
Gradient |
Steep, irregular |
Moderate, Smooth |
Low, smooth |
|
Valley profile |
Narrow, V-shaped |
Broad, moderately U- shaped |
Very broad |
|
Valley depth |
Deep |
Deep, moderate, shallow |
Shallow |
|
Meanders |
Absent |
Common |
Extremely common |
|
Floodplain |
Absent or small |
Equals width of meander belt |
Wider than width of meander belt |
|
Natural levees |
Absent |
May be present |
Abundant |
|
Tributaries |
Few, small |
Many |
Few, large |
|
Velocity |
High |
Moderate |
Sluggish |
|
Waterfalls |
Many |
Few |
None |
|
Erosion |
Downward cutting |
Downward and lateral cutting equal |
Lateral cutting |
|
Deposition |
Absent or transitory |
Present, but partly transitory |
Much and fairly permanent |
|
Culture |
Steep-walled valleys are barriers to road and railroads |
Flat valley floors are good transportation routes |
Large rivers and nearby swamps are barriers |
|
Summary of Regional Erosion Cycle |
|||
|
Dissection |
Partial |
Complete |
None |
|
Divides |
Broad, flat, high |
Knife-edged |
Low, broad, rounded |
|
Valley development |
Youthful to mature |
Mostly mature |
Old age |
|
Number of streams |
Few |
Maximum |
Few |
|
Relief |
Great |
Maximum |
Minimum |
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Surficial Geology
Figure 3-4. A youthful stream valley
25 September 2012 TM 3-34.64/MCRP 3-17.7G 3-7
Chapter 3
Figure 3-5. A mature stream valley
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Surficial Geology
Figure 3-6. An old age stream valley
STREAM DEPOSITS
3-20. Coarse-grained (gravels and sands) and fine-grained (silts and clays) deposits can be found by map reconnaissance. Certain surficial features are comprised of coarse-grained materials, others are made up of medium-sized particles, and still others of fine-grained sediments. However, if the source area for a stream is composed only of fine-grained materials, then the resulting depositional features will also contain fine- grained sediments, regardless of their usual composition.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 3-9
Chapter 3
3-21. The following surficial features can be identified by their topographic expressions on military maps and are likely sources of construction materials.
Point Bars
3-22. Meandering is the process by which a stream is gradually deflected from a straight-line course by slight irregularities. Most streams that flow in wide, flat-floored valleys tend to meander (bend). These streams are alternately cutting and filling their channels, and as the deflection progresses, the force of the flowing water concentrates against the channel wall on the outside of the curve. This causes erosion on that wall (a body in motion tends to remain in motion in the same direction and with the same velocity until acted on by an external force) (see figure 3-7). Consequently, there is a decrease in velocity and carrying power of the water on the inside of the curve, and the gravels and sands are deposited, forming point bar deposits (see figure 3-8). Point bar deposits on many maps will not be apparent but can be inferred to be at the inside of each meander loop.
Figure 3-7. Meander erosion and deposition
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Surficial Geology
Figure 3-8. Point bar deposits designated by gravel symbols
Channel Bars
3-23. When a stream passes through a meander loop, its speed increases on the outer bank due to the greater volume of water that is forced to flow on the outside of the loop. When the stream leaves the meander and the channel straightens out, the forces that caused the stream to move faster are no longer in control and the stream slows down and deposits materials. These materials are coarse-grained (gravels and sands) and are on the opposite bank and downstream of the point bar. If there is a series of meander loops, these deposits may or may not be present between point bars, depending on the spacing of the meanders. However, a channel bar can be expected after the last meander loop. Figure 3-9, page 3-13, shows channel bar deposits, oxbow lakes, and backswamp/floodplain deposits in the vicinity of Fort Leavenworth, Kansas.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 3-11
Chapter 3
A prominent channel bar is located north of Stigers Island. Mud Lake, Burns Lake, and Horseshoe Lake are oxbow lakes. Backswamps on the floodplain are represented by swampy ground symbols.
Oxbow Lakes
3-24. During high-water stages, a stream that normally flows through a meander loop may cut through the neck of a point, thus separating the loop. When this happens, the stream has taken the path of least resistance and has isolated the bend. The cutoff meander bend is eventually sealed from the main stream by fine deposits. The bend itself then forms an oxbow lake (see figure 3-10, page 3-14). These deserted loops may become stagnant lakes or bogs, or the water may evaporate completely leaving a U-shaped depression in the ground. Fine-grained deposits (silts and clays) are normally located in oxbow lakes. An old point bar deposit can be found on the inside of the U (see figure 3-11, page 3-14). In figure 3-9, Horseshoe Lake is an example of the topographic expression of an oxbow lake.
Natural Levees
3-25. Stream velocity increases during flooding as the stream swells within the confines of its bank to move a greater volume of water. As the stream moves faster, it has the ability to carry more material. If the volume of water becomes so great that the water cannot stay in the channel, the stream spills over its banks onto the surrounding floodplain, which is a flat expanse of land adjacent to a stream or river. Once the stream spills over its banks, the water velocity decreases as the water spreads out to occupy a larger area. As the velocity decreases, sediment carried by the floodwater is deposited. The size of this material depends primarily on the character of the material in the source area upstream and the velocity of the water in the stream channel. Generally, gravels and sands can be found in a natural levee, with the larger material deposited near the stream bank and a gradual gradation to smaller sand particles away from the stream.
Backswamps/Floodplains
3-26. After a flood ends and the stream regresses into its channel, much of the water that spilled over the banks onto the floodplain is trapped on the outside of the natural levees. The fine materials (silts and clays) that are suspended in this water settle onto the floodplain. Consequently, these areas are often used for agricultural production. In the lower-lying areas of the floodplain, a large amount of fines may accumulate, inhibit drainage, and form swamplike conditions called a backswamp (see figure 3-9).
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Surficial Geology
Figure 3-9. Channel bar deposits, oxbow lakes, and backswamp/floodplain deposits
25 September 2012 TM 3-34.64/MCRP 3-17.7G 3-13
Chapter 3
Figure 3-10. Meander development and cutoff
Figure 3-11. Oxbow lake deposits, natural levees, and backswamp deposits
Alluvial Terraces
3-27. A depositing stream tends to fill its valley with a fair amount of granular alluvial material. If a change in the geological situation results in the uplift of a large area or rejuvenation of the stream, an
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Surficial Geology
increase in the stream velocity by other means, or a change in the sedimentation and erosion process, the stream may begin to erode away the material it had deposited previously. As the eroding stream meanders about in its new valley, it may leave benchlike remnants of the preexisting valley fill material perched against the valley walls as terraces. This action of renewed downcutting may occur several times, leaving several terrace levels (see figure 3-12). These are easily recognized on a topographic map because they show up as flat areas with no contour lines, alternating with steeply sloping regions with many contour lines. Alluvial terraces usually occur on one side of the stream but can be found on both sides. They are a normal feature of the history of any fluvial valley. They are usually a good source of sands and gravels. Figure 3-13, page 3-16, shows alluvial terraces in the vicinity of Souris River, North Dakota.
Figure 3-12. Alluvial terraces
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Chapter 3
Figure 3-13. Topographic expression of alluvial terraces
Deltas
3-28. When streams carrying sediments in suspension flow into a body of standing water, the velocity of the stream is immediately and drastically reduced. As a result, the sedimentary load begins to settle out of suspension, with the heavier particles settling first. If the conditions in the body of water (sea or lake) are such that these particles are not spread out over a large area by wave action, or if they are not carried away by currents, they continue to accumulate at the mouth of the stream. Large deposits of these sediments gradually build up to just above the water level to form deltas (see figure 3-14). These assume three general forms, depending mainly on the relative influence of waves, fluvial processes, and tides. These forms are—
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Surficial Geology
Figure 3-14. Growth of a simple delta
Figure 3-15. Arcuate, bird’s foot, and elongate deltas
3-29. Arcuate deltas are arc- or fan-shaped and are formed when waves are the primary force acting on the deposited material. Arcuate deltas usually result from deposition by streams carrying relatively coarse material (sands and gravels) with some occasional fine material. Arcuate deltas consisting primarily of coarse material have very good internal drainage; therefore, they have few minor channels. On the other hand, an arcuate delta having a considerable amount of fine material (silts and clays) mixed with the coarse material does not have good internal drainage. In this case, a larger number of minor channels develop.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 3-17
Chapter 3
Generally, arcuate deltas are considered good sources of sands and gravels. An example of an arcuate delta is the Nile Delta in Egypt.
3-30. Bird’s-foot deltas are formed in situations where fluvial processes have a major influence on deposited sediments. Bird’s-foot deltas resemble a bird’s foot from the air, hence the name. They are generally composed of fine-grained material and have very poor internal drainage. These deltas are flat with vegetation, have many small outlets, and are a good source of fine materials. The Mississippi Delta is a classic example of this delta type.
3-31. Elongate deltas form where tidal currents have a major impact on sediment deposition. They contain only a few distributaries, but the distributaries that occur are large.
Alluvial Fans
3-32. These are the dry land counterpart of deltas. They are formed by streams flowing from rough terrain, such as mountains or steep faults, onto a flat plain. This type of deposit is found in regions that have an arid to semiarid type of climate, such as the western interior, the Basin and Range Province of the United States, and the desert mountain areas worldwide. The valleys in these areas are normally dry much of the year, with streams resulting only after torrential rainstorms or following the spring snow melt. The mountains themselves are devoid of vegetation, and erosion by the streams is not impeded. These streams rush down a steep gradient, and when they meet the valley floor, there is a sudden reduction in velocity. The sediment load is deposited at the foot of the rough terrain. This deposit is in the form of a broad “semicone” with the apex pointing upstream. Coalescing alluvial fans consist of a series of fans that have joined to form one large feature. This is typical in arid areas. Figure 3-16 depicts alluvial and coalescing alluvial fans. Alluvial fans may be readily identified by their topographic expressions of concentric half-circular contour lines. Figure 3-17 is a topographic map showing the Cedar Creek alluvial fan in the vicinity of Ennis Lake, Montana. This alluvial fan is approximately four miles in radius. Figure 3-18, page 3-20, shows coalescing alluvial fans in the vicinity of Las Vegas, Nevada.
Figure 3-16. Alluvial fan and coalescing alluvial fans
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Surficial Geology
Figure 3-17. Cedar Creek alluvial fan
25 September 2012 TM 3-34.64/MCRP 3-17.7G 3-19
Chapter 3
Figure 3-18. Coalescing alluvial fans
3-33. The types of materials found in alluvial fans are gravels, sands, and fines based on a ⅓ rule. The first
⅓, the area adjacent to the highland, is primarily composed of gravels; the middle ⅓ is composed of sands; and the final ⅓, the area farthest from the highland, is composed of fines.
3-34. Fluvial features are found throughout the world and are the primary source of borrow pit materials for military engineers. Table 3-2, and figure 3-19, present a summary of fluvial features. Figure 3-20, page 3-22, shows a generalized distribution of fluvial surficial features throughout the world.
Table 3-2. Fluvial surficial features
|
Feature |
Description |
|
Point bar |
A low, crescent-shaped mound located at the inside of many bends in rivers or streams. |
|
Channel bar |
A Low, streamlined mound in braided streams or just downstream from point bars and on the opposite bank |
|
Floodplain |
A flat valley floor, leveled by back and forth erosion of the river or stream between the valley walls. |
|
Alluvial Terrace |
A platform or flat surface higher than the floodplain and generally close to the valley walls. It is all that remains of what was a floodplain many years before. |
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Surficial Geology
Table 3-2. Fluvial surficial features
|
Feature |
Description |
|
Oxbow lake |
A horseshoe-shaped, abandoned section of a stream or river channel still containing water. |
|
Clay plug |
A clay-filled, abandoned section of a stream or river at the ends of horseshoe-shaped oxbow lakes. |
|
Natural levee |
Wide, low mounds (5 to 15 feet high), paralleling the river along both banks with sloping sides away from the river. |
|
Backswamp |
A swampy, level portion of a floodplain with very poor drainage and a high water table |
|
Delta |
Natural land extension into a body of water, visible as a low, almost level, land mass protruding into the body of water. |
|
Alluvial fan |
A cone-shaped mound formed against a valley wall, appearing fan- shaped from above. |
|
Lake bed deposit |
A layer formed from the settling of sediments to the bottom of lakes. The layers are thick at the center of the lake bed and thin near the lake margins. |
Figure 3-19. Major floodplain features
25 September 2012 TM 3-34.64/MCRP 3-17.7G 3-21
Chapter 3
Figure 3-20. World distribution of fluvial landforms
GLACIAL PROCESS
3-35. Between ten and twenty-five thousand years ago, much of North America, Europe, and Northern Asia was covered by glaciers. Significant ice sheets still cover Greenland and Antarctica, and lesser ice sheets can be found at high elevations and latitudes (see figure 3-21).
Figure 3-21. Ice sheets of North America and Europe
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Surficial Geology
3-36. Glaciation produces great changes in the existing topography by reshaping the land surface and depositing new surficial features that may serve as a source of construction aggregate for military engineers.
TYPES OF GLACIATION
3-37. The glaciation process may be described as either continental or alpine glaciation.
Continental
3-38. Continental glaciation occurs on a large, regional scale affecting vast areas. It may be characterized by the occurrence of more depositional features than erosional features. Continental glaciers can be of tremendous thickness and extent. They move slowly in a plastic state with the ice churning the soil and rocks beneath it as well as crushing and plucking rocks from the ground and incorporating large amounts of material within the glacier itself. The overall range of particle size of these materials is from clays through cobbles and boulders (see figure 3-22).
Figure 3-22. Contintental glaciation
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Chapter 3
Alpine
3-39. Alpine or mountain glaciation takes place in mountainous areas and generally results in the creation of mainly erosional forms. Alpine glacial features are very distinctive and easy to recognize. In the past, glaciers scooped out and widened the valleys through which they moved, producing valleys with a U- shaped profile in contrast to the V-shaped profile produced by fluvial erosion (see figure 3-23).
Figure 3-23. Alpine glaciation
GLACIAL DEPOSITS
3-40. Materials deposited by glaciers are frequently differentiated into two types. They are—
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Surficial Geology
Stratified
3-41. The features composed of stratified deposits are actually the result of deposition of sediment by glacial streams (glaciofluvial) and not by the movement of the ice itself. These features are—
3-42. They result when the material in the glacier has been carried and deposited by meltwater from the glacier. The water selectively deposits the coarsest materials, carrying the fines away from the area. The end result is essentially deposits of sands and gravels.
3-43. Outwash plains result when melting ice at the edge of the glacier creates a great volume of water that flows through the end moraine as a number of streams rather than as a continuous sheet of water. Each of the streams builds an alluvial fan and each of the fans joins together and forms a plain that slopes gently away from the end moraine area. The coarsest material is deposited nearest the end moraine, and the fines are deposited at greater distances. Much of the prairie land in the United States consists of outwash plains. Drainage and trafficability in the outwash plains are much better than in a ground moraine; however, kettles can be formed in outwash plains due to large masses of ice left during the recession of the ice front. If the kettles are numerous, the outwash area is called a pitted plain (see figure 3-22, page 3-23).
3-44. Eskers are winding ridges of irregularly stratified sands and gravels that are found within the area of the ground moraine. The ridges are usually several miles long but are rarely more than 45 to 60 feet wide or more than 150 feet high. They are formed by water that flowed in tunnels or ice-walled gorges in or beneath the ice. They branch and wind like stream valleys but are not like ordinary valleys in that they may cross normal drainage patterns at an angle, and they may also pass over hills (see figures 3-22 and figure 3-24, page 3-26). Figure 3-24 shows kettle lakes, swamps, and eskers.
3-45. A similar feature that resembles an esker, but is rarely more than a mile in length, is a ridge known as a crevasse filling. A crevasse is a large, deep crevice or fissure on the surface of a glacier. Unsorted debris washes into the crevasse, and when the surrounding ice melts, a ridge containing a considerable amount of fines is left standing.
3-46. Kames are conical hills of sands and gravels deposited by heavily laden glacial streams that flowed on top of or off of the glacier. They are usually isolated hills that are associated with the end or recessional moraine; kettle lakes are commonly found in the same area. The formation of kames normally occurred when meltwater streams deposited relatively coarse materials in the form of a glacioalluvial fan at the edge of the ice; the fine particles were washed away. This material accumulated along the side of the ice, and when the ice receded, the material slumped back on the side formerly in contact with the glacier.
3-47. Delta kames are another type of kame that may be formed when the meltwater flows into a marginal lake and forms a delta. After the lake and the ice disappear, deltas are left as flat-topped, steep-sided hills of well-sorted sands and gravels (see figure 3-22).
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Chapter 3
Figure 3-24. Moraine topographic expression with kettle lakes, swamps, and eskers
3-48. Kame terraces are features associated with alpine glaciation. When the ice moves down a valley, it is in contact with the sides of the valley. As the glacier melts away from the valley wall, glacial water flows into the space created between the side of the glacier and the valley wall. The void is filled with gravels and sands, while the fines are carried away by the stream water. A terrace is left where the ice was in contact with the valley; gravels and sands can be found at the base of the terrace (see figure 3-25).
3-49. Glacial lake deposits occur during the melting of the glacier when many lakes and ponds are created by the meltwater in the outwash areas. The streams that fed these waters were laden with glacial material. Most of the gravels and sands that were not deposited before reaching the lake accumulated as a delta (later to be called a delta kame) after melting of the ice. The fines that remained suspended in the water were, on the other hand, deposited throughout the lake. During the summer, a band consisting of light-colored, coarse silt was deposited, whereas a thinner band of darker, finer-grained material was deposited in the winter. The two bands together represent a time span of one year and are referred to as a varve.
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Surficial Geology
Figure 3-25. Valley deposits from melting ice
Unstratified
3-50. Unstratified glacial deposits (sediments deposited by the ice itself) are the most common of the of glacial deposits. They comprise the following surfical features:
3-51. Unstratified deposits make up landforms that may be readily identified in the field, on aerial photographs, and from topographic and other maps. Unstratified deposits are composed of a heterogeneous mixture of particle types and sizes ranging from clays to boulders. Till is the name given to this mixture of materials. It is the most widespread of all the forms of glacial debris. In general, features comprised of till are undesirable as sources of military construction aggregate since the material must be washed and screened to provide proper gradation.
3-52. Ground moraines, sometimes called till plains, are deposits that are laid down as the glacier recedes. Melting ice drops material that blankets the area over which the glacier traveled. A deposit of this kind forms gently rolling plains. The deposit itself may be a thin veneer of material lying on the bedrock, or it may be hundreds of feet thick. Moraine soil composition is complex and often indeterminate. This variation in sediment makeup is due to the large variety of rocks and soil picked up by the moving glacier (see figure 3-22, page 3-23).
3-53. Morainic areas have a highly irregular drainage pattern because of the haphazard arrangement of ridges and hills, although older till plains tend to develop dendritic patterns. Frequent features associated with ground moraines are kettle holes and swamps. Kettles are usually formed by the melting of ice that had been surrounded by or embedded in the moraine material. Large amounts of fines in the till prevent water from percolating down through the soil. This may allow for the accumulation of water in the kettle holes forming kettle lakes or, in low-lying areas, swamps. Figure 3-22 and figure 3-24, show ground moraine with an esker.
3-54. End moraines, sometimes called terminal moraines, are ridges of till material that were pushed to their locations at the limit of the glacier’s advance by the forceful action of the ice sheet. Generally, there is no one linear element, such as a continuous ridge, evident in either the field or on aerial photos. Normally, this deposit appears as a discontinuous chain of elongated to oval hills. These hills vary in height from tens to hundreds of feet. The till material is, at times, quite clayey. Kettle lakes are sometimes associated with terminal moraine deposits also (see figure 3-22).
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Chapter 3
3-55. Recessional moraines, which are similar to end moraines, are produced when a receding glacier halts its retreat for a considerable period of time. The stationary action allows for the accumulation of till material along the glacier’s edge. A series of these moraines may result during the retreat of a glacier (see figure 3-22, page 3-23).
3-56. Drumlins are asymmetrical, streamlined hills of gravel till deposited at the base of a glacier and oriented in a direction parallel to ice flow. The stoss side (the side from which the ice flowed) of the drumlin is steeper and blunter than the lee side. The overall appearance of a drumlin resembles an inverted spoon if viewed from above. Drumlins commonly occur in groups of two or more. Individual drumlins are seldom more than ½ mile long, and they can rise to heights of 75 to 100 feet (see figure 3-26 and figure 3-27).
Figure 3-26. Idealized cross section of a drumlin
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Surficial Geology
Figure 3-27. Topographic expression of a drumlin field
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Chapter 3
3-57. It is important to understand that features formed from the glacial process only occur in certain areas of the world. Figures 3-28 and 3-29 illustrate the regions of the United States and the world where glacial landforms occur. Table 3-3, page 3-32, is a summary of glacial surficial features.
Figure 3-28. Distribution of major groups of glacial landforms across the United States
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Surficial Geology
Figure 3-29. World distribution of glacial landforms
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Chapter 3
Table 3-3. Glacial surficial features
|
Feature |
Description |
|
Grand Moraine |
A blanket of till covering the bedrock or an old soil layer extending for miles in all directions |
|
End moraine |
A long, irregularly rounded, narrow ridge or parallel ridges of till formed at the edges of a glacier. |
|
Drumlin |
A streamlined hill of till (teardrop-shaped when viewed from above), generally from 20 to 200 feet tall, 50 to 400 feet wide, and 500 to 3,000 feet long. Drumlins are commonly found in groups but may exist alone. |
|
Esker |
A winding, rounded ridge that is generally 100 to 200 feet wide, with a fairly constant height of 50 to 100 feet, trending for distances of several thousand feet to several miles |
|
Kame |
An irregularly shaped mound of sand and gravel found near or in ridge moraines. Distinguished from mounds of till by slightly greater relief, slightly steeper sides, and better drainage (drier). |
|
Kame Terrace |
A platform or flat surface (in U-shaped, glaciated valleys) higher than the valley floor and butting against the valley wall. |
|
Outwash Plain |
A fairly flat, well-drained plain, formed by large quantities of meltwater running away from the edge of the glacier. Outwash plains are commonly found in front of end moraines |
|
Note. Till is not a feature. It is the material picked up, moved, and deposited by glaciers. It is largely boulders and silt but also contains gravels, sands, and clays. |
|
EOLIAN PROCESS
3-58. In arid areas where water is scarce, wind takes over as the main erosional agent. When a strong wind passes over a soil, it carries many particles of soil with it. The height and distance the materials are transported is a function of the particle size and the wind velocity. The subsequent decrease in the wind velocity gives rise to a set of wind-borne deposits called eolian features.
TYPES OF EOLIAN EROSION
3-59. There are two types of wind erosion. They are—
Deflation
3-60. Deflation occurs when loose particles are lifted and removed by the wind. This results in a lowering of the land surface as materials are carried away. Unlike stream erosion, in which downcutting is limited by a “base level” (usually sea level), deflation can continue lowering a land surface as long as it has loose material to carry away. Deflation may be terminated if the land surface is cut down to the water table (moist soil is not carried away as easily) or if vegetation is sufficient to hold the soil in place. In addition, deflation may be halted when the supply of fine material has been depleted. This makes a surface of gravel in the area where deflation has taken place. This gravel surface is known as desert pavement (see figure 3-30).
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Surficial Geology
Figure 3-30. Three stages illustrating the development of desert armor
Abrasion
3-61. Abrasion occurs when hard particles are blown against a rock face causing the rocks to break down. As fragments are broken off, they are carried away by the wind. This process can grind down and polish rock surfaces. A rock fragment with facets that have been cut in this way is called a ventifact (see figure 3-31).
Figure 3-31. Cutting of a ventifact
MODES OF TRANSPORTATION
3-62. Soil particles can be carried by the wind in the following ways:
Bed Load
3-63. Material that is too heavy to be carried by the wind for great distances at a time (mainly sand-sized particles) bounces along the ground, rarely higher than two feet.
Suspended Load
3-64. These are fines (mostly silts) that are easily carried by the wind. Suspended loads extend to high altitudes (sometimes thousands of feet) and can be transported for thousands of miles. During a particularly bad dust storm in the mid-western “dust bowl” on 20 March 1935, the suspended load extended to altitudes of over 12,000 feet. The lowermost mile of the atmosphere was estimated to contain over 166,000 tons of suspended particles per cubic mile. Enough material was transported to bring temporary twilight to New York and New England (over 2,000 miles away) on 21 March.
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Chapter 3
EOLIAN FEATURES
3-65. Eolian surficial features may consist of gravels, sands, or fines. The three main types of eolian features are—
3-66. Figure 3-32 illustrates the origin of these deposits.
Figure 3-32. Eolian features
Lag Deposits or Desert Pavement
3-67. As the wind billows across the ground, sands and fines are continually removed. Eventually, gravels and pebbles that are too large to be carried by the wind cover the surface. These remnants accumulate into a sheet that ultimately covers the finer-grained material beneath and protects it from further deflation. Desert pavement usually develops rapidly on alluvial fan and alluvial terrace surfaces. The exposed surface of the gravels may become coated with a black, glittery substance termed desert varnish. In some locations, the evaporation of water, brought to the surface by the capillary action of the soil, may leave behind a deposit of calcium carbonate (caliche) or gypsum. It acts as a cement, hardening the pavement into a conglomeratelike slab.
3-68. Although desert pavement contains good gravel material, the layers are normally too thin to supply the quantity required for construction. However, it does provide a rough but very trafficable surface for all types of vehicles and also provides excellent airfields.
Sand Dunes
3-69. Dunes may take several forms, depending on the supply of sand, the lay of the land, vegetation restrictions, and the steady direction of the wind. Their general expressions are as follows:
3-70. Transverse dunes are wavelike ridges that are separated by troughs; they resemble sea waves during a storm. These dunes, which are oriented perpendicular to the prevailing wind direction, occur in desert locations where a great supply of sand is present over the entire surface. A collection of transverse dunes is known as a sand sea (see figure 3-33, a).
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Surficial Geology
3-71. Longitudinal dunes have been elongated in the direction of the prevailing winds. They usually occur where strong winds blow across areas of meager amounts of sand or where the winds compete with the stabilizing effect of grass or small shrubs (see figure 3-33, b).
3-72. Barchan dunes are the simplest and most common of the dunes. A barchan is usually crescent- shaped, and the windward side has a gentle slope rising to a broad dome that cuts off abruptly to the leeward side. Barchans form in open areas where the direction of the wind is fairly constant and the ground is flat and unrestricted by vegetation and topography (see figure 3-33, c).
Figure 3-33. Sand dune types
3-73. Parabolic, or U-shaped, dunes have tips that point upwind. They typically form along coastlines where the vegetation partially covers the sand or behind a gap in an obstructing ridge. Later, a parabolic dune may detach itself from the site of formation and migrate independently (see figure 3-33, d).
3-74. Complex dunes lack a distinct form and develop where wind directions vary, sand is abundant, and vegetation may interfere. These can occur locally when other dune types become overcrowded and overlap, thereby losing their characteristic shapes in a disorder of varying slopes (see figure 3-33, e).
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Chapter 3
Loess Deposits
3-75. In a number of regions of the world, thick accumulations of yellowish-brown material composed primarily of windblown silts make up a substantial amount of surface area. These deposits are known as loess. The material that makes up these deposits originated mainly from dried glacial outwash, floodplains, or desert area fines. Loess is composed of physically ground rock rather than of chemically weathered material. The source and deposition point for the material may be many miles apart, and the deposits may range in thickness from a few feet to hundreds of feet. Thickness tends to decrease with distance from the source. In the United States, most of Kansas, Nebraska, Iowa, and Illinois are covered by loess. After a loess has been laid down, it is rarely picked up again. This is due to a very thin layer of fines that interlock after wetting. While dry loess is trafficable, it loses all strength with a slight amount of water (see figure 3-34).
Figure 3-34. Loess landforms
3-76. Eolian features occur worldwide and may consist of areas of sand dunes and desert pavement or loess; however, their topographic expressions vary. In general, dune areas are specified on maps by special topographic symbols since they are continually changing unless stabilized by vegetation. Figure 3-35, is a topographic expression, using special symbols, of sand dunes and desert pavement (Summan). Figure 3-36, page 3-38, shows the generalized distribution of eolian landforms throughout the world.
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Surficial Geology
Figure 3-35. Topographic expression of dunes and desert pavement
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Chapter 3
Figure 3-36. Worldwide distribution of eolian landforms
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SOURCES OF CONSTRUCTION AGGREGATE
Surficial Geology
3-77. Military engineers use their knowledge of surficial features to develop borrow pits and provide construction aggregate to meet mission requirements. Generally, engineer units attempt to develop borrow pit operations in fluvial features since they are easy to identify and are normally accessible. In arid and semiarid regions, eolian deposits and alluvial fans provide large amounts of aggregate. In mountainous regions and continentally glaciated regions, fluvial-glacial deposits can provide large quantities of quality aggregate. Therefore, their presence should not necessarily be discounted in preference to fluvial materials. Table 3-4 summarizes the types of aggregates found in common fluvial, glacial, and eolian surficial features.
Table 3-4. Aggregate types by feature
|
Feature |
Cobbles |
Gravels |
Sands |
Silts |
Clays |
|
Point bar |
|
X |
X |
|
|
|
Channel bar |
|
X |
X |
|
|
|
Alluvial Terrace |
X |
X |
X |
|
|
|
Oxbow Lake, Clay plug |
|
|
|
X |
X |
|
Natural levee |
|
X |
X |
X |
|
|
Backswamp |
|
|
|
X |
X |
|
Delta, Bird’s-foot |
|
|
|
X |
X |
|
Delta, Arcuate |
|
X |
X |
X |
X |
|
Alluvial fan |
X |
X |
X |
X |
X |
|
Lake bed deposits |
|
|
|
X |
X |
|
Esker |
X |
X |
X |
|
|
|
Kame |
X |
X |
X |
|
|
|
Kame Terrace |
X |
X |
X |
|
|
|
Outwash Plains |
|
X |
X |
|
|
|
Desert Pavement |
X |
X |
|
|
|
|
Sand Dunes |
|
|
X |
|
|
|
Loess |
|
|
|
X |
|
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Chapter 4
Soil Formation and Characteristics
The term “soil,” as used by the US Army, refers to the entire unconsolidated material that overlies and is distinguishable from bedrock. Soil is composed principally of the disintegrated and decomposed particles of rock. It also contains air and water as well as organic matter derived from the decomposition of plants and animals. Bedrock is considered to be the solid part of the earth’s crust, consisting of massive formations broken only by occasional structural failures. Soil is a natural conglomeration of mineral grains ranging from large boulders to single mineral crystals of microscopic size. Highly organic materials, such as river bottom mud and peat, are also considered soil. To help describe soils and predict their behavior, the military engineer should understand the natural processes by which soils are formed from the parent materials of the earth’s crust. As soils are created, by the process of rock weathering and often by the additional processes of transportation and disposition, they often acquire distinctive characteristics that are visible both in the field and on maps and photographs.
SECTION I – SOIL FORMATION
WEATHERING
4-1. Weathering is the physical or chemical breakdown of rock. It is this process by which rock is converted into soil. Weathering is generally thought of as a variety of physical or chemical processes that are dependent on the environmental conditions present.
PHYSICAL PROCESSES
4-2. Physical weathering is the disintegration of rock. Physical weathering processes break rock masses into smaller and smaller pieces without altering the chemical composition of the pieces. Therefore, the disintegrated fragments of rock exhibit the same physical properties as their sources. Processes that produce physical weathering are—
Unloading
4-3. When rock layers are buried under the surface, they are under compressive stress from the weight of overlying materials. When these materials are removed, the resulting stress reduction may allow the rock unit to expand, forming tensional cracks (jointing) and causing extensive fracturing.
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Chapter 4
Frost Action
4-4. Most water systems in rocks are open to the atmosphere, but freezing at the surface can enclose the system. When the enclosed water freezes, it expands nearly one-tenth of its volume, creating pressures up to 4,000 pounds per square inch (psi). The expanding ice fractures the rock.
Organism Growth
4-5. Trees and plants readily grow in the joints of rock masses near the surface. The wedging action caused by their root growth hastens the disintegration process.
Temperature Changes
4-6. Daily or seasonal temperature changes can cause differential expansion and contraction of rocks near the earth’s surface. This results in a tensional failure called spalling or exfoliation. As the rock’s surface heats up, it expands; as it cools, it contracts. The jointing patterns of igneous rock are often the result of temperature changes.
Crystal Growth
4-7. The growth of minerals precipitating from groundwater can apply pressure similar to that of expanding ice. Soluble minerals, such as halite (salt), readily crystalize out of solution.
Abrasion
4-8. Sediments suspended in wind or fast-moving water can act as abrasives to physically weather rock masses. Rock particles carried by glacial ice can also be very abrasive.
CHEMICAL PROCESSES
4-9. Chemical weathering is the decomposition of rock through chemical processes. Chemical reactions take place between the minerals of the rock and the air, water, or dissolved or suspended chemicals in the atmosphere. Processes that cause chemical weathering are—
Oxidation
4-10. Oxidation is the chemical union of a compound with oxygen. An example is rusting, which is the chemical reaction of oxygen, water, and the iron mineral pyrite (FeS2) to form ferrous sulfate (FeSO4). Oxidation is responsible for much of the red and yellow coloring of soils and surface rock bodies. This type of reaction is important in the decomposition of rocks, primarily those with metallic minerals.
Hydration
4-11. Hydration is the chemical union of a compound with water. For example, the mineral anhydrite (CaSO4) incorporates water into its structure to form the new mineral gypsum (CaSO42H2O).
Hydrolysis
4-12. This decomposition reaction is related to hydration in that it involves water. It is a result of the partial dissociation of water during chemical reactions that occur in a moist environment. It is one of the types of weathering in a sequence of chemical reactions that turns feldspars into clays. An example of hydrolysis is the altering of sodium carbonate (Na2CO3) to sodium hydroxide (NaOH) and carbonic acid (H2CO3).
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Soil Formation and Characteristics
Carbonation
4-13. This is the chemical process in which carbon dioxide from the air unites with various minerals to form carbonates. A copper penny eventually turns green from the union of copper with carbon dioxide in the air to form copper carbonate. Carbonate rocks, in turn, are susceptible to further weathering processes, namely solution.
Solution
4-14. Carbon dioxide dissolved in water forms a weak acid called carbonic acid (H2CO3). Carbonic acid acts as a solvent to dissolve carbonates, such as limestone, and carry them away. This creates void spaces, or caves, in the subsurface. Areas that have undergone extensive solutioning are known as karst regions.
DISCONTINUITIES AND WEATHERING
4-15. Jointing and other discontinuities increase the surface area of the rock mass exposed to the elements and thereby enhance chemical weathering. Discontinuities, such as joints, faults, or caverns, act as conduits into the rock mass for the weathering agents (air and water) to enter. Weathering occurs on exposed surfaces, such as excavation walls, road cuts, and the walls of discontinuities. The effect of weathering along discontinuities is a general weakening in a zone surrounding the wall surface.
EFFECTS ON CLIMATE
4-16. The climate determines largely whether a type of rock weathers mostly by chemical or mostly by physical processes. Warm, wet (tropical or subtropical) climates favor chemical weathering. In such climates, there is abundant water to support the various chemical processes. Also, in warm, wet climates, the temperature is high enough to allow the chemical reactions to occur rapidly. Cold, dry climates discourage chemical weathering of rock but not physical weathering. The influence of climate on the weathering of the many rock types varies; however, most rock types weather more rapidly in warm, wet climates than in cold and/or dry climates.
EFFECTS ON RELIEF FEATURES
4-17. Weathering combined with erosion (the transportation of weathered materials) is responsible for most of the relief features on the earth’s surface. For example, the subsurface cavities so predominant in karst regions develop along the already existing joints and bedding planes and commonly form an interlacing network of underground channels. If the ceiling of one of these subterranean void spaces should collapse, a sinkhole forms at the earth’s surface. The sinkhole may range in size from a few feet to several miles in diameter. It may be more than a hundred feet deep, and it may be dry or contain water. Extensive occurrences of sinkholes result in the formation of karst topography, which is characterized by a pitted or pinnacle ground surface with numerous depressions and a poorly developed drainage pattern. Other features associated with karst topography include:
4-18. Solution cavities and sinkholes can be detrimental to foundations for horizontal and vertical construction and should be identified and evaluated for military operations.
SOIL FORMATION METHODS
4-19. Soils may be divided into two groups based on the method of formation—residual soils and transported soils.
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Chapter 4
RESIDUAL SOILS
4-20. Where residual soils are formed, the rock material has been weathered in place. While mechanical weathering may occur, chemical weathering is the dominant factor. As a result of this process, and because the rock material may have an assorted mineral structure, the upper layers of soils are usually fine-grained and relatively impervious to water. Under this fine-grained material is a zone of partially disintegrated parent rock. It may crumble easily and break down rapidly when exposed to loads, abrasions, or further weathering. The boundary line between soil and rock is usually not clearly defined. Lateritic soils (highly weathered tropical soils containing significant amounts of iron or iron and aluminum) are residual. Residual soils generally present both drainage and foundation problems. Residual soil deposits are characteristically erratic and variable in nature. Figure 4-1, shows a typical residual soil.
Figure 4-1. Residual soil forming from the in-place weathering of igneous rock
TRANSPORTED SOILS
4-21. By far, most soils the military engineer encounters are materials that have been transported and deposited at a new location. Three major forces—glacial ice, water, and wind—are the transporting agents. These forces have acted in various ways and have produced a wide variety of soil deposits. Resulting foundations and construction problems are equally varied. These soils may be divided into glacial deposits, sedimentary or water-laid deposits, and eolian or wind-laid deposits. Useful construction material can be located by being able to identify these features on the ground or on a map (see chapter 3).
SOIL PROFILES
4-22. As time passes, soil deposits undergo a maturing process. Every soil deposit develops a characteristic profile because of weathering and the leaching action of water as it moves downward from the surface. The profile developed depends not only on the nature of the deposit but also on factors such as temperature, rainfall amounts, and vegetation type. Under certain conditions, complex profiles may be developed, particularly with old soils in humid regions. In dry regions, the profile may be obscured.
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Soil Formation and Characteristics
4-23. Typical soil profiles have at least three layers, known as horizons (see figure 4-2). They are—
Figure 4-2. Soil profile showing characteristic soil horizons
4-24. The A horizon, or upper layer, contains a zone of accumulation of organic materials in its upper portion and a lower portion of lighter color from which soil colloids and other soluble constituents have been removed. The B horizon represents the layer where soluble materials accumulate that have washed out of the A horizon. This layer frequently contains much clay and may be several feet thick. The C horizon is the weathered parent material. The development of a soil profile depends on the downward movement of water. In arid and semiarid regions, the movement of water may be reversed and water may be brought to the surface because of evaporation. Soluble salts may thus be brought to the surface and deposited.
4-25. The study of the maturing of soils and the relationship of the soil profile to the parent material and its environment is called pedology. As will be explained later, soils may be classified on the basis of their soil profiles. This approach is used by agricultural soil scientists and some engineering agencies. This system is of particular interest to engineers who are concerned with road and airfield problems.
4-26. Soils not only characteristically vary with depth, but several soil types can and often do exist within a relatively small area. These variations may be important from an engineering standpoint. The engineering properties of a soil are a function not only of the kind of soil but also of its conditions.
SECTION II – SOIL CHARACTERISTICS
PHYSICAL PROPERTIES
4-27. The engineering characteristics of soil vary greatly, depending on such physical properties as—
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Chapter 4
4-28. These properties are defined, in most cases numerically, as a basis for the systematic classification of soil types. Such a classification system, used in connection with a common descriptive vocabulary, permits the ready identification of soils that may be expected to behave similarly.
4-29. The nature of any given soil can be changed by manipulation. Vibration, for example, can change a loose sand into a dense one. Therefore, the behavior of a soil in the field depends not only on the significant properties of the individual constituents of the soil mass but also on properties due to particle arrangement within the mass.
4-30. Frequently, the available laboratory equipment or other considerations only permit the military engineer or the engineer’s soil technician to determine some of the soil’s properties and then only approximately. Hasty field identification often permits a sufficiently accurate evaluation for the problem at hand. However, the engineer cannot rely solely on experience and judgment in estimating soil conditions or identifying soils. He must make as detailed a determination of the soil properties as possible and subsequently correlate these identifying properties with the observed behavior of the soil.
GRAIN OR PARTICLE SIZE
4-31. In a natural soil, the soil particles or solids form a discontinuous mass with spaces or voids between the particles. These spaces are normally filled with water and/or air. Organic material may be present in greater or lesser amounts. The following paragraphs are concerned with the soil particles themselves. The terms “particle” and “grain” are used interchangeably.
Determination
4-32. Soils may be grouped on the basis of particle size. Particles are defined according to their sizes by the use of sieves, which are screens attached to metal frames. Figure 4-3 shows sieves used for the Unified Soil Classification System (USCS). If a particle will not pass through the screen with a particular size opening, it is said to be “retained on” that sieve. By passing a soil mixture through several different size sieves, it can be broken into its various particle sizes and defined according to the sieves used. Many different grain-size scales have been proposed and used. Coarse gravel particles are comparable in size to a lemon, an egg, or a walnut, while fine gravel is about the size of a pea. Sand particles range in size from that of rock salt, through table salt or granulated sugar, to powdered sugar. Below a Number 200 sieve, the particles (fines) are designated as silts or clays, depending on their plasticity characteristics.
4-33. Other grain-size scales apply other limits of size to silts and clays. For example, some civil engineers define silt as material less than 0.05 mm in diameter and larger than 0.005 mm. Particles below 0.005 mm are clay sizes. A particle 0.07 mm in diameter is about as small as can be detected by the unaided eye. It must be emphasized that below the Number 200 sieve (0.074-mm openings), particle size is relatively unimportant in most cases compared to other properties. Particles below 0.002 mm (0.001 mm in some grain-size scales) are frequently designated as soil colloids. The organic materials that may be present in a soil mass have no size boundaries.
4-34. Several methods may be used to determine the size of soil particles contained in a soil mass and the distribution of particle sizes. Dry sieve analysis has sieves stacked according to size, the smallest being on the bottom. Numbered sieves designate the number of openings per lineal inch. Dimensioned sieves indicate the actual size of the opening. For example, the Number 4 standard sieve has four openings per lineal inch (or 16 openings per square inch), whereas the ¼-inch sieve has a sieve opening of ¼ inch.
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Soil Formation and Characteristics
Figure 4-3. Dry sieve analysis
4-35. The practical lower limit for the use of sieves is the Number 200 sieve, with 0.074-millimeter-square openings. In some instances, determining the distribution of particle sizes below the Number 200 sieve is desirable, particularly for frost susceptibility determination. This may be done by a process known as wet mechanical analysis, which employs the principle of sedimentation. Grains of different sizes fall through a liquid at different velocities. The wet mechanical analysis is not a normal field laboratory test. It is not particularly important in military construction, except that the percentage of particles finer than 0.02 mm has a direct bearing on the susceptibility of soil to frost action. A field method for performing a wet mechanical analysis for the determination of the percentage of material finer than 0.02 mm is given in Technical Manual (TM) 5-530. The procedure is called decantation.
4-36. The procedures that have been described above are frequently combined to give a more complete picture of grain-size distribution. The procedure is then designated as a combined mechanical analysis.
4-37. Other methods based on sedimentation are frequently used in soils laboratories, particularly to determine particle distribution below the Number 200 sieve. One such method is the hydrometer analysis. A complete picture of grain-size distribution is frequently obtained by a combined sieve and hydrometer analysis. This method is described in TM 5-530 (section V).
Reports
4-38. Test results may be recorded in one of the following forms:
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Chapter 4
The form in this publication is obsolete. See http://www.apd.army.mil for current form.
Figure 4-4. Data sheet, example of dry sieve analysis
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Soil Formation and Characteristics
The form in this publication is obsolete. See http://www.apd.army.mil for current form.
Figure 4-5. Grain-size distribution curve from sieve analysis
4-39. The tabular form is used most often on soil consisting predominantly of coarse particles. This method is frequently used when the soil gradation is being checked for compliance with a standard specification, such as for a gravel base or a wearing course.
4-40. The graphic form permits the plotting of a grain-size distribution curve. This curve affords ready visualization of the distribution and range of particle sizes. It is also particularly helpful in determining the soil classification and the soil’s use as a foundation or construction material.
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Chapter 4
GRADATION
4-41. The distribution of particle sizes in a soil is known as its gradation. Gradation and other associated factors, primarily as applicable to coarse-grained soils, are discussed in the following paragraphs.
Effective Size
4-42. The grain size corresponding to 10 percent passing on a grain-size distribution curve (see figure 4-5, page 4-9) is called Hazen’s effective size. It is designated by the symbol D10. For the soil shown, D10 is
0.13 mm. The effective sizes of clean sands and gravels can be related to their permeability.
Coefficient of Uniformity
4-43. The coefficient of uniformity (Cu) is defined as the ratio between the grain diameter (in millimeters) corresponding to 60 percent passing on the curve (D60) divided by the diameter of the 10 percent (D10) passing. Hence, Cu – D60/D10
4-44. For the soil shown on figure 4-5—
𝐷60 = 2.4 𝑚𝑚 𝑎𝑛𝑑 𝐷10 = 0.13𝑚𝑚
𝑡ℎ𝑒𝑛 𝐶u = 2.4/0.13 = 18.5
4-45. The uniformity coefficient is used to judge gradation.
Coefficient of Curvature
4-46. Another quantity that may be used to judge the gradation of a soil is the coefficient of curvature, designated by the symbol Cc.
4-47. D10 and D60 have been defined, while D30 is the grain diameter corresponding to 30 percent passing on the grain-size distribution curve. For the soil shown in figure 4-5:
Well-Graded Soils
4-48. A well-graded soil is defined as having a good representation of all particle sizes from the largest to the smallest (see figure 4-6), and the shape of the grain-size distribution curve is considered “smooth.” In the USCS, well-graded gravels must have a Cu value > 4, and well-graded sands must have a Cu value > 6. For well-graded sands and gravels, a Cc value from 1 to 3 is required. Sands and gravels not meeting these conditions are termed poorly graded.
Figure 4-6. Well-graded soil
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Soil Formation and Characteristics
Poorly Graded Soils
4-49. The two types of poorly graded soils are—
4-50. A uniformly graded soil consists primarily of particles of nearly the same size (see figure 4-7). A gap-graded soil contains both large and small particles, but the gradation continuity is broken by the absence of some particle sizes (see figure 4-8).
Figure 4-7. Uniformly graded soil
Figure 4-8. Gap-graded soil
4-51. Figure 4-9 shows typical examples of well-graded and poorly graded sands and gravels. Well-graded soils ((GW) and (SW) curves) would be represented by a long curve spanning a wide range of sizes with a constant or gently varying slope. Uniformly graded soils ((SP) curve) would be represented by a steeply sloping curve spanning a narrow range of sizes; the curve for a gap-graded soil ((GP) curve) flattens out in the area of the grain-size deficiency.
Figure 4-9. Typical grain-size distribution curves for well-graded and poorly graded soils
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Chapter 4
Bearing Capacity
4-52. Coarse materials that are well-graded are usually preferable for bearing from an engineering standpoint, since good gradation usually results in high density and stability. Specifications for controlling the percentage of the various grain-size groups required for a well-graded soil have been established for engineering performance and testing. By proportioning components to obtain a well-graded soil, it is possible to provide for maximum density. Such proportioning develops an “interlocking” of particles with smaller particles filling the voids between larger particles, making the soil stronger and more capable of supporting heavier loads. Since the particles are “form-fitted”, the best load distribution downward will be realized. When each particle is surrounded and “locked” by other particles, the grain-to-grain contact is increased and the tendency for displacement of the individual grains is minimized.
PARTICLE SHAPE
4-53. The shape of individual particles affects the engineering characteristics of soils. Three principal shapes of soil grains have been recognized. They are—
Bulky
4-54. Bulky grains are nearly equal in all three dimensions. This shape characterizes sands and gravels and some silts. Bulky grains may be described by such terms as—
4-55. These four subdivisions of the bulky particle shape depend on the amount of weathering that has occurred (see figure 4-10). These subdivisions are discussed in the order of desirability for construction.
Figure 4-10. Bulky grains
4-56. Angular particles are particles that have recently been broken up. They are characterized by jagged projections, sharp ridges, and flat surfaces. The interlocking characteristics of angular gravels and sands generally make them the best materials for construction. Such particles are seldom found in nature because weathering processes normally wear them down in a relatively short time. Angular material may be produced artificially by crushing, but because of the time and equipment required for such an operation, natural materials with other grain shapes are frequently used.
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Soil Formation and Characteristics
4-57. Subangular particles have been weathered until the sharper points and ridges of their original angular shape have been worn off. The particles are still very irregular in shape with some flat surfaces and are excellent for construction.
4-58. Subrounded particles are those on which weathering has progressed even further. Still somewhat irregular in shape, they have no sharp corners and few flat areas. Subrounded particles are frequently found in stream beds. They may be composed of hard, durable particles that are adequate for most construction needs.
4-59. Rounded particles are those in which all projections have been removed and few irregularities in shape remain. The particles approach spheres of varying sizes. Rounded particles are usually found in or near stream beds, beaches, or dunes. Perhaps the most extensive deposits exist at beaches where repeated wave action produces almost perfectly rounded particles that may be uniform in size. Rounded particles also exist in arid environments due to wind action and the resulting abrasion between particles. They are not desirable for use in asphalt or concrete construction until the rounded shape is altered by crushing.
Platy
4-60. Platy grains are extremely thin compared to their length and width. They have the general shape of a flake of mica or a sheet of paper. Some coarse particles, particularly those formed by the mechanical breakdown of mica, are flaky or scalelike in shape. However, most particles that fall in the range of clay sizes, including the so-called clay minerals, have this characteristic shape. As will be explained in more detail later, the presence of these extremely small platy grains is generally responsible for the plasticity of clay. This type of soil is also highly compressible under static load.
Needlelike
4-61. These grains rarely occur.
STRUCTURE
4-62. Soils have a three-phase composition, the principal ingredients being the soil particles, water, and air. Organic materials are also found in the surface layer of most soils. Basic concepts regarding volume and weight relationships in a solid mass are shown in figure 4-11. These relationships form the basis of soil testing, since they are used in both quantitative and qualitative reporting of soils. It must be recognized that the diagram merely represents soil mass for studying the relationships of the terms to be discussed. All void and solid volumes cannot be segregated as shown.
Figure 4-11. Volume-weight relationships of a soil mass
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Chapter 4
Specific Gravity
4-63. The specific gravity, designated by the symbol G, is defined as the ratio between the weight per unit volume of the material at a stated temperature (usually 20 degrees Celsius (C)) and the weight per unit volume of water.
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑔𝑟𝑎𝑣𝑖𝑡𝑦 = 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑖𝑛 𝑎𝑖𝑟 (𝑔𝑟𝑎𝑚𝑠)
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑖𝑛 𝑎𝑖𝑟 (𝑔𝑟𝑎𝑚𝑠) −
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑖𝑛 𝑤𝑎𝑡𝑒𝑟 (𝑔𝑟𝑎𝑚𝑠)
4-64. Test procedures are contained in TM 5-530. The specific gravity of the solid substance of most inorganic soils varies between 2.60 and 2.80. Tropical iron-rich laterite soils generally have a specific gravity of 3.0 or more. Clays can have values as high as 3.50. Most minerals, of which the solid matter of soil particles is composed, have a specific gravity greater than 2.60. Therefore, smaller values of specific gravity indicate the possible presence of organic matter.
Volume Ratios
4-65. The total volume (V) of a soil mass consists of the volume of voids (Vv) and the volume of solids (Vs). The volume of voids in turn consists of the volume of air (Vs), and the volume of water (Vw) (see figure 4-11, page 4-13). The most important volume ratio is the void ratio (e). It is expressed:
fx
4-66. The volume of solids is the ratio of the dry weight (Wd) of a soil mass, in pounds, to the product of its specific gravity (G) and the unit weight of water (62.4 pounds per cubic foot (pcf). It is expressed:
fx
4-67. The volume of the water is the ratio of the weight of the water (Ww), in pounds, to the unit weight of the water. It is expressed:
fx
4-68. The degree of saturation (S) expresses the relative volume of water in the voids and is always expressed as a percentage. It is expressed:
fx
4-69. A soil is saturated if S equals 100 percent, which means all void volume is filled with water.
Weight Ratios
4-70. The total weight (W) of a soil mass consists of the weight of the water (Ww) and the weight of the solids (Ws), the weight of the air being negligible. Weight ratios widely used in soil mechanics are moisture content, unit weight, dry unit weight, and wet unit weight.
4-71. Moisture content (w) expressed as a percentage is the ratio of the weight of the water to the weight of the solids. It is expressed:
fx
4-72. The moisture content may exceed 100 percent. By definition, when a soil mass is dried to constant weight in an oven maintained at a temperature of 105 + 5 degrees C, Ww = 0, and the soil is said to be oven dry or dry. If a soil mass is cooled in contact with the atmosphere, it absorbs some water. This water absorbed from the atmosphere is called hygroscopic moisture. TM 5-530 contains testing procedures for determining moisture content.
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Soil Formation and Characteristics
4-73. Unit weight (γ) is the expression given to the weight per unit volume of a soil mass. It is expressed:
fx
4-74. In soils terminology, the terms “unit weight” and “density” are used interchangeably.
4-75. Wet unit weight (gm), also expressed as wet density, is the term used if the moisture content is anything other than zero. The wet unit weight of natural soils varies widely. Depending on denseness and gradation, a sandy soil may have a wet unit weight or density of 115 to 135 pcf. Some very dense glacial tills may have wet unit weights as high as 145 pcf. Wet unit weights for most clays range from 100 to 125 pcf. Density of soils can be greatly increased by compaction during construction. In foundation problems, the density of a soil is expressed in terms of wet unit weight.
4-76. Dry unit weight (gd), also expressed as dry density, is the term used if the moisture content is zero. Since no water is present and the weight of air is negligible, it is written:
fx
4-77. Dry unit weight normally is used in construction problems. The general relationship between wet unit weight and dry unit weight is expressed:
fx
4-78. A numerical example of volume-weight relationships follows: GIVEN:
A soil mass with:
wet unit weight = 125 pcf
moisture content = 18 percent
specific gravity = 2.65
volume = 1 cubic foot (cu ft)
FIND:
SOLUTION:
fx
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Chapter 4
If Vv = V – Vs, then
Vv = 1.00 – 0.64 = 0.36 cu ft
Thus, if e = Vv/Vs, substituting computed values, then
𝑒 = 0.36/0.64 = 0.56
4-79. The relationships discussed previously are used in calculations involved in soil construction work. They are used along with the necessary soil tests to classify and to help determine engineering characteristics of soil.
RELATIVE DENSITY
4-80. Use of the void ratio is not very effective in predicting the soil behavior of granular or unrestricted soils. More useful in this respect is the term relative density, expressed as Dr. Relative density is an index of the degree to which a soil has been compacted. Values range from 0 (e = emax) to 1.0 (e = emin). It is written:
fx
emax = the void ratio in the loosest possible condition
emin = the void ratio in the most dense condition possible.
4-81. The limiting ranges of emax may be found by pouring the soil loosely into a container and determining its weight and volume. The limiting ranges of emin may be found by tamping and shaking the soil until it reaches a minimum volume and recording its weight and volume at this point. Relative density is important for gravels and sands.
SOIL-MOISTURE CONDITIONS
4-82. Coarse-grained soils are much less affected by moisture than are fine-grained soils. Coarser soils have larger void openings and generally drain more rapidly. Capillarity is no problem in gravels having only very small amounts of fines mixed with them. These soils will not usually retain large amounts of water if they are above the groundwater table. Also, since the particles in sandy and gravelly soils are relatively large (in comparison to silt and clay particles), they are heavy in comparison to the films of moisture that might surround them. Conversely, the small, sometimes microscopic, particles of a fine- grained soil weigh so little that water within the voids has a considerable effect on them. The following phenomena are examples of this effect:
4-83. These effects are very important to an engineer and are functions of changing water content. The Army’s emphasis on early achievement of proper drainage in horizontal construction stems from these properties of cohesive soils.
Adsorbed Water
4-84. In general terms, adsorbed water is water that may be present as thin films surrounding the separate soil particles. When the soil is in an air-dry condition, the adsorbed water present is called hygroscopic moisture. Adsorbed water is present because the soil particles carry a negative electrical charge. Water is
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dipolar; it is attracted to the surface of the particle and bound to it (see figure 4-12). The water films are affected by the chemical and physical structure of the soil particle and its relative surface area. The relative surface area of a particle of fine-grained soil, particularly if it has a flaky or needlelike shape, is much greater than for coarse soils composed of bulky grains. The electrical forces that bind adsorbed water to the soil particle also are much greater. Close to the particle, the water contained in the adsorbed layer has properties quite different from ordinary water. In the portion of the layer immediately adjacent to the particle, the water may behave as a solid, while only slightly farther away it behaves as a viscous liquid. The total thickness of the adsorbed layer is very small, perhaps on the order of 0.00005 mm for clay soils. In coarse soils, the adsorbed layer is quite thin compared with the thickness of the soil particle. This, coupled with the fact that the contact area between adjacent grains is quite small, leads to the conclusion that the presence of the adsorbed water has little effect on the physical properties of coarse-grained soils. By contrast, for finer soils and particularly in clays, the adsorbed water film is thick in comparison with particle size. The effect is very pronounced when the particles are of colloidal size.
Figure 4-12. Layer of adsorbed water surrounding a soil particle
Plasticity and Cohesion
4-85. Two important aspects of the engineering behavior of fine-grained soils are directly associated with the existence of adsorbed water films. These aspects are plasticity and cohesion.
4-86. Plasticity is the ability of a soil to deform without cracking or breaking. Soils in which the adsorbed films are relatively thick compared to particle size (such as clays) are plastic over a wide range of moisture contents. This is presumably because the particles themselves are not in direct contact with one another. Plastic deformation can take place because of distortion or shearing of the outside layer of viscous liquid in the moisture films. Coarse soils (such as clean sands and gravels) are nonplastic. Silts also are essentially nonplastic materials, since they are usually composed predominantly of bulky grains; if platy grains are present, they may be slightly plastic.
4-87. A plasticity index (PI) is used to determine if soils are cohesive. Not all plastic soils are cohesive; soil is considered cohesive if its PI is 5. That is, they possess some cohesion or resistance to deformation because of the surface tension present in the water films. Thus, wet clays can be molded into various shapes without breaking and will retain these shapes. Gravels, sands, and most silts are not cohesive and are called cohesionless soils. Soils of this general class cannot be molded into permanent shape and have little or no strength when dry and unconfined. Some of these soils may be slightly cohesive when damp. This is attributed to what is sometimes called apparent cohesion, which is also due to the surface tension in the water films between the grains.
Clay Minerals and Base Exchange
4-88. The very fine (colloidal) particles of clay soils consist of clay minerals, which are crystalline in structure. These minerals are complex compounds of hydrous aluminum silicates and are important because their presence greatly influences a soil’s physical properties. X rays have been used to identify several different kinds of clay minerals that have somewhat different properties. Two extreme types are kaolinite
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Chapter 4
and montmorillonite. Both have laminated crystalline structures, but they behave differently. Kaolinite has a very rigid crystalline structure, while montmorillonite can swell by taking water directly into its lattice structure. Later, the flakes themselves may decrease in thickness as the water is squeezed out during drying; the flakes are thus subject to detrimental shrinkage and expansion. An example of this type of material is bentonite, largely made up of the montmorillonite type of clay mineral. Because of its swelling characteristics, bentonite is widely used commercially in the construction of slurry walls and temporary dam cores. Most montmorillonites have much thicker films of adsorbed water than do kaolinites. Kaolinites tend to shrink and swell much less than montmorillonites with changes in moisture content. In addition, the adsorbed water film may contain disassociated ions. For example, metallic cations, such as sodium, calcium, or magnesium, may be present. The presence of these cations also affects the physical behavior of the soil. A montmorillonite clay, for example, in which calcium cations predominate in the adsorbed layer may have properties quite different from a similar clay in which sodium cations predominate. The process of replacing cations of one type with cations of another type in the surface of the adsorbed layer is called base (or cation) exchange. It is possible to effect this replacement and thereby alter the physical properties of a clay soil. For example, the soil may swell, the plasticity may be reduced, or the permeability may be increased by this general process.
Capillary Phenomena
4-89. Capillary phenomena in soils are important for two reasons. First, water moves by capillary action into a soil from a free-water surface. This aspect of capillary phenomena is not discussed here but is covered in chapter 8. Second, capillary phenomena are closely associated with the shrinkage and expansion (swelling) of soils.
The capillary rise of water in small tubes is a common phenomenon, which is caused by surface tension (see figure 4-13). The water that rises upward in a small tube is in tension, hanging on the curved boundary between air and water (meniscus) as if from a suspending cable. The tensile force in the meniscus is balanced by a compressive force in the walls of the tube. Capillary phenomena in small tubes can be simply analyzed and equations derived for the radius of the curved meniscus, the capillary stress (force per unit of area), and the height of capillary rise (see hc in figure 4-13). A soil mass may be regarded as being made up of a bundle of small tubes formed by the interconnected void spaces. These spaces form extremely irregular, tortuous paths for the capillary movement of water. An understanding of capillary action in soils is thus gained by analogy. Theoretical analyses indicate that maximum possible compressive pressure that can be exerted by capillary forces is inversely proportional to the size of the capillary openings.
Figure 4-13. Capillary rise of water in small tubes
Shrinkage
4-90. Many soils undergo a very considerable reduction in volume when their moisture content is reduced. The effect is most pronounced when the moisture content is reduced from that corresponding to complete
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saturation to a very dry condition. This reduction in volume is called shrinkage and is greatest in clays. Some of these soils show a reduction in volume of 50 percent or more while passing from a saturated to an oven-dry condition. Sands and gravels, in general, show very little or no change in volume with change in moisture content. An exception to this is the bulking of sands, which is discussed below. The shrinkage of a clay mass may be attributed to the surface tension existing in the water films created during the drying process. When the soil is saturated, a free-water surface exists on the outside of the soil mass, and the effects of surface tension are not important. As the soil dries out because of evaporation, the surface water disappears and innumerable meniscuses are created in the voids adjacent to the surface of the soil mass. Tensile forces are created in each of these boundaries between water and air. These forces are accompanied by compressive forces that, in a soil mass, act on the soil structure. For the typical, fairly dense structure of a sand or gravel, the compressive forces are of little consequence; very little or no shrinkage results. In fine-grained soils, the soil structure is compressible and the mass shrinks. As drying continues, the mass attains a certain limiting volume. At this point, the soil is still saturated. The moisture content at this stage is called the shrinkage limit. Further drying will not cause a reduction in volume but may cause cracking as the meniscuses retreat into the voids. In clay soils, the internal forces created during drying may become very large. The existence of these forces also principally accounts for the rocklike strength of a dried clay mass. Both silt and clay soils may be subject to detrimental shrinkage with disastrous results in some practical situations. For example, the uneven shrinkage of a clay soil may deprive a concrete pavement of the uniform support for which it is designed; severe cracking or failure may result when wheel loads are applied to the pavement.
Swelling and Slaking
4-91. If water is again made available to a still-saturated clay soil mass that has undergone shrinkage, water enters the soil’s voids from the outside and reduces or destroys the internal forces previously described. Thus, a clay mass will absorb water and expand or swell. If expansion is restricted, as by the weight of a concrete pavement, the expansion force may be sufficient to cause severe pavement cracking. If water is made available to the soil after it has dried below the shrinkage limit, the mass generally disintegrates or slakes. Slaking may be observed by putting a piece of dry clay into a glass of water. The mass will fall completely apart, usually in a matter of minutes. Construction problems associated with shrinkage and expansion are generally solved by removing the soils that are subject to these phenomena or by taking steps to prevent excessive changes in moisture content.
Bulking of Sands
4-92. Bulking is a phenomenon that occurs in dry or nearly dry sand when a slight amount of moisture is introduced into the soil and the soil is disturbed. Low moisture contents cause increased surface tension, which pulls the grains together and inhibits compaction. As a result, slightly moist sands can have lower compacted densities than totally dry or saturated sands. Commonly in sands, this problem is made worse because a slight addition of moisture above the totally dry state increases the sliding coefficient of the particles. The U-shaped compaction curve, with characteristic free-draining soils (sands and gravels), illustrates the concept of bulking. Adding sufficient water to saturate the sand eliminates surface tension, and the sand can be compacted to its densest configuration (see figure 4-14, page 4-20).
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Chapter 4
Figure 4-14. U-shaped compaction curve
CONSISTENCY (ATTERBERG) LIMITS
4-93. A fine-grained soil can exist in any one of several different states, depending on the amount of water in the soil. The boundaries between these different soil states are moisture contents called consistency limits. They are also called Atterberg limits after the Swedish soil scientist who first defined them in 1908. The shrinkage limit is the boundary between the semisolid and solid states. The plastic limit (PL) is the boundary between the semisolid and plastic states. The liquid limit (LL) is the boundary between the plastic and liquid states. Above the LL, the soil is presumed to behave as a liquid. The numerical difference between the LL and the PL is called the PI and is the range of moisture content over which the soil is in a plastic condition. The Atterberg limits are important index properties of fine-grained soils. They are particularly important in classification and identification. They are also widely used in specifications to control the properties, compaction, and behavior of soil mixtures.
TEST PROCEDURES
4-94. The limits are defined by more or less arbitrary and standardized test procedures that are performed on the portion of the soil that passes the Number 40 sieve. This portion of soil is sometimes called the soil binder. TM 5-530 contains detailed test procedures to be used in determining the LL and the PL. The tests are performed with the soil in a disturbed condition.
Liquid Limit
4-95. The LL (or wL) is defined as the minimum moisture content at which a soil will flow upon application of a very small shearing force. With only a small amount of energy input, the soil will flow under its own weight. In the laboratory, the LL is usually determined by use of a mechanical device (see figure 4-15). The detailed testing procedure is described in TM 5-530.
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Soil Formation and Characteristics
Figure 4-15. Liquid limit test
Plastic Limit
4-96. The PL (or wp) is arbitrarily defined as the lowest moisture content at which a soil can be rolled into a thread ⅛ inch in diameter without crushing or breaking. If a cohesive soil has a moisture content above the PL, a thread may be rolled to less than ⅛ inch in diameter without breaking. If the moisture content is below the PL, the soil will crumble when attempts are made to roll it into ⅛-inch threads. When the moisture content is equal to the PL, a thread can be rolled out by hand to ⅛ inch in diameter; then it will crumble or break into pieces ⅛ to ⅜ inch long when further rolling is attempted. Some soils (for example, clean sands) are nonplastic and the PL cannot be determined. A clean sand or gravel will progress immediately from the semisolid to the liquid state.
PLASTICITY INDEX
4-97. The PI (or Ip) of a soil is the numerical difference between the LL and the PL. For example, if a soil has a LL of 57 and a PL of 23, then the PI equals 34 (PI = LL – PL). Sandy soils and silts have characteristically low PIs, while most clays have higher values. Soils that have high PI values are highly plastic and are generally highly compressible and highly cohesive. The PI is inversely proportional to the permeability of a soil. Soils that do not have a PL, such as clean sands, are reported as having a PI of zero.
4-98. Relationships between the LLs and PIs of many soils were studied by Arthur Casagrande of Harvard University and led to the development of the plasticity chart. The chart’s development and use in classifying and identifying soils and selecting the best of the available soils for a particular construction application are discussed in chapter 5.
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Chapter 5
Soil Classification
Early attempts to classify soils were based primarily on grain size. These are the textural classification systems. In 1908, a system that recognized other factors was developed by Atterberg in Sweden and primarily used for agricultural purposes. Somewhat later, a similar system was developed and used by the Swedish Geotechnical Commission. In the United States, the Bureau of Public Roads System was developed in the late twenties and was in widespread use by highway agencies by the middle thirties. This system has been revised over time and is widely used today. The Airfield Classification System was developed by Professor Arthur Casagrande of Harvard University during World War II. A modification of this system, the USCS, was adopted by the US Army Corps of Engineers and the Bureau of Reclamation in January 1952. A number of other soil classification systems are in use throughout the world, and the military engineer should be familiar with the most common ones.
The principal objective of any soil classification system is predicting the engineering properties and behavior of a soil based on a few simple laboratory or field tests. Laboratory and/or field test results are then used to identify the soil and put it into a group that has soils with similar engineering characteristics. Probably no existing classification system completely achieves the stated objective of classifying soils by engineering behavior because of the number of variables involved in soil behavior and the variety of soil problems encountered. Considerable progress has been made toward this goal, particularly in relationship to soil problems encountered in highway and airport engineering. Soil classification should not be regarded as an end in itself but as a tool to further your knowledge of soil behavior.
SECTION I – UNIFIED SOIL CLASSIFICATION SYSTEM
SOIL CATEGORIES
5-1. Soils seldom exist in nature separately as sand, gravel, or any other single component. Usually they occur as mixtures with varying proportions of particles of different sizes. Each component contributes its characteristics to the mixture. The USCS is based on the characteristics of the soil that indicate how it will behave as a construction material.
5-2. In the USCS, all soils are placed into one of three major categories. They are—
5-3. The USCS further divides soils that have been classified into the major soil categories by letter symbols, such as—
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Chapter 5
5-4. A soil that meets the criteria for a sandy clay would be designated (SC). There are cases of borderline soils that cannot be classified by a single dual symbol, such as GM for silty gravel. These soils may require four letters to fully describe them. For example, (SM-SC) describes a sand that contains appreciable amounts of silt and clay.
COARSE-GRAINED SOILS
5-5. Coarse-grained soils are defined as those in which at least half the material is retained on a Number 200 sieve. They are divided into two major divisions, which are—
5-6. A coarse-grained soil is classed as gravel if more than half the coarse fraction by weight is retained on a Number 4 sieve. The symbol G is used to denote a gravel and the symbol S to denote a sand. No clearcut boundary exists between gravelly and sandy soils; as far as soil behavior is concerned, the exact point of division is relatively unimportant. Where a mixture occurs, the primary name is the predominant fraction and the minor fraction is used as an adjective. For example, a sandy gravel would be a mixture containing more gravel than sand by weight. Additionally, gravels are further separated into either coarse gravel or fine gravel with the ¾-inch sieve as the dividing line and sands are either coarse, medium, or fine with the Number 10 and Number 40 sieves, respectively. The coarse-grained soils may also be further divided into three groups on the basis of the amount of fines (materials passing a Number 200 sieve) they contain. These amounts are—
5-7. Coarse-grained soils with less than 5 percent passing the Number 200 sieve may fall into the following groups:
5-8. Coarse-grained soils containing more than 12 percent passing the Number 200 sieve fall into the following groups:
5-9. Gradation of these materials is not considered significant. For both of these groups, the Atterberg limits must plot below the A-line of the plasticity chart shown in figure 5-1. A dual symbol system allows more precise classification of soils based on gradation and Atterberg limits.
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Soil Classification
Figure 5-1. Sample plasticity chart
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Chapter 5
5-10. Gradation of these materials is not considered significant. For both of these groups, the Atterberg limits plot above the A-line.
5-11. The use of the symbols M and C is based on the plasticity characteristics of the material passing the Number 40 sieve. The LL and PI are used in determining the plasticity of the fine materials. If the plasticity chart shown in figure 5-1, page 5-3, is analyzed with the LL and PI, it is possible to determine if the fines are clayey or silty. The symbol M is used to indicate that the material passing the Number 40 sieve is silty in character. M usually designates a fine-grained soil of little or no plasticity. The symbol C is used to indicate that the binder soil is clayey in character. A dual symbol system allows more precise classification of soils based on gradation and Atterberg limits.
5-12. For example, coarse-grained soils with between 5 and 12 percent of material passing the Number 200 sieve, and which meet the criteria for well-graded soil, require a dual symbol, such as—
5-13. Similarly, coarse-grained soils containing more than 12 percent of material passing the Number 200 sieve, and for which the limits plot in the hatched portion of the plasticity chart (see figure 5-1), are borderline between silt and clay and are classified as (SM-SC) or (GM-GC).
5-14. In rare instances, a soil may fall into more than one borderline zone. If appropriate symbols were used for each possible classification, the result would be a multiple designation using three or more symbols. This approach is unnecessarily complicated. It is considered best to use only a double symbol in these cases, selecting the two believed most representative of probable soil behavior. If there is doubt, the symbols representing the poorer of the possible groupings should be used. For example, a well-graded sandy soil with 8 percent passing the Number 200 sieve, with an LL of 28 and a PI of 9, would be designated as (SW-SC). If the Atterberg limits of this soil were such as to plot in the hatched portion of the plasticity chart (for example, an LL of 20 and a PI of 5), the soil could be designated either (SW-SC) or (SW-SM), depending on the judgment of the soils technician.
FINE-GRAINED SOILS
5-15. Fine-grained soils are those in which more than half the material passes a Number 200 sieve. The fine-grained soils are not classified by grain size but according to plasticity and compressibility. Laboratory classification criteria are based on the relationship between the LL and the PI, determined from the plasticity chart shown in figure 5-1. The chart indicates two major groupings of fine-grained soils. These are—
5-16. The symbols L and H represent low and high compressibility, respectively. Fine-grained soils are further divided based on their position above or below the A-line of the plasticity chart.
5-17. Typical soils of the (ML) and (MH) groups are inorganic silts. Those of low plasticity are in the (ML) group; others are in the (MH) group. Atterberg limits of these soils all plot below the A-line. The (ML) group includes—
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SoilClassification
5-18. Micaceous and diatomaceous soils generally fall into the (MH) group but may extend into the (ML) group with LLs < 50. The same statement is true of certain types of kaolin clays, which have low plasticity. Plastic silts fall into the (MH) group.
5-19. In (CL) and (CH) groups, the C stands for clay, with L and H denoting low or high compressibility. These soils plot above the A-line and are principally inorganic clays. The (CL) group includes gravelly clays, sandy clays, silty clays, and lean clays. In the (CH) group are inorganic clays of high plasticity, including fat clays, the gumbo clays of the southern United States, volcanic clays, and bentonite. The glacial clays of the northern United States cover a wide band in the (CL) and (CH) groups.
5-20. Soils in the (OL) and (OH) groups are characterized by the presence of organic matter, hence the symbol O. The Atterberg limits of these soils generally plot below the A-line. Organic silts and organic silt clays of low plasticity fall into the (OL) group, while organic clays plot in the (OH) zone of the plasticity chart. Many organic silts, silt-clays, and clays deposited by rivers along the lower reaches of the Atlantic seaboard have LLs between 40 and 100 and plot below the A-line. Peaty soils may have LLs of several hundred percent and their Atterberg limits generally plot below the A-line.
5-21. Fine-grained soils having limits that plot in the shaded portion of the plasticity chart are given dual symbols (for example, (CL-ML)). Several soil types exhibiting low plasticity plot in this general region on the chart and no definite boundary between silty and clayey soils exists.
HIGHLY ORGANIC SOILS
5-22. A special classification, (Pt), is reserved for the highly organic soils, such as peat, which have many undesirable engineering characteristics. No laboratory criteria are established for these soils, as they generally can be easily identified in the field by their distinctive color and odor, spongy feel, and frequently fibrous texture. Particles of leaves, grass, branches, or other fibrous vegetable matter are common components of these soils.
5-23. Table 5-1, page 5-6, and table 5-2, page 5-8, are major charts which present information applicable to the USCS and procedures to be followed in identifying and classifying soils under this system. Principal categories shown in the chart include—
5-24. These charts are valuable aids in soil classification problems. They provide a simple systematic means of soil classification.
LABORATORY TESTING
5-25. Usually soil samples are obtained during the soil survey and are tested in the laboratory to determine test properties for classifying the soils. The principal tests are—
5-26. These tests are used for all soils except those in the (Pt) group. With the percentages of gravel, sand, and fines and the LL and PI, the group symbol can be obtained from the chart in table 5-2 by reading the diagram from top to bottom. For the gravels and sands containing 5 percent (or less) fines, the shape of the grain-size distribution curve can be used to establish whether the material is well-graded or poorly graded. For the fine-grained soils, it is necessary to plot the LL and PI in the drawing on figure 5-1, to establish the proper symbol. Organic silts or clays (ML) and (MH) are subjected to LL and PL tests before and after oven drying. An organic silt or clay shows a radical drop in these limits as a result of oven drying. An inorganic soil shows a slight drop that is not significant. Where there is an appreciable drop, the predrying values should be used when the classification is determined from table 5-2.
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Chapter 5
Table 5-1. Unified soil classification (including identification and description)
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Table 5-1. Unified soil classification (including identification and description)
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Table 5-2. Auxiliary laboratory identification procedure
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SoilClassification
DESIRABLE SOIL PROPERTIES FOR ROAD AND AIRFIELDS
5-27. The properties desired in soils for foundations under roads and airfields are—
5-28. Some of these properties may be supplied by proper construction methods. For instance, materials having good drainage characteristics are desirable, but if such materials are not available locally, adequate drainage may be obtained by installing a properly designed water-collecting system. Strength requirements for base course materials are high, and only good quality materials are acceptable. However, low strengths in subgrade materials may be compensated for in many cases by increasing the thickness of overlying base materials or using a geotextile (see chapter 11). Proper design of road and airfield pavements requires the evaluation of soil properties in more detail than possible by use of the general soils classification system. However, the grouping of soils in the classification system gives an initial indication of their behavior in road and airfield construction, which is useful in site or route selection and borrow source reconnaissance.
5-29. General characteristics of the soil groups pertinent to roads and airfields are in the soil classification sheet in table 5-3, page 5-13, as follows:
STRENGTH
5-31. In column 3 of table 5-3 the basic soil groups (GM) and (SM) have each been subdivided into two groups designated by the following suffixes:
5-32. This subdivision applies to roads and airfields only and is based on field observation and laboratory tests on soil behavior in these groups. The basis for the subdivision is the LL and PI of the fraction of the soil passing the Number 40 sieve. The suffix d is used when the LL is 25 and the PI is 5; the suffix u is used otherwise.
5-33. The descriptions in columns 7, 8, and 9 generally indicate the suitability of the soil groups for use as subgrade, subbase, or base materials not subjected to frost action. In areas where frost heaving is a problem,
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Chapter 5
the value of materials as subgrades is reduced, depending on the potential frost action of the material (see column 10). Proper design procedures should be used in situations where frost action is a problem.
Coarse-Grained Soils
5-34. Generally, the coarse-grained soils make the best subgrade, subbase, and base materials. The (GW) group has excellent qualities as a base material. The adjective “excellent” is not used for any of these soils for base courses, because “excellent” should only be used to describe a high quality processed crushed stone. Poorly graded gravels and some silty gravels (groups (GP) and (GMd)) are usually only slightly less desirable as subgrade or subbase materials. Under favorable conditions, these gravels may be used as base materials; however, poor gradation and other factors sometimes reduce the value of these soils so they offer only moderate strength. For example—
Fine-Grained Soils
5-35. The fine-grained soils range from fair to very poor subgrade materials as follows—
5-36. These qualities are compensated for in flexible pavement design by increasing the thickness of overlying base material. In rigid pavement design, these qualifications are compensated for by increasing the pavement thickness or by adding a base course layer. None of the fine-grained soils are suitable as a subbase under bituminous pavements, but soils in the (ML) and (CL) groups may be used as select material. The fibrous organic soils (group (Pt)) are very poor subgrade materials and should be removed wherever possible; otherwise, special construction measures should be adopted. They are not suitable as subbase and base materials. The CBR values shown in column 15 give a relative indication of the strength of the various soil groups when used in flexible pavement design. Similarly, values of subgrade modulus
(k) in column 16 are relative indications of strengths from plate-bearing tests when used in rigid pavement design. Actual test values should be used for this purpose instead of the approximate values shown in the tabulation.
5-37. For wearing surfaces on unsurfaced roads, slightly plastic sand-clay-gravel mixtures (GC) are generally considered the most satisfactory. However, they should not contain too large a percentage of fines, and the PI should be in the range of 5 to about 15.
FROST ACTION
5-38. The relative effects of frost action on the various soil groups are shown in column 10. Regardless of the frost susceptibility of the various soil groups, two conditions must be present simultaneously before frost action is a major consideration. These are—
5-39. Water necessary for the formation of ice lenses may become available from a high groundwater table, a capillary supply, water held within the soil voids, or through infiltration. The degree of ice formation that will occur is markedly influenced by physical factors, such as—
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Soil Classification
5-40. In general, the silts and fine silty sands are most susceptible to frost. Coarse-grained materials with little or no fines are affected only slightly or not at all. Clays ((CL) and (CH)) are subject to frost action, but the loss of strength of such materials may not be as great as for silty soils. Inorganic soils containing less than 3 percent (by weight) of grains finer than 0.02 mm in diameter are considered nonfrost-susceptible. Where frost-susceptible soils occur in subgrades and frost is a problem, two acceptable methods of pavement design are available:
5-41. In the second case, design is based on the reduced strength of the subgrade during the frost-melting period. Often an appropriate drainage measure to prevent the accumulation of water in the soil pores helps limit ice development in the subgrade and subbase.
COMPRESSION
5-42. The compression or consolidation of soils becomes a design factor primarily when heavy fills are made on compressible soils. The two types of compression are—
5-43. If adequate provision is made for this type of settlement during construction, it will have little influence on the load-carrying capacity of the pavement. However, when elastic soils subject to compression and rebound under wheel loads are encountered, adequate protection must be provided. Even small movements of this type soil may be detrimental to the base and wearing course of pavements. Fortunately, the free-draining, coarse-grained soils ((GW), (GP), (SW), and (SP)), which generally make the best subgrade and subbase materials, exhibit almost no tendency toward high compressibility or expansion. In general, the compressibility of soil increases with an increasing LL. However, compressibility is also influenced by soil structure, grain shape, previous loading history, and other factors not evaluated in the classification system. Undesirable compression or expansion characteristics may be reduced by distributing the load through a greater thickness of overlying material. These factors are adequately handled by the CBR method of design for flexible pavements. However, rigid pavements may require the addition of an acceptable base course under the pavement.
DRAINAGE
5-44. The drainage characteristics of soils are a direct reflection of their permeability. The evaluation of drainage characteristics for use in roads and runways is shown in column 12 of table 5-3, page 5-13. The presence of water in base, subbase, and subgrade materials, except for free-draining, coarse-grained soils, may cause pore water pressures to develop resulting in a loss of strength. The water may come from infiltration of groundwater or rainwater or by capillary rise from an underlying water table. While free- draining materials permit rapid draining of water, they also permit rapid ingress of water. If free-draining materials are adjacent to less pervious materials and become inundated with water, they may serve as reservoirs. Adjacent, poorly drained soils may become saturated. The gravelly and sandy soils with little or no fines (groups (GW), (GP), (SW), (SP)) have excellent drainage characteristics. The (GMd) and (SMd) groups have fair to poor drain-age characteristics, whereas the (GMu), (GC), (SMu), and (SC) groups have very poor drainage characteristics or are practically impervious. Soils of the (ML), (MH), and (Pt) groups have fair to poor drainage characteristics. All other groups have poor drainage characteristics or are practically impervious.
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Chapter 5
COMPACTION
5-45. Compacting soils for roads and airfields requires attaining a high degree of density during construction to prevent detrimental consolidation from occurring under an embankment’s weight or under traffic. In addition, compaction reduces the detrimental effects of water. Processed materials, such as crushed rock, are often used as a base course and require special treatment during compaction. Types of compaction equipment that may be used to achieve the desired soil densities are shown in table 5-3, column
5-46. Suggested minimum weights of the various types of equipment are shown in note 2 of table 5-3. Column 14 shows ranges of unit dry weight for soil compacted according to the moisture-density testing procedures outlined in Military Standard 621A, method 100. These values are included primarily for guidance; base design or control of construction should be based on laboratory test results.
DESIRABLE SOIL PROPERTIES FOR EMBANKMENTS AND FOUNDATIONS
5-47. Table 5-4, page 5-14, lists the soil characteristics pertinent to embankment and foundation construction. After the soil has been classified, look at column 3 and follow it downward to the soil class. Table 5-4 contains the same type of information as table 5-3 except that column 8 lists the soil permeability and column 12 lists possible measures to control seepage. Material not pertinent to embankments and foundations, such as probable CBR values, are not contained in table 5-4. Both tables are used in the same manner. Read the notes at the bottom of both tables carefully.
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Soil Classification
Table 5-3. Characteristics pertinent to roads and airfields
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Chapter 5
Table 5-4. Classifications pertinent to embankment and foundation construction
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SOIL GRAPHICS
SoilClassification
5-48. It is customary to present the results of soils explorations on drawings as schematic representations of the borings or test pits or on soil profiles with the various soils encountered shown by appropriate symbols. One approach is to write the group letter symbol in the appropriate section of the log. As an alternative, hatching symbols shown in column 4 of table 5-3, page 5-13, may be used. In addition, show the natural water content of fine-grained soils along the side of the log. Use other descriptive remarks as appropriate. Colors may be used to delineate soil types on maps and drawings. A suggested color scheme to show the major soil groups is described in column 5. Boring logs are discussed in more detail in chapter 3. Soil graphics generated in terrain studies usually use numeric symbols, each of which represents a USCS soil type.
FIELD IDENTIFICATION
5-49. The soil types of an area are an important factor in selecting the exact location of airfields and roads. The military engineer, construction foreman, and members of engineer reconnaissance parties must be able to identify soils in the field so that the engineering characteristics of the various soil types encountered can be compared. Because of the need to be economical in time, personnel, equipment, materiel, and money, selection of the project site must be made with these factors in mind. Lack of time and facilities often make laboratory soil testing impossible in military construction. Even where laboratory tests are to follow, field identification tests must be made during the soil exploration to distinguish between the different soil types encountered so that duplication of samples for laboratory testing is minimized. Several simple field identification tests are described in this manual. Each test may be performed with a minimum of time and equipment, although seldom will all of them be required to identify a given soil. The number of tests required depends on the type of soil and the experience of the individual performing them. By using these tests, soil properties can be estimated and materials can be classified. Such classifications are approximations and should not be used for designing permanent or semipermanent construction.
PROCEDURES
5-50. The best way to learn field identification is under the guidance of an experienced soils technician. To learn without such assistance, systematically compare laboratory test results for typical soils in each group with the “feel” of these soils at various moisture contents.
5-51. An approximate identification of a coarse-grained soil can be made by spreading a dry sample on a flat surface and examining it, noting particularly grain size, gradation, grain shape, and particle hardness. All lumps in the sample must be thoroughly pulverized to expose individual grains and to obtain a uniform mixture when water is added to the fine-grained portion. A rubber-faced or wooden pestle and a mixing bowl is recommended for pulverizing. Lumps may also be pulverized by placing a portion of the sample on a firm, smooth surface and using the foot to mash it. If an iron pestle is used for pulverizing, it will break up the mineral grains and change the character of the soil; therefore, using an iron pestle is discouraged.
5-52. Tests for identification of the fine-grained portion of any soil are performed on the portion of the material that passes a Number 40 sieve. This is the same soil fraction used in the laboratory for Atterberg limits tests, such as plasticity. If this sieve is not available, a rough separation may be made by spreading the material on a flat surface and removing the gravel and larger sand particles. Fine-grained soils are examined primarily for characteristics related to plasticity.
EQUIPMENT
5-53. Practically all the tests to be described may be performed with no equipment or accessories other than a small amount of water. However, the accuracy and uniformity of results is greatly increased by the proper use of certain equipment. The following equipment is available in nearly all engineer units (or may be improvised) and is easily transported:
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Chapter 5
FACTORS
5-54. The soil properties that form the basis for the Unified Soil Classification System are the—
5-55. These same properties are to be considered in field identification. Other characteristics observed should also be included in describing the soil, whether the identification is made by field or laboratory methods.
5-56. Properties normally included in a description of a soil are—
5-57. An example of a soil description using the sequence and considering the properties referred to above might be—
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Soil Classification
5-58. A complete description with the proper classification symbol conveys much more to the reader than the symbol or any other isolated portion of the description used alone.
TESTS
5-59. The following tests can be performed to aid in field identification of soils:
Visual Examination Test
5-60. Determine the color, grain size, and grain shape of the coarse-grained portion of a soil by visual examination. The grain-size distribution may be estimated. To observe these properties, dry a sample of the material and spread it on a flat surface.
5-61. In soil surveys in the field, color is often helpful in distinguishing among various soil strata, and from experience with local soils, color may aid in identifying soil types. Since the color of a soil often varies with its moisture content, the condition of the soil when color is determined must always be recorded. Generally, more contrast occurs in these colors when the soil is moist, with all the colors becoming lighter as the moisture contents are reduced. In fine-grained soils, certain dark or drab shades of gray or brown (including almost-black colors) are indicative of organic colloidal matter ((OL) and (OH)). In contrast, clean and bright-looking colors (including medium and light gray, olive green, brown, red, yellow, and white) are usually associated with inorganic soils. Soil color may also indicate the presence of certain chemicals. Red, yellow, and yellowish-brown soil may be a result of the presence of iron oxides. White to pinkish colors may indicate the presence of considerable silica, calcium carbonate, or (in some cases) aluminum compounds . Grayish-blue, gray, and yellow mottled colors frequently indicate poor drainage.
5-62. Estimate the maximum particle size for each sample, thereby establishing the upper limit of the grain-size distribution curve for that sample. The naked eye can normally distinguish the individual grains of soil down to about 0.07 mm. All particles in the gravel and sand ranges are visible to the naked eye. Most of the silt particles are smaller than this size and are invisible to the naked eye. Material smaller than
0.75 mm will pass the Number 200 sieve.
5-63. Perform the laboratory mechanical analysis whenever the grain-size distribution of a soil sample must be determined accurately; however, the grain-size distribution can be approximated by visual inspection. The best way to evaluate a material without using laboratory equipment is to spread a portion of the dry sample on a flat surface. Then, using your hands or a piece of paper, separate the material into its various grain-size components. By this method, the gravel particles and some of the sand particles can be separated from the remainder. This will at least give you an opportunity to estimate whether the total sample is to be considered coarse-grained or fine-grained, depending on whether or not more than 50 percent of the material would pass the Number 200 sieve. Percentage of values refers to the dry weight of the soil fractions indicated as compared to the dry weight of the original sample. A graphical summary of the procedure is shown in figure 5-2, page 5-18.
5-64. If you believe the material is coarse-grained, then consider the following criteria:
5-65. If both criteria can be satisfied and there appears to be a good representation of all grain sizes from largest to smallest, without an excessive deficiency of any one size, the material may be said to be well- graded ((GW) or (SW)). If any intermediate sizes appear to be missing or if there is too much of any one size, then the material is poorly graded ((GP) or (SP)). In some cases, it may only be possible to take a few of the standard sieves into the field. When this is the case, take the Number 4, Number 40, and Number 200 sieves. The sample may be separated into gravels, sands, and fines by use of the Number 4 and Number 200 sieves. However, if there is a considerable quantity of fines, particularly clay particles, separation of the fines can only be readily accomplished by washing them through the Number 200 sieve. In such cases, a determination of the percentage of fines is made by comparing the dry weight of the original sample with that retained on the Number 200 sieve after washing. The difference between these two is the weight of the fines lost in the washing process. To determine the plasticity, use only that portion of the soil passing through a Number 40 sieve.
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The form in this publication is obsolete. See http://www.apd.army.mil for current form.
Figure 5-2. Graphical summary of grain-size distribution
5-66. Estimating the grain-size distribution of a sample using no equipment is probably the most difficult part of field identification and places great importance on the experience of the individual making the estimate. A better approximation of the relative proportions of the components of the finer soil fraction may sometimes be obtained by shaking a portion of this sample into a jar of water and allowing the material to settle. It will settle in layers, with the gravel and coarse sand particles settling out almost immediately. The fine sand particles settle within a minute; the silt particles require as much as an hour; and the clay particles remain in suspension indefinitely or until the water is clear. In using this method, remember that the gravels and sands settle into a much more dense formation than either the silts or clays.
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Soil Classification
5-67. The grain shape of the sand and gravel particles can be determined by close examination of the individual grains. The grain shape affects soil stability because of the increased resistance to displacement found in the more irregular particles. A material with rounded grains has only the friction between the surfaces of the particles to help hold them in place. An angular material has this same friction force, which is increased by the roughness of the surface. In addition, an interlocking action is developed between the particles which gives the soil much greater stability.
5-68. A complete description of a soil should include prominent characteristics of the undisturbed material. The aggregate properties of sand and gravel are described qualitatively by the terms “loose,” “medium,” and “dense.” Clays are described as “hard,” “stiff,” “medium,” and “soft.”
5-69. These characteristics are usually evaluated on the basis of several factors, including the relative ease or difficulty of advancing the drilling and sampling tools and the consistency of the samples. In soils that are described as “soft,” there should be an indication of whether the material is loose and compressible, as in an area under cultivation, or spongy (elastic), as in highly organic soils. The moisture condition at the time of evaluation influences these characteristics and should be included in the report.
Breaking or Dry Strength Test
5-70. The breaking test is performed only on material passing the Number 40 sieve. This test, as well as the roll test and the ribbon test, is used to measure the cohesive and plastic characteristics of the soil. The test is normally made on a small pat of soil about ½ inch thick and about 2 inches in diameter. The pat is prepared by molding a portion of the soil in the wet plastic state into the size and shape desired and then allowing the pat to dry completely. Samples may be tested for dry strength in their natural conditions. Such a test may be used as an approximation; however, it should be verified later by testing a carefully prepared sample.
5-71. After the prepared sample is thoroughly dry, attempt to break it using the thumb and forefingers of both hands (see figure 5-3). If it can be broken, try to powder it by rubbing it with the thumb and fingers of one hand.
Figure 5-3. Breaking or dry strength test
5-72. Typical reactions obtained in this test for various types of soils are described below.
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Chapter 5
5-73. The breaking or dry strength test is one of the best tests for distinguishing between plastic clays and nonplastic silts or fine sands. However, a word of caution is appropriate. Dry pats of highly plastic clays quite often display shrinkage cracks. Breaking the sample along one of these cracks gives an indication of only a very small part of the true dry strength of the clay. It is important to distinguish between a break along such a crack and a clean, fresh break that indicates the true dry strength of the soil.
Roll or Thread Test
5-74. The roll or thread test is performed only on material passing the Number 40 sieve. Prepare the soil sample by adding water to the soil until the moisture content allows easy remolding of the soil without sticking to the fingers. This is sometimes referred to as being just below the “sticky limit.” Using a nonabsorbent surface, such as glass or a sheet of heavy coated paper, rapidly roll the sample into a thread approximately ⅛ inch in diameter figure 5-4.
Figure 5-4. Roll or thread test
5-75. A soil that can be rolled into a ⅛-inch- diameter thread at some moisture content has some plasticity. Materials that cannot be rolled in this manner are nonplastic or have very low plasticity. The number of times that the thread may be lumped together and the rolling process repeated without crumbling and breaking is a measure of the degree of plasticity of the soil. After the PL is reached, the degree of plasticity may be described as follows:
5-76. From this test, the cohesiveness of the material near the PL may also be described as weak, firm, or tough. The higher the position of a soil on the plasticity chart, the stiffer are the threads as they dry out and the tougher are the lumps if the soil is remolded after rolling.
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Soil Classification
Ribbon Test
5-77. The ribbon test is performed only on the material passing the Number 40 sieve. The sample prepared for use in this test should have a moisture content slightly below the sticky limit. Using this material, form a roll of soil about ½ or ¾ inch in diameter and about 3 to 5 inches long. Place the material in the palm of the hand and, starting with one end, flatten the roll, forming a ribbon ⅛ to ¼ inch thick by squeezing it between the thumb and forefinger (see figure 5-5). The sample should be handled carefully to form the maximum length of ribbon that can be supported by the cohesive properties of the material. If the soil sample holds together for a length of 8 to 10 inches without breaking, the material is considered to be both plastic and highly compressive (CH).
5-78. If soil cannot be ribboned, it is nonplastic (ML) or (MH). If it can be ribboned only with difficulty into short lengths, the soil is considered to have low plasticity (CL). The roll test and the ribbon test complement each other in giving a clearer picture of the degree of plasticity of a soil.
Figure 5-5. Ribbon test (highly plastic clay)
Wet Shaking Test
5-79. The wet shaking test is performed only on material passing the Number 40 sieve. In preparing a portion of the sample for use in this test, moisten enough material with water to form a ball of material about ¾ inch in diameter. This sample should be just wet enough so that the soil will not stick to the fingers when remolding (just below the sticky limit) (see figure 5-6, page 5-22, a).
5-80. Place the sample in the palm of your hand and shake vigorously (see figure 5-6, b). Do this by jarring the hand on the table or some other firm object or by jarring it against the other hand. The soil has reacted to this test when, on shaking, water comes to the surface of the sample producing a smooth, shiny appearance (see figure 5-6, c). This appearance is frequently described as “livery.” Then, squeeze the sample between the thumb and forefinger of the other hand. The surface water will quickly disappear, and the surface will become dull. The material will become firm and resist deformation. Cracks will occur as pressure is continued, with the sample finally crumbling like a brittle material. The vibration caused by the shaking of the soil sample tends to reorient the soil grains, decrease voids, and force water that had been within these voids to the surface. Pressing the sample between the fingers tends to disarrange the soil grains, increase the void space, and draw the water into the soil. If the water content is still adequate, shaking the broken pieces will cause them to liquefy again and flow together. This process only occurs when the soil grains are bulky and cohesionless.
5-81. Very fine sands and silts are readily identified by the wet shaking test. Since it is rare that fine sands and silts occur without some amount of clay mixed with them, there are varying degrees of reaction to this
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Chapter 5
test. Even a small amount of clay tends to greatly retard this reaction. Descriptive terms applied to the different rates of reaction to this test are—
Figure 5-6. Wet shaking test
5-82. A sudden or rapid reaction to the shaking test is typical of nonplastic fine sands and silts. A material known as rock flour, which has the same size range as silt, also gives this type of reaction.
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Soil Classification
5-83. A sluggish or slow reaction indicates slight plasticity, such as that which might be found from a test of some organic or inorganic silts or from silts containing a small amount of clay.
5-84. Obtaining no reaction at all to this test does not indicate a complete absence of silt or fine sand. Even a slight content of colloidal clay imparts some plasticity and slows the reaction to the shaking test. Extremely slow or no reaction is typical of all inorganic clays and highly plastic clays.
Odor Test
5-85. Organic soils of the (OL) and (OH) groups have a distinctive musty, slightly offensive odor, which can be used as an aid in identifying such material. This odor is especially apparent from fresh samples. Exposure to air gradually reduces the odor, but heating a wet sample rejuvenates the odor. Organic soils are undesirable as foundation or base course material and are usually removed from the construction site.
Bite or Grit Test
5-86. The bite or grit test is a quick and useful method of distinguishing among sands, silts, or clays. In this test, a small pinch of the soil material is ground lightly between the teeth and identified.
5-87. A sandy soil may be identified because the sharp, hard particles of sand grate very harshly between the teeth and will be highly objectionable. This is true even of the fine sand.
5-88. The silt grains of a silty soil are so much smaller than sand grains that they do not feel nearly so harsh between the teeth. Silt grains are not particularly objectionable, although their presence is still easily detected.
5-89. The clay grains of a clayey soil are not gritty but feel smooth and powdery, like flour, between the teeth. Dry lumps of clayey soils stick when lightly touched with the tongue.
Slaking Test
5-90. The slaking test is useful in determining the quality of certain shales and other soft rocklike materials. The test is performed by placing the soil in the sun or in an oven to dry and then allowing it to soak in water for a period of at least 24 hours. The strength of the soil is then examined. Certain types of shale completely disintegrate, losing all strength.
5-91. Other materials that appear to be durable rocks may be crumbled and readily broken by hand after such soaking. Materials that have a considerable reduction in strength are undesirable for use as base course materials.
Acid Test
5-92. The acid test is used to determine the presence of calcium carbonate. It is performed by placing a few drops of HCl on a piece of soil. A fizzing reaction (effervescence) to this test indicates the presence of calcium carbonate.
CAUTION
HC1 may cause burns. Use appropriate measures to protect the skin and eyes. If it is splashed on the skin or in the eyes, immediately flush with water and seek medical attention.
5-93. Calcium carbonate is normally desirable in a soil because of the cementing action it provides to add to the soil’s stability. In some very dry noncalcareous soils, the absorption of the acid creates the illusion of effervescence. This effect can be eliminated in dry soils by moistening the soil before applying the acid.
5-94. Since cementation is normally developed only after a considerable curing period, it cannot be counted on for strength in most military construction. This test permits better understanding of what
25 September 2012 TM 3-34.64/MCRP 3-17.7G 5-23
Chapter 5
appears to be abnormally high strength values on fine-grained soils that are tested in-place where this property may exert considerable influence.
Shine Test
5-95. The shine test is another means of measuring the plasticity characteristics of clays. A slightly moist or dry piece of highly plastic clay will give a definite shine when rubbed with a fingernail, a pocket knife blade, or any smooth metal surface. On the other hand, a piece of lean clay will not display any shine but will remain dull.
Feel Test
5-96. The feel test is a general-purpose test and one that requires considerable experience and practice before reliable results can be obtained. This test will be used more as familiarity with soils increases. Moisture content and texture can be readily estimated by using the feel test.
5-97. The natural moisture content of a soil indicates drainage characteristics, nearness to a water table, or other factors that may affect this property. A piece of undisturbed soil is tested by squeezing it between the thumb and forefinger to determine its consistency. The consistency is described by such terms as “hard,” stiff,” “brittle,” “friable,” “sticky,” “plastic,” or “soft.” Remold the soil by working it in the hands and observing any changes. By this test, the natural water content is estimated relative to the LL or PL of the soil. Clays that turn almost liquid on remolding are probably near or above the LL. If the clay re-mains stiff and crumbles on being remolded, the natural water content is below the PL.
5-98. The term “texture,” as applied to the fine-grained portion of a soil, refers to the degree of fineness and uniformity. Texture is described by such expressions as “floury,” “smooth,” “gritty,” or “sharp,” depending on the feel when the soil is rubbed between the fingers. Sensitivity to this sensation may be increased by rubbing some of the material on a more tender skin area, such as the inside of the wrist. Fine sand will feel gritty. Typical dry silts will dust readily and feel relatively soft and silky to the touch. Clay soils are powdered only with difficulty but become smooth and gritless like flour.
HASTY FIELD IDENTIFICATION
5-99. With the standard methods of field identification supplemented with a few simplified field tests, an approximate and hasty classification of almost any soil can be obtained. The simple or hasty tests outlined in figure 5-7, pages 5-25 through 5-27, will, for the most part, eliminate the need for specialized equipment such as sieves. The results will give at least a tentative classification to almost any soil. The schematic diagram in figure 5-7, may be used as a guide to the testing sequence in the process of assigning a symbol to a sample of soil.
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Soil Classification
Figure 5-7. Suggested procedure for hasty field identification
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Chapter 5
Figure 5-7. Suggested procedure for hasty field identification (continued)
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Soil Classification
Figure 5-7. Suggested procedure for hasty field identification (continued)
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Chapter 5
OPTIMUM MOISTURE CONTENT (OMC)
5-100. To determine whether a soil is at or near OMC, mold a golf-ball-size sample of the soil with your hands. Then squeeze the ball between your thumb and forefinger. If the ball shatters into several fragments of rather uniform size, the soil is near or at OMC. If the ball flattens out without breaking, the soil is wetter than OMC. If, on the other hand, the soil is difficult to roll into a ball or crumbles under very little pressure, the soil is drier than OMC.
SECTION II – OTHER SOIL CLASSIFICATION SYSTEMS
COMMONLY USED SYSTEMS
5-101. Information about soils is available from many sources, including publications, maps, and reports. These sources may be of value to the military engineer in studying soils in a given area. For this reason, it is important that the military engineer have some knowledge of other commonly used systems. Table 5-5 gives approximate equivalent groups for the USCS, Revised Public Roads System, and the Federal Aviation Administration (FAA) System.
Table 5-5. Comparison of the USCS, revised public roads system, and FAA system
|
USCS |
Revised Public Roads System |
Federal Aviation Administration System |
|
|
GW |
A-1-a |
|
|
|
GP GM |
A-1-a A-1-a, A-2-4 or 5 |
Gravelly soils not included directly |
|
|
GC |
A-2-6 or 7 |
|
|
|
SW SP SM |
A-1-b A-3 A-1-b, A-2-4 or 5 |
E-1, 2 or 3 |
E-4 or 5 (usually SM or SC) |
|
SC |
A-2-6 or 7 |
|
|
|
MI |
A-4 |
E-6 |
|
|
CL |
A-6, A-7-5 |
|
|
|
OL |
A-4, A-7-5 |
E-6 |
|
|
MH |
A-5 |
|
E-8 (usually L group) |
|
CH |
A-7 |
E-10, 11, or 12 |
E-9 (usually not CH) |
|
OH |
A-7 |
|
|
|
Pt |
|
E-13 |
|
|
Note. Groups are only approximately equivalent, since different limiting values are used in each system. |
|||
REVISED PUBLIC ROADS SYSTEM
5-102. Most civil agencies concerned with highways in the United States classify soil by the Revised Public Roads System. This includes the Bureau of Public Roads and most of the state highway departments. The Public Roads System was originated in 1931. Part of the original system, which applied to uniform subgrade soils, used a number of tables and charts based on several routine soil tests to permit placing of a given soil into one of eight principal groups, designated A-1 through A-8. The system was put into use by many agencies. As time passed, it became apparent that some of the groups were too broad in coverage because somewhat different soils were classed in the same group. A number of the agencies using the system modified it to suit their purposes. Principal modifications included breaking down some of the broad groups into subgroups of more limited scope. The revisions culminated in a comprehensive committee report that appeared in the Proceedings of the 25th Annual Meeting of the Highway Research Board (1945). This same report contains detailed information relative to the Airfield Classification System
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Soil Classification
and the Federal Aviation Administration System. The Revised Public Roads System is primarily designed for the evaluation of subgrade soils, although it is useful for other purposes also.
Basis
5-103. Table 5-6, page 5-30, shows the basis of the Revised Public Roads System. Soils are classed into one of two very broad groups. They are—
5-104. There are seven major groups, numbered A-1 through A-7, together with a number of suggested subgroups. The A-8 group of the original system, which contained the highly organic soils such as peat, is not included in the revised system. The committee felt that no group was needed for these soils because of their ready identification by appearance and odor. Whether a soil is silty or clayey depends on its PI. “Silty” is applied to material that has a PI < 10 and “clayey” is applied to a material that has a PI ≥ 10.
5-105. Figure 5-8, page 5-31, shows the formula for group index and charts to facilitate its computation. The group index was devised to provide a basis for approximating within-group evaluations. Group indexes range from 0 for the best subgrade soils to 20 for the poorest. Increasing values of the group index within each basic soil group reflect the combined effects of increasing LLs and PIs and decreasing percentages of coarse material in decreasing the load-carrying capacity of subgrades. Figure 5-9, page 5-32, graphically shows the ranges of LL and PI for the silt-clay groups. It is particularly useful for subdividing the soils of the A-7 group.
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Chapter 5
Table 5-6. Revised public roads system of soil classification
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Soil Classification
Figure 5-8. Group index formula and charts, revised public roads system
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Chapter 5
Figure 5-9. Relationship between LL and PI for silt-clay groups, revised public roads system
Procedure
5-106. Table 5-6, page 5-30, is used in a left-to-right elimination process, and the given soil is placed into the first group or subgroup in which it fits. In order to distinguish the revised from the old system, the group symbol is given, followed by the group index in parentheses (for example, A-4(5)). The fact that the A-3 group is placed ahead of the A-2 group does not imply that it is a better subgrade material. This arrangement is used to facilitate the elimination process. The classification of some borderline soils requires judgment and experience. The assignment of the group designation is often accompanied by the writing of a careful description, as in the USCS. Detailed examples of the classification procedure are given later in this chapter.
AGRICULTURAL SOIL CLASSIFICATION SYSTEM
5-107. Many reports published for agricultural purposes can be useful to the military engineer. Two phases of the soil classification system used by agricultural soil scientists are discussed here. These are—
Textural Classification
5-108. Information about the two textural classification systems of the US Department of Agriculture is contained in figure 5-10 and table 5-7, page 5-36. The chart and table are largely self-explanatory. The grain-size limits, which are applicable to the categories shown, are as follows:
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Soil Classification
Figure 5-10. US Department of Agriculture textural classification chart
Pedological Classification
5-109. Soil profile and pedology were discussed in chapter 1. Agricultural soil scientists have devised a complete and complex system for describing and classifying surface soils. No attempt will be made to discuss this system in detail here. The portion of the system in which engineers are principally interested refers to the terms used in mapping limited areas. Mapping is based on—
5-110. The designation known as soil series is applied to soils that have the same genetic horizons, possess similar characteristics and profiles, and are derived from the same parent material. Series names follow no particular pattern but are generally taken from the geographical place near where they were first found. The soil type refers to the texture of the upper portion of the soil profile. Several types may, and usually do, exist within a soil series. Phase is variation, usually of minor importance, in the soil type.
5-111. A mapping unit may be “Emmet loamy sand, gravelly phase” or any one of the large number of similar designations. Soils given the same designation generally have the same agricultural properties wherever they are encountered. Many of their engineering properties may also be the same. The mapping
25 September 2012 TM 3-34.64/MCRP 3-17.7G 5-33
Chapter 5
unit in which a given soil is placed is determined by a careful examination of the soil sample obtained by using an auger to bore or by observing highway cuts, natural slopes, and other places where the soil profile is exposed. Particularly important factors are—
5-112. Other important factors are—
5-113. Agricultural soil maps prepared from field surveys show the extent of each important soil type and its geographical location. Reports that accompany the maps contain word descriptions of the various types, some laboratory test results, typical profiles, and soil properties important to agricultural use. Frequently prepared on a county basis, soil surveys (maps and reports) are available for many areas in the United States and in many foreign countries. For installation projects, check with the Natural Resources Branch or Land Management Branch of the Directorate of Engineering and Housing for a copy of the soil survey. For off-post projects, request a copy of the soil survey from the county Soil Conservation Service (SCS). The information contained in the soil survey is directly useful to engineers.
GEOLOGICAL SOIL CLASSIFICATION
5-114. Geologists classify soils according to their origin (process of formation) following a pattern similar to that used in chapter 1. TM 5-545 gives a geological classification of soil deposits and related information.
TYPICAL SOIL CLASSIFICATION
5-115. The following paragraphs concern the classification of four inorganic soil types on the basis of laboratory test data. Each soil is classified under the Unified Soil Classification System, the Revised Public Roads System, and the Agricultural Soil Classification System. Table 5-8, page 5-37, shows the information known about each soil.
UNIFIED SOIL CLASSIFICATION SYSTEM
5-116. The results of the classification of the four soil types under this system are as follows:
Soil Number 1
5-117. The soil is fine-grained since more than half passes the Number 200 sieve. The LL is less than 50; therefore, it must be (ML) or (CL) since it is inorganic. On the plasticity chart, it falls below the A-line; therefore, it is a sandy silt (ML).
Soil Number 2
5-118. The soil is fine-grained since more than half passes the Number 200 sieve. The LL is more than 50; therefore, it must be (MH) or (CH). On the plasticity chart, it falls above the A-line; therefore, it is a sandy clay (CH).
Soil Number 3
5-119. The soil is coarse-grained since very little passes the Number 200 sieve. It must be a sand since it all passes the Number 10 sieve. The soil contains less than 5 percent passing the Number 200 sieve;
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Soil Classification
therefore, it must be either an (SW) or an (SP) (see table 5-2, page 5-8). The value of Cu = 2 will not meet requirements for (SW); therefore, it is a poorly graded sand (SP).
Soil Number 4
5-120. The soil is coarse-grained since again very little passes the Number 200 sieve. It must be a gravel since more than half of the coarse fraction is larger than a Number 4 sieve. Since the soil contains less than 5 percent passing the Number 200 sieve, it is either (GW) or (GP) (see table 5-2). It meets the gradation requirements relative to Cu and Cc</sub>; therefore, it is a well-graded gravel (GW).
REVISED PUBLIC ROADS CLASSIFICATION SYSTEM
5-121. The results of the classification of the four soil types under this system are as follows:
Soil Number 1
5-122. To calculate the group index, refer to figure 5-8, page 5-31. From chart A, read 0; from chart B, read 3; therefore, the group index = 0 + 3 = 3. Table 5-6, page 5-30, shows by a left-to-right elimination process, that the soil cannot be in one of the granular materials groups, since more than 35 percent passes a Number 200 sieve. It meets the requirements of the A-4 group; therefore, it is A-4(3).
Soil Number 2
5-123. The group index = 8 + 10 = 18. Table 5-6 shows that the soil falls into the A-7 group, since this is the only group that will permit a group index value as high as 18. Figure 5-9, page 5-32, shows that it falls in the A-7-6 subgroup; therefore, A-7-6(18).
Soil Number 3
5-124. The group index = 0 + 0 = 0. This is one of the soils described as granular material. It will not meet the requirements of an A-1 soil, since it contains practically no fines. It does not meet the requirements of the A-3 group; therefore, it is A-3(0) (see table 5-6).
Soil Number 4
5-125. The group index = 0 + 0 = 0. This is obviously a granular material and meets the requirements of the A-1-a (0) (see table 5-6).
AGRICULTURAL SOIL CLASSIFICATION SYSTEM
5-126. Although the values are not given in the previous tabulation, assume that 12 percent of Soil Number 1 and 35 percent of Soil Number 2 are in the range of clay sizes that is below 0.005 mm.
Soil Number 1
5-127. This soil contains 100 – 48.2 = 51.8 percent sand, since the opening of a Number 270 sieve is 0.05 mm. The soil is then composed of 52 percent sand, 36 percent silt, and 12 percent clay. Figure 5-10, page 5-33, classifies this soil as a sandy loam.
Soil Number 2
5-128. This soil contains approximately 35 percent sand, 30 percent silt, and 35 percent clay. Figure 5-10 classifies this soil as clay.
Soil Number 3
5-129. This soil is 99 percent sand; therefore, it can only be classified as sand.
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Chapter 5
Soil Number 4
5-130. This soil contains approximately 70 percent coarse gravel, 14 percent fine gravel, 13 percent sand, and 3 percent silt and clay combined. It cannot be classified by using figure 5-10, page 5-33, because the chart does not cover gravels and gravelly sands. Table 5-7, classifies the material as gravel and sand.
Table 5-7. Agricultural soil classification system
|
Gradation Limits of Textural Soil Groups* |
|||||||
|
First place the soil in Textural Group I, II, or III according to clay content (column 7) or silt and clay content combined (column 5); and finally the sand content and gravel content (columns 1 to 4 inclusive). When clay content approaches the upper limit for that group, it is called “heavy,” such as “heavy clay loam,” and a “light” soil when approaching the lower clay limit. |
|||||||
|
Soil texture |
(1) Fine and coarse gravel, percent |
(2) Coarse sand and gravel, percent |
(3) Sand andgravel, percent |
(4) Fine sand, percent |
(5) Sill and clay, percent |
(6) Silt, percent |
(7) Clay, percent |
|
Group IA: Sands and Gravels |
|||||||
|
Gravel |
85 to 100 |
— |
— |
— |
0 to 15 |
— |
0 to 20 |
|
Gravel and sand |
50 to 85 |
— |
— |
— |
0 to 15 |
— |
0 to 20 |
|
Sand and gravel |
25 to 50 |
— |
— |
— |
0 to 15 |
— |
0 to 20 |
|
Coarse sand |
0 to 25 |
50 to 100 |
— |
— |
0 to 15 |
— |
0 to 20 |
|
Sand |
0 to 25 |
0 to 50 |
— |
0 to 50 |
0 to 15 |
— |
0 to 20 |
|
Fine sand |
0 to 25 |
— |
— |
0 to 100 |
0 to 15 |
— |
0 to 20 |
|
Group IB: Loamy Sands |
|||||||
|
Gravelly, loamy, coarse sand |
25 to 85 |
50 to 85 |
— |
— |
15 to 20 |
— |
0 to 20 |
|
Gravelly, loamy sand |
25 to 50 |
25 to 50 |
— |
0 to 50 |
15 to 20 |
— |
0 to 20 |
|
Gravelly, loamy, fine sand |
25 to 35 |
— |
— |
50 to 60 |
15 to 20 |
— |
0 to 20 |
|
Loamy, coarse sand |
0 to 25 |
50 to 85 |
— |
— |
15 to 20 |
— |
0 to 20 |
|
Loamy sand |
0 to 25 |
0 to 50 |
— |
0 to 50 |
15 to 20 |
— |
0 to 20 |
|
Loamy, fine sand |
0 to 25 |
— |
— |
50 to 85 |
15 to 20 |
— |
0 to 20 |
|
Group IC Sandy Loams |
|||||||
|
Gravelly, coarse, sandy loam |
25 to 85 |
30 to 80 |
— |
— |
20 to 50 |
— |
0 to 20 |
|
Gravelly, sandy loam |
25 to 50 |
25 to 50 |
— |
0 to 50 |
20 to 50 |
— |
0 to 20 |
|
Gravelly, fine, sandy loam |
25 to 30 |
— |
— |
50 to 55 |
20 to 50 |
— |
0 to 20 |
|
Coarse, sandy loam |
0 to 25 |
50 to 80 |
— |
|
20 to 50 |
— |
0 to 20 |
|
Sandy loam |
0 to 25 |
0 to 50 |
— |
0 to 50 |
20 to 50 |
— |
0 to 20 |
|
Fine, sandy loam |
0 to 25 |
— |
— |
50 to 80 |
20 to 50 |
— |
0 to 20 |
|
Groups ID: Loams and Silt Loams |
|||||||
|
Gravelly loam |
25 to 50 |
— |
— |
— |
50 to 70 |
30 to 50 |
0 to 20 |
|
Gravelly, silt loam |
25 to 50 |
— |
— |
— |
50 to 100 |
50 to 80 |
0 to 20 |
|
Loam |
0 to 25 |
— |
— |
— |
50 to 70 |
30 to 50 |
0 to 20 |
|
Silt loam |
0 to 25 |
— |
— |
— |
50 to 100 |
50 to 80 |
0 to 20 |
|
Silt |
0 to 25 |
— |
— |
— |
50 to 100 |
80 to 100 |
0 to 20 |
|
Groups II: Clay Loams |
|||||||
|
Gravelly, sandy, clay loam |
25 to 80 |
— |
50 to 80 |
— |
— |
— |
20 to 30 |
|
Gravelly, clay loam |
50 to 50 |
— |
25 to 50 |
— |
— |
20 to 50 |
20 to 30 |
|
Gravelly, silty, clay loam |
25 to 30 |
— |
0 to 30 |
— |
— |
50 to 55 |
20 to 30 |
|
Sandy, clay loam |
0 to 25 |
— |
50 to 80 |
— |
— |
0 to 30 |
20 to 30 |
|
Clay loam |
0 to 25 |
— |
20 to 50 |
— |
— |
20 to 50 |
20 to 30 |
|
Silty, clay loam |
0 to 25 |
— |
0 to 30 |
— |
— |
50 to 80 |
20 to 30 |
|
Group III: Clays |
|||||||
|
Gravelly, sandy clay |
25 to 70 |
— |
50 to 70 |
— |
— |
— |
30 to 100 |
|
Gravelly clay |
25 to 50 |
— |
25 to 50 |
— |
— |
0 to 45 |
30 to 100 |
|
Sandy clay |
0 to 25 |
— |
50 to 70 |
— |
— |
0 to 20 |
30 to 100 |
|
Silty clay |
0 to 25 |
— |
0 to 20 |
— |
— |
50 to 70 |
30 to 100 |
|
Clay |
0 to 25 |
— |
0 to 50 |
— |
— |
0 to 20 |
30 to 100 |
|
|||||||
5-36 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Classification
Table 5-8. Classification of four inorganic soil types
|
Soil Number |
||||
|
|
1 |
2 |
3 |
4 |
|
Mechanical Analysis (Percent Passing by Weight) |
||||
|
3-inch sieve |
— |
— |
— |
100.0 |
|
¾-inch sieve |
— |
— |
— |
56.0 |
|
Number 4 sieve |
— |
— |
— |
30.0 |
|
Number 10 sieve |
100.0 |
100.0 |
100.0 |
16.4 |
|
Number 40 sieve |
85.2 |
97.6 |
85.0 |
7.2 |
|
Number 60 sieve |
— |
— |
20.0 |
5.0 |
|
Number 200 sieve |
52.1 |
69.8 |
1.2 |
3.5 |
|
Number 270 sieve |
48.2 |
65.0 |
— |
— |
|
Numerical Values |
||||
|
Cu |
— |
— |
2.0 |
12.5 |
|
Cc |
— |
— |
— |
2.2 |
|
Plasticity Characteristics |
||||
|
Liquid limit |
29 |
67 |
21 |
— |
|
Plasticity index |
5 |
39 |
NP* |
— |
|
* Nonplastic |
||||
COMPARISON OF CLASSIFICATION SYSTEMS
5-131. Table 5-9 is a summary of the classification of the soils in question under the three different classification systems considered.
Table 5-9. Comparison of soils under three classification system
|
Soil Number |
Unified Soil Classification System |
Revised Public Roads System |
Agricultural Soil Classification System |
|
1 |
ML |
A-4(3) |
Sandy loam |
|
2 |
CH |
A-7-6 (18) |
Clay |
|
3 |
SP |
A-3(0) |
Sand |
|
4 |
GW |
A-1-a (0) |
Gravel and sand |
25 September 2012 TM 3-34.64/MCRP 3-17.7G 5-37
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Chapter 6
Concepts of Soil Engineering
The military engineer encounters a wide variety of soils in varying physical states. Because of the inherent variability of the physical properties of soil, several tests and measurements within soils engineering were developed to quantify these differences and to enable the engineer to apply the knowledge to economical design and construction. This chapter deals with soil engineering concepts, to include settlement, shearing resistance, and bearing capacity, and their application in the military construction arena.
SECTION I – SETTLEMENT
FACTORS
6-1. The magnitude of a soil’s settlement depends on several factors, including—
6-2. Unless otherwise stated, it is assumed that the soil mass undergoing settlement is completely confined, generally by the soil that surrounds it.
COMPRESSIBILITY
6-3. Compressibility is the property of a soil that permits it to deform under the action of an external compressive load. Loads discussed in this chapter are primarily static loads that act, or may be assumed to act, vertically downward. Brief mention will be made of the effects of vibration in causing compression. The principal concern here is with the property of a soil that permits a reduction in thickness (volume) under a load like that applied by the weight of a highway or airfield. The compressibility of the underlying soil may lead to the settlement of such a structure.
COMPRESSIVE LOAD BEHAVIOR
6-4. In a general sense, all soils are compressible. That is, they undergo a greater or lesser reduction in volume under compressive static loads. This reduction in volume is attributed to a reduction in volume of the void spaces in the soil rather than to any reduction in size of the individual soil particles or water contained in the voids.
6-5. If the soil is saturated before the load is applied, some water must be forced from the voids before settlement can take place. This process is called consolidation. The rate of consolidation depends on how quickly the water can escape, which is a function of the soil’s permeability.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 6-1
Chapter 6
COHESIONLESS SOILS
6-6. The compressibility of confined coarse-grained cohesionless soils, such as sand and gravel, is rarely a practical concern. This is because the amount of compression is likely to be small in a typical case, and any settlement will occur rapidly after the load is applied. Where these soils are located below the water table, water must be able to escape from the stratum. In the case of coarse materials existing above the water table and under less than saturated conditions, the application of a static load results in the rearrangement of soil particles. This produces deformation without regard to moisture escape. So, generally speaking, settlement occurs during the period of load application (construction). Deformations that are thus produced in sands and gravels are essentially permanent in character. There is little tendency for the soil to return to its original dimensions or rebound when the load is removed. A sand mass in a compact condition may eventually attain some degree of elasticity with repeated applications of load.
6-7. The compressibility of a loose sand deposit is much greater than that of the same sand in a relatively dense condition. Generally, structures should not be located on loose sand deposits. Avoid loose sand deposits if possible, or compact to a greater density before the load is applied. Some cohesionless soils, including certain very fine sands and silts, have loose structures with medium settlement characteristics. Both gradation and grain shape influence the compressibility of a cohesionless soil. Gradation is of indirect importance in that a well-graded soil generally has a greater natural density than one of uniform gradation. Soils that contain platy particles are more compressible than those composed entirely of bulky grains. A fine sand or silt that contains mica flakes may be quite compressible.
6-8. Although soils under static loads are emphasized here, the effects of vibration should also be mentioned. Vibration during construction may greatly increase the density of cohesionless soils. A loose sand deposit subjected to vibration after construction may also change to a dense condition. The latter change in density may have disastrous effects on the structures involved. Cohesionless soils are usually compacted or “densified” as a planned part of construction operations. Cohesive soils are usually insensitive to the effects of vibrations.
CONSOLIDATION
6-9. Consolidation is the time-dependent change in volume of a soil mass under compressive load that occurs when water slowly escapes from the pores or voids of the soil. The soil skeleton is unable to support the load and changes structure, reducing its volume and producing vertical settlement.
COHESIVE SOILS
6-10. The consolidation of cohesive, fine-grained soils (particularly clays) is quite different from the compression of cohesionless soils. Under comparable static loads, the consolidation of a clay may be much greater than coarse-grained soils and settlement may take a very long time to occur. Structures often settle due to consolidation of a saturated clay stratum. The consolidation of thick, compressible clay layers is serious and may cause structural damage. In uniform settlement, the various parts of a structure settle approximately equal amounts. Such uniform settlement may not be critical. Nonuniform, or differential, settlement of parts of a structure due to consolidation causes serious structural damage. A highway or airfield pavement may be badly damaged by the nonuniform settlement of an embankment founded on a compressible soil.
CONSOLIDATION TESTS
6-11. The consolidation characteristics of a compressible soil should be determined for rational design of many large structures founded on or above soils of this type. Consolidation characteristics generally are determined by laboratory consolidation tests performed on undisturbed samples. The natural structure, void ratio, and moisture content are preserved as carefully as possible for undisturbed samples. However, military soils analysts are not equipped or trained to perform consolidation tests.
6-12. Information on consolidation tests and settlement calculations for the design of structures to be built on compressible soils may be found in Naval Facilities Engineering Command Design Manual (NAVFAC DM)-7.1.
6-2 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Concepts of Soil Engineering
SECTION II – SHEARING RESISTANCE
IMPORTANCE
6-13. From an engineering viewpoint, one of the most important properties a soil possesses is shearing resistance or shear strength. A soil’s shearing resistance under given conditions is related to its ability to withstand loads. The shearing resistance is especially important in its relation to the supporting strength, or bearing capacity, of a soil used as a base or subgrade beneath a road, runway, or other structure. The shearing resistance is also important in determining the stability of the slopes used in a highway or airfield cut or embankment and in estimating the pressures exerted against an earth-retaining structure, such as a retaining wall.
LABORATORY TESTS
6-14. Three test procedures are commonly used in soil mechanics laboratories to determine the shear strength of a soil. These are—
6-15. The basic principles involved in each of these tests are illustrated in the simplified drawings of figure 6-1, page 6-4. Military soils analysts are not equipped or trained to perform the direct shear or triaxial compression tests. For most military applications, the CBR value of a soil is used as a measure of shear strength (see TM 5-530).
6-16. A variation of the unconfined compression test can be performed by military soils analysts, but the results are ordinarily used only in evaluation of soil stabilization. Shear strength for a soil is expressed as a combination of an apparent internal angle of friction (normally associated with cohesive soils).
CALIFORNIA BEARING RATIO
6-17. The CBR is a measure of the shearing resistance of a soil under carefully controlled conditions of density and moisture. The CBR is determined by a penetration shear test and is used with empirical curves for designing flexible pavements. Recommended design procedures for flexible pavements are presented in TM 5-330. The CBR test procedure for use in design consists of the following steps:
6-18. Although a standardized procedure has been established for the penetration portion of the test, one standard procedure for the preparation of test specimens cannot be established because soil conditions and construction methods vary widely. The soil test specimen is compacted so it duplicates as nearly as possible the soil conditions in the field.
6-19. In a desert environment, soil may be compacted and tested almost completely dry. In a wet area, soil should probably be tested at 100 percent saturation. Although penetration tests are most frequently performed on laboratory-compacted test specimens, they may also be performed on undisturbed soil samples or on in-place soil in the field. Detailed procedures for conducting CBR tests and analyzing the data are in TM 5-330. Appendix A describes the procedure for applying CBR test data in designing roads and airfields.
6-20. Column 15, table 5-3, page 5-13, shows typical ranges in value of the field CBR for soils in the USCS. Values of the field CBR may range from as low as 3 for highly plastic, inorganic clays (CH) and some organic clays and silts (OH) to as high as 80 for well-graded gravel and gravel-sand mixtures.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 6-3
Chapter 6
Figure 6-1. Laboratory shear tests
AIRFIELD INDEX (AI)
6-21. Engineering personnel use the airfield cone penetrometer to determine an index of soil strengths (called Airfield Index) for various military applications.
AIRFIELD CONE PENETROMETER
6-22. The airfield cone penetrometer is compact, sturdy, and simple enough to be used by military personnel inexperienced in soil strength determination. If used correctly, it can serve as an aid in maintaining field control during construction operations; however, this use is not recommended, because more accurate methods are available for use during construction.
Description
6-23. The airfield cone penetrometer is a probe-type instrument consisting of a right circular cone with a base diameter of ½ inch mounted on a graduated staff. On the opposite end of the staff are a spring, a load
6-4 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Concepts of Soil Engineering
indicator, and a handle. The overall length of the assembled penetrometer is 36 ⅛ inches. For ease in carrying, the penetrometer can be disassembled into three main pieces. They are—
6-25. The airfield cone penetrometer must not be confused with the trafficability penetrometer, a standard military item included in the Soil Test Set. The cone penetrometer used for trafficability has a dial-type load indicator (zero to 300 range) and is equipped with a cone ½ inch in diameter and a cross-sectional area of 0.2 square inch and another cone 0.8 inch in diameter and a cross-sectional area of 0.5 square inch. If the trafficability penetrometer is used to measure the AI, the readings obtained with the 0.2-square-inch cone must be divided by 20; the reading obtained with the 0.5-square-inch cone must be divided by 50.
Operation
6-26. Before the penetrometer is used, inspect the instrument to see that all joints are tight and that the load indicator reads zero. To operate the penetrometer, place your palms down symmetrically on the handle. Steady your arms against your thighs and apply force to the handle until a slow, steady, downward movement of the instrument occurs. Read the load indicator at the moment the base of the cone enters the ground (surface reading) and at desired depths at the moment the corresponding depth mark on the shaft reaches the soil surface. The reading is made by shifting the line of vision from the soil surface to the indicator just a moment before the desired depth is reached. Maximum efficiency is obtained with a two- person team in which one person operates and reads the instrument while the other acts as a recorder. One person can operate the instrument and record the measurements by stopping the penetration at any intermediate depth, recording previous readings, and then resuming penetration. Observe the following rules to obtain accurate data:
Maintenance
6-27. The airfield cone penetrometer is simply constructed of durable metals and needs little care other than cleaning and oiling. The calibration should be checked occasionally. If an error in excess of about 5 percent is noted, recalibrate the penetrometer.
SOIL-STRENGTH EVALUATION
6-28. The number of measurements to be made, the location of the measurements, and other such details vary with each area to be examined and with the time available. For this reason, hard and fast rules for evaluating an airfield are not practical, but the following instructions are useful:
Fine-Grained Soils
6-29. A reading near zero can occur in a very wet soil; it cannot support traffic. A reading approaching 15 occurs in dry, compact clays or silts and tightly packed sands or gravels. Most aircraft that might be required to use an unpaved area could easily be supported for a substantial number of landings and takeoffs on a soil having an AI of 15.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 6-5
Chapter 6
6-30. Soil conditions are extremely variable. As many penetrometer measurements should be taken as time and circumstances permit. The strength range and uniformity of an area controls the number of measurements necessary. Areas obviously too soft for emergency landing strips will be indicated after a few measurements, as will areas with strengths that are more than adequate. In all areas, the spots that appear to be softest should be tested first, since the softest condition of an area controls suitability. Soft spots are not always readily apparent. If the first test results indicate barely adequate strength, the entire area should be examined. Penetrations in areas that appear to be firm and uniform may be few and widely spaced, perhaps every 50 feet along the proposed centerline. In areas of doubtful strength, the penetrations should be more closely spaced, and areas on both sides of the centerline should be investigated. No fewer than three penetrations should be made at each location and usually five are desirable. If time permits, or if inconsistencies are apparent, as many as 10 penetrations should be made at each test location.
6-31. Soil strength usually increases with depth; but in some cases, a soil has a thin, hard crust over a deep, soft layer or has thin layers of hard and soft material. For this reason, each penetration should be made to a 24-inch depth unless prevented by a very firm condition at a lesser depth. When penetration cannot be made to the full 24-inch depth, a hole should be dug or augured through the firm materials, and penetrometer readings should be taken in the bottom of the hole to ensure that no soft layer underlies the firm layer. If possible, readings should be taken every 2 inches from the surface to a depth of 24 inches. Generally, the surface reading should be disregarded when figures are averaged to obtain a representative AI.
6-32. In the normal soil condition, where strength increases with depth, the readings at the 2- to 8-inch depths (4 to 10 inches for dry sands and for larger aircraft) should be used to designate the soil strength for airfield evaluation. If readings in this critical layer at any one test location do not differ more than 3 or 4 units, the arithmetic average of these readings can be taken as the AI for the areas represented by the readings. When the range between the highest and lowest readings is more than about 4, the engineer must use judgment in arriving at a rating figure. For conservatism, the engineer should lean toward the low readings.
6-33. In an area in which hard crust less than about 4 inches thick overlies a much softer soil, the readings in the crust should not be used in evaluating the airfield. For example, if a 3-inch-thick crust shows average readings of 10 at the 2-inch depth and average readings of 5 below 3 inches, the area should be evaluated at
5. If the crust is more than about 4 inches thick, it will probably play an important part in aircraft support. If the crust in the above instance is 5 inches thick, the rating of the field would then be about halfway between the 10 of the crust and the 5 of the underlying soil or, conservatively, 7. Innumerable combinations of crust thickness and strength and underlying soil strength can occur. Sound reasoning and engineering judgment should be used in evaluating such areas.
6-34. In an area in which a very soft, thin layer is underlain by a firmer layer, the evaluation also is a matter of judgment. If, for example, there are 1 to 2 inches of soil with an index averaging about 5 overlying a soil with an index of 10, rate the field at 10; but if this soft layer is more than about 4 inches thick, rate the field at 5. Areas of fine-grained soils with very low readings in the top 1 inch or more are likely to be slippery or sticky, especially if the soil is a clay.
Coarse-Grained Soils
6-35. When relatively dry, many sands show increasing AIs with depth, but the 2-inch depth index will often be low, perhaps about 3 or 4. Such sands usually are capable of supporting aircraft that require a much higher AI than 3 to 4, because the strength of the sand actually increases under the confining action of the aircraft tires. Generally, any dry sand or gravel is adequate for aircraft in the C-130 class, regardless of the penetrometer readings. All sands and gravel in a “quick” condition (water moving upward through them) must be avoided. Evaluation of wet sands should be based on the penetrometer readings obtained as described earlier.
6-36. Once the strength of the soil, in terms of AI, has been established by use of the airfield cone penetrometer, the load-carrying capability of this soil can be determined for each kind of forward, support, or rear-area airfield through use of the subgrade strength requirements curves. These curves are based on correlations of aircraft performance and AIs. Unfortunately, these are not exact correlations uniquely
6-6 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Concepts of Soil Engineering
relating aircraft performance to AI. As soils vary in type and condition from site to site, so varies the relation of AI to aircraft performance. For this reason, the curves may not accurately reflect performance in all cases. These relations were selected so that in nearly all cases aircraft performance will be equal to or better than that indicated.
CORRELATION BETWEEN CBR AND AI
6-37. Expedient soil strength measurements in this manual are treated in terms of AI. Measurement procedures using the airfield penetrometer are explained; however, in the references listed at the end of this manual, which cover less expedient construction methods, soil strength is treated in terms of CBR. To permit translation between the CBR and the AI, a correlation is presented in figure 6-2. This figure can be used for estimating CBR values from AI determinations. This correlation has been established to yield values of CBR that generally are conservative. The tendency toward conservatism is necessary because there is no unique relationship between these measurements over a wide range of soil types. It follows that the curve should not be used to estimate AI values from CBR determinations since these would not be conservative.
Figure 6-2. Correlation of CBR and AI
SECTION III – BEARING CAPACITY
IMPORTANCE
6-38. The bearing capacity of a soil is its ability to support loads that may be applied to it by an engineering structure, such as—
6-39. A soil with insufficient bearing capacity to support the loads applied to it may simply fail by shear, allowing the structure to move or sink into the ground. Such a soil may fail because it undergoes excessive deformation, with consequent damage to the structure. Sometimes the ability of a soil to support loads is simply called its stability. Bearing capacity is directly related to the allowable load that may be safely placed on a soil. This allowable load is sometimes called the allowable soil pressure.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 6-7
Chapter 6
6-40. Types of failure that may take place when the ultimate bearing capacity is exceeded are illustrated in figure 6-3. Such a failure may involve tipping of the structure, with a bulge at the ground surface on one side of the structure. Failure may also take place on a number of surfaces within the soil, usually accompanied by bulging of the ground around the foundation. The ultimate bearing capacity not only is a function of the nature and condition of the soil involved but also depends on the method of application of the load.
Figure 6-3. Typical failure surfaces beneath shallow foundations
FOUNDATIONS
6-41. The principle function of a foundation is to transmit the weight of a structure and the loads that it carries to the underlying soil or rock. A foundation must be designed to be safe against a shear failure in the underlying soil. This means that the load placed on the soil must not exceed its ultimate bearing capacity.
SHALLOW FOUNDATIONS
6-42. A shallow foundation is one that is located at, or slightly below, the surface of the ground. A typical foundation of this type is seen in the shallow footings, either of plain or reinforced concrete, which may support a building. Footings are generally square or rectangular. Long continuous or strip footings are also used, particularly beneath basement or retaining walls. Another type of shallow foundation is the raft or mat; it may cover a large area, perhaps the entire area occupied by a structure.
DEEP FOUNDATIONS
6-43. When the surface soils at the site of a proposed structure are too weak and compressible to provide adequate support, deep foundations are frequently used to transfer the load to underlying suitable soils. Two common types of deep foundations are—
Piles
6-44. Piles and pile foundations are very commonly used in both military and civil construction. By common usage, a pile is a load-bearing member made of timber, concrete, or steel, which is generally forced into the ground. Piles are used in a variety of forms and for a variety of purposes. A pile foundation
6-8 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Concepts of Soil Engineering
is one or more piles used to support a pier, or column, or a row of piles under a wall. Piles of this type are normally used to support vertical loads, although they may also be used to support inclined or lateral forces.
6-45. Piles driven vertically and used for the direct support of vertical loads are commonly called bearing piles. They may be used to transfer the load through a soft soil to an underlying firm stratum. These are called end-bearing piles. Bearing piles may also be used to distribute the load through relatively soft soils that are not capable of supporting concentrated surface loads. These are called friction piles. A bearing pile may sometimes receive its support from a combination of end bearing and friction. Bearing piles also may be used where a shallow foundation would likely be undetermined by scour, as in the case of bridge piers. Bearing piles are illustrated in figure 6-4, page 6-10.
6-46. A typical illustration of an end-bearing pile is when a pile driven through a very soft soil, such as a loose silt or the mud of a river bottom, comes to rest on firm stratum beneath. The firm stratum may, for example, be rock, sand, or gravel. In such cases, the pile derives practically all its support from the underlying firm stratum.
6-47. A friction pile develops its load-carrying capacity entirely, or principally, from skin friction along the sides of the pile. The load is transferred to the adjoining soil by friction between the pile and the surrounding soil. The load is thus transferred downward and laterally to the soil. The soil surrounding the pile or group of piles, as well as that beneath the points of the piles is stressed by the load.
6-48. Some piles carry a load by a combination of friction and end bearing. A pile of this sort may pass through a fairly soft soil that provides some frictional resistance; then it may pass into a firm layer that develops load-carrying capacity through a combination of friction over a relatively short length of embedment and end bearing.
6-49. Piles are used for many purposes other than support for vertical loads. Piles that are driven at an angle with the vertical are commonly called batter piles. They may be used to support inclined loads or to provide lateral loads. Piles are sometimes used to support lateral loads directly, as in the pile fenders that may be provided along waterfront structures to take the wear and shock of docking ships. Sometimes piles are used to resist upward, tensile forces. These are frequently called anchor piles. Anchor piles may be used, for example, as anchors for bulkheads, retaining walls, or guy wires. Vertical piles are sometimes driven for the purpose of compacting loose cohesionless deposits. Closely spaced piles, or sheet piles, may be driven to form a wall or a bulkhead that restrains a soil mass.
Piers
6-50. Piers are much less common than piles and are normally used only for the support of very heavy loads.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 6-9
Chapter 6
Figure 6-4. Bearing piles
SECTION IV – EARTH-RETAINING STRUCTURES
PURPOSE
6-51. Earth-retaining structures must be used to restrain a mass of earth that will not stand unsupported. Such structures are commonly required when a cut is made or when an embankment is formed with slopes too steep to stand alone.
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Concepts of Soil Engineering
6-52. Earth-retaining structures are subjected to lateral thrust from the earth masses that they support. The pressure of the earth on such a structure is commonly called lateral earth pressure. The lateral earth pressure that may be exerted by a given soil on a given structure is a function of many variables. It must be estimated with a reasonable degree of accuracy before an earth-retaining structure may be properly designed. In many cases, the lateral earth pressure may be assumed to be acting in a horizontal direction, or nearly so.
TYPES
6-53. Earth-retaining structures discussed in this section are retaining walls and bracing systems used in temporary excavations.
RETAINING WALLS
6-54. A retaining wall is a wall constructed to support a vertical or nearly vertical earth bank that, in turn, may support vertical loads. Generally, retaining walls are classified into the following five types (see figure 6-5, page 6-12):
6-55. When a retaining wall is used to support the end of a bridge span, as well as retain the earth backfill, it is called an abutment. There are several types of gravity retaining walls, such as—
6-56. Retaining walls are used in many applications. For example, a structure of this sort may be used in a highway or railroad cut to permit the use of a steep slope and avoid excessive amounts of excavation. Retaining walls are similarly used on the embankment side of sidehill sections to avoid excessive volumes of fill. Bridge abutments and the headwalls of culverts frequently function as retaining walls. In the construction of buildings and various industrial structures, retaining walls are often used to provide support for the side of deep, permanent excavations.
6-57. Permanent retaining walls are generally constructed from plain or reinforced concrete; stone masonry walls are also used occasionally. In military construction, timber crib retaining walls are important. Their design is discussed later.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 6-11
Chapter 6
Figure 6-5. Principal types of retaining walls
Backfills
6-58. The design of the backfill for a retaining wall is as important as the design of the wall itself. The backfill must be materials that are—
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Concepts of Soil Engineering
6-59. The best materials for backfills behind retaining walls are clean sands, gravels, and crushed rock. In the USCS, the (GW) and (SW) soils are preferred, if they are available. The (GP) and (SP) soils are also satisfactory. These granular materials require compaction to make them stable against the effects of vibration. Compaction also generally increases the angle of internal friction, which is desirable in that it decreases the lateral pressure exerted on the wall. Materials of the (GM), (GC), (SM), and (SC) groups may be used for backfills behind retaining walls, but they must be protected against frost action and may require elaborate drainage provisions. Fine-grained soils are not desirable as backfills because they are difficult to drain. If clay soil must be used, the wall should be designed to resist earth pressures at rest. Ideal backfill materials are purely granular soils containing < 5 percent of fines.
6-60. Backfills behind retaining walls are commonly put in place after the structure has been built. The method of compaction depends on the—
6-61. Since most backfills are essentially cohesionless, they are best compacted by vibration. Equipment suitable for use with these soils is discussed in chapter 8. Common practice calls for the backfill to be placed in layers of loose material that, when compacted, results in a compacted layer thickness of from 6 to 8 inches. Each layer is compacted to a satisfactory density. In areas inaccessible to rollers or similar compacting equipment, compaction may be done by the use of mechanical air tampers or hand tools.
Drainage
6-62. Drainage of the backfill is essential to keep the wall from being subjected to water pressure and to prevent frost action. Common drainage provisions used on concrete walls are shown in figure 6-6, page 6-14.
6-63. When the backfill is composed of clean, easily drained materials, it is customary to provide for drainage by making weep holes through the wall. Weep holes are commonly made by embedding pipes 4 to
6 inches in diameter into the wall. These holes are spaced from 5 to 10 feet center to center both horizontally and vertically. A filter of granular material should be provided around the entrance to each weep hole to prevent the soil from washing out or the drain from becoming clogged. If possible, this material should conform to the requirements previously given for filter materials.
6-64. Weep holes have the disadvantage of discharging the water that seeps through the backfill at the toe of the wall where the soil pressures are greatest. The water may weaken the soil at this point and cause the wall to fail. A more effective solution, which is also more expensive, is to provide a longitudinal back drain along the base of the wall (see figure 6-6). A regular pipe drain should be used, surrounded with a suitable filter material. The drainage may be discharged away from the ends of the wall.
6-65. If a granular soil, which contains considerable fine material and is poorly drained (such as an (SC) soil) is used in the backfill, then more elaborate provisions may be installed to ensure drainage. One such approach is to use a drainage blanket (see figure 6-6). If necessary, a blanket of impervious soil or bituminous material may be used on top of the backfill to prevent water from entering the fill from the top. Such treatments are relatively expensive.
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Chapter 6
Figure 6-6. Common types of retaining wall drainage
Frost Action
6-66. Conditions for detrimental frost action on retaining walls include the following:
6-67. If these conditions are present in the backfill, steps must be taken to prevent the formation of ice lenses and the resultant severe lateral pressures that may be exerted against the wall. The usual way to prevent frost action is to substitute a thick layer of clean, granular, nonfrost-susceptible soil for the backfill material immediately adjacent to the wall. The width of the layer should be as great as the maximum depth of frost penetration in the area (see figure 6-7). As with other structures, the bottom of a retaining wall should be located beneath the line of frost penetration.
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Concepts of Soil Engineering
Figure 6-7. Eliminating frost action behind retaining walls
Timber Crib
6-68. A very useful type of retaining wall for military purposes in a theater of operations is timber cribbing. The crib or cells are filled with earth, preferably clean, coarse, granular material. A wall of this sort gains its stability through the weight of the material used to fill the cells, along with the weight of the crib units themselves. The longitudinal member in a timber crib is called a stretcher, while a transverse member is a header.
6-69. A principal advantage of a timber crib retaining wall is that it may be constructed with unskilled labor and a minimum of equipment. Suitable timber is available in many military situations. Little foundation excavation is usually required and may be limited to shallow trenching for the lower part of the crib walls. The crib may be built in short sections, one or two cribs at a time. Where the amount of excavation is sufficient and suitable, the excavated soil may be used for filling the cells.
6-70. A crib of this sort may be used on foundation soils that are weak and might not be able to support a heavy wall, since the crib is fairly flexible and able to undergo some settlement and shifting without distress. However, this should not be misunderstood, as the foundation soil must not be so soft as to permit excessive differential settlement that would destroy the alignment of the crib.
6-71. Experience indicates that a satisfactory design will generally be achieved if the base width is a minimum of 4 feet at the top and bottom or 50 percent of the height of the wall, provided that the wall does not carry a surcharge and is on a reasonably firm foundation. If the wall carries a heavy surcharge, the base width should be increased to a minimum of 65 percent of the height. In any case, the width of the crib at the top and bottom should not be less than 4 feet.
6-72. Timber crib walls may be built with any desired batter (receding upward slope) or even vertical. The batter most often used and recommended is one horizontal to four vertical (see figure 6-8, page 6-16). If less batter is used, the base width must be increased to ensure that the resultant pressure falls within the middle third of the base. The desired batter is normally achieved by placing the base on a slope equal to the batter. The toe may be placed on sills; this is frequently done with high walls. Sometimes double-cell construction is used to obtain the necessary base width of high walls. The wall is then decreased in width, or “stepped-back,” in the upper portions of the wall, above one third height. Additional rows of bottom stretchers may be used to decrease the pressure on the soil or to avoid detrimental settlement.
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Chapter 6
Figure 6-8. Typical timber crib retaining wall
6-73. The front and rear wall of the crib should be connected at each panel point. The crib must be kept an essentially flexible structure and must be free to move somewhat in any direction, so as to adjust itself to thrusts and settlements.
6-74. The material used in filling the cells should be placed in thin layers and should be well compacted. Backfill behind the wall should also be compacted and kept close to, but not above, the level of the material in the cribs. Drainage behind timber crib walls may or may not be required, depending on local conditions and wall construction.
6-75. Figure 6-8 shows the elevation and cross section of a timber crib retaining wall, which may be used to a maximum height of about 16 feet. A similar arrangement may be used for heights up to about 8 feet, with a minimum width of 4 feet. For heights above 16 feet, the headers are usually 6-inch by 12-inch timbers and the stretchers 12-inch by 12-inch timbers. Timbers are normally connected by means of heavy (¾-inch diameter) driftpins.
Other Timber Walls
6-76. Other types of timber retaining walls are used for low heights, particularly in connection with culverts and bridges. A wall of this sort may be built by driving timber posts into the ground and attaching planks or logs. Details on retaining walls, used in conjunction with bridge abutments, are given in TM 5-312. Figure 6-9 illustrates two other types of timber retaining walls.
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Concepts of Soil Engineering
Figure 6-9. Other timber retaining walls
Gabions
6-77. Gabions are large, steel-wire-mesh baskets usually rectangular in shape and variable in size (see figure 6-10, page 6-18). They are designed to solve the problem of erosion at a low cost. Gabions were used in sixteenth century fortifications, and experience in construction with factory-produced prefabricated gabions dates back to 1894 in Italy. Gabions have been widely used in Europe and are now becoming accepted in the United States as a valuable and practical construction tool. They can be used in place of sheet piling, masonry construction, or cribbing. Gabions may be used as—
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Chapter 6
Figure 6-10. Typical gabion
6-78. The best use of gabions as retaining walls is where flexibility and permeability are important considerations, especially where unstable ground and drainage conditions impose problems difficult to solve with rigid and impervious material. Use of gabions does require ready access to large-size stones, such as those found in mountainous areas. Areas that are prone to landslides have used gabions successfully. Gabion walls have been erected in mountainous country to trap falling rocks and debris and in some areas to act as longitudinal drainage collectors.
6-79. The best filling material for a gabion is one that allows flexibility in the structure but also fills the gabion compartments with the minimum of voids and with the maximum weight. Ideally, the stone should be small, just slightly larger than the size of the mesh. The stone must be clean, hard, and durable to withstand abrasion and resistance to weathering and frost action. The gabions are filled in three lifts, one foot at a time. Rounded stone, if available, reduces the possibility of damage to the wire during mechanical filling as compared with sharp quarry stone. If stone is not available, gabions can be filled with a good quality soil. To hold soil, hardware cloth inserts must be placed inside the gabions. For use in gabions, backfill material should meet the following Federal Highway Administration criteria:
EXCAVATION BRACING SYSTEMS
6-80. Bracing systems may be required to protect the sides of temporary excavations during construction operations. Such temporary excavations may be required for several purposes but are most often needed in connection with the construction of foundations for structures and the placing of utility lines, such as sewer and water pipes.
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Concepts of Soil Engineering
Shallow Excavations
6-81. The term “shallow excavation” refers to excavations made to depths of 12 to 20 feet below the surface, depending principally on the soil involved. The lower limit applies to fairly soft clay soils, while the upper limit generally applies to sands and sandy soils.
6-82. Shallow excavations may be made as open cuts with unsupported slopes, particularly when the excavation is being done above the water table. Chapter 10 gives recommendations previously given relative to safe slopes in cuts that are applicable here if the excavation is to remain open for any length of time. If the excavation is purely temporary in nature, most sandy soils above the water table will stand at somewhat steeper slopes (as much as ½ to 1 for brief periods), although some small slides may take place. Clays may be excavated to shallow depths with vertical slopes and will remain stable briefly. Generally, bracing cuts in clay that extend to depths of 5 feet or more below the surface are safer unless flat slopes are used.
6-83. Even for relatively shallow excavations, using unsupported cuts may be unsatisfactory for several reasons. Cohesive soils may stand on steep slopes temporarily, but bracing is frequently needed to protect against a sudden cave-in. Required side slopes, particularly in loose, granular soils, may be so flat as to require an excessive amount of excavation. If the excavation is being done close to other structures, space may be limited, or the consequences of the failure of a side slope may be very serious. Considerable subsidence of the adjacent ground may take place, even though the slope does not actually fail. Finally, if the work is being done below the water table, the excavation may have to be surrounded with a temporary structure that permits the excavation to be unwatered.
Narrow Shallow Excavations
6-84. Several different schemes may be used to brace the sides of a narrow shallow excavation. Two of these schemes are shown in figure 6-11, page 6-20).
6-85. In the first scheme, timber planks are driven around the boundary of the excavation to form what is called vertical sheeting. The bottom of the sheeting is kept at or near the bottom of the pit or trench as excavation proceeds. The sheeting is held in place by means of horizontal beams called wales. These wales are usually supported against each other by means of horizontal members called struts, which extend from one side of the excavation to the other. The struts may be cut slightly long, driven into place, and held by nails or cleats. They may also be held in position by wedges or shims. Hydraulic or screw-type jacks can be used as struts.
6-86. The second scheme uses horizontal timber planks to form what is called horizontal lagging. The lagging, in turn, is supported by vertical solid beams and struts. If the excavation is quite wide, struts may have to be braced horizontally or vertically or both.
6-87. Bracing systems for shallow excavations are commonly designed on the basis of experience. Systems of this sort represent cases of incomplete deformation, since the bracing system prevents deformation at some points while permitting some deformation at others.
6-88. Members used in bracing systems should be strong and stiff. In ordinary work, struts vary from 4-inch to 6-inch timbers for narrow cuts up to 8-inch by 8-inch timbers for excavations 10 or 12 feet wide. Heavier timbers are used if additional safety is desired. Struts are commonly spaced about 8 feet horizontally and from 5 to 6 feet vertically. Lagging or sheeting is customarily made from planks from 6 to 12 inches wide, with the minimum thickness usually being about 2 inches.
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Chapter 6
Figure 6-11. Bracing a narrow shallow excavation
Wide Shallow Excavations
6-89. If the excavation is too wide to be cross braced by the use of struts, vertical sheeting may be used (see figure 6-12). The wales are supported by inclined braces, which are sometimes called rakes. The rakes, in turn, react against kicker blocks that are embedded in the soil. As the excavation is deepened, additional wales and braces may be added as necessary to hold the sheeting firmly in position. The success of this system depends on the soil in the bottom of the excavation being firm enough to provide adequate support for the blocks.
Figure 6-12. Bracing a wide shallow excavation
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Chapter 7
Movement of Water Through Soils
The movement of water into or through a soil mass is a phenomenon of great practical importance in engineering design and construction. It is probably the largest single factor causing soil failures. For example, water may be drawn by capillarity from a free water surface or infiltrate through surface cracks into the subgrade beneath a road or runway. Water then accumulated may greatly reduce the bearing capacity of subgrade soil, allowing the pavement to fail under wheel loads if precautions have not been taken in design. Seepage flow may be responsible for the erosion or failure of an open cut slope or the failure of an earth embankment. This chapter concerns the movement of water into and through soils (and, to some extent, about the practical measures undertaken to control this movement) and the problems associated with frost action.
SECTION I – WATER
7-1. Knowledge of the earth’s topography and characteristics of geologic formations help the engineer find and evaluate water sources. Water in the soil may be from a surface or a subsurface source. Surface water sources (streams, lakes, springs) are easy to find. Finding subsurface water sources could require extensive searching. Applying geologic principles can help eliminate areas where no large groundwater supplies are present and can indicate where to concentrate a search.
HYDROLOGIC CYCLE
7-2. Water covers 75 percent of the earth’s surface. This water represents vast storage reservoirs that hold most of the earth’s water. Direct radiation from the sun causes water at the surface of rivers, lakes, oceans, and other bodies to change from a liquid to a vapor. This process is called evaporation. Water vapor rises in the atmosphere and accumulates in clouds. When enough moisture accumulates in the clouds and the conditions are right, water is released as precipitation (rain, sleet, hail, snow). Some precipitation occurs over land surfaces and represents the early stage of the land hydrologic cycle. Precipitation that falls on land surfaces is stored on the surface, flows along the surface, or flows into the ground as infiltration. Infiltration is a major source of groundwater and is often referred to as recharge, because it replenishes or recharges groundwater resources.
7-3. The hydrologic cycle (figure 7a, page 7-2) consists of several processes. It does not usually progress through a regular sequence and can be interrupted or bypassed at any point. For example, rain might fall in an area of thick vegetation, and a certain amount of this moisture will remain on the plants and not reach the ground. The moisture could return to the atmosphere by direct evaporation, thereby causing a break in the hydrologic cycle. For the military engineer, the two most important states of this cycle are those pertaining to surface runoff and water infiltration.
SURFACE WATER
7-4. Streams and lakes are the most available and most commonly used sources for military water supplies. However, other subsurface water sources should be considered because long-term droughts could occur. Water naturally enters through soil surfaces unless they are sealed; and sealed surfaces may have cracks, joints, or fissures that allow water penetration. Surface water may also enter from the sides of construction projects, such as roads or airfields.
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Chapter 7
Figure 7a. Hydrologic cycle
7-5. Surface water is not chemically pure. It may contain sediment, bacteria, or dissolved salts that make the water unfit for consumption. Natural contamination and the pollution that man causes are also dangerous and occur in surface water. Test all surface water for purity and take proper precautions before using it.
STREAMS
7-6. Streams normally supply an abundant quantity of water for the initial phase of a field operation. The water supply needs to be adequate for the time of year the operation is planned. For a long-term supply, you must learn the permanent status of the stream flow.
LAKES
7-7. Most lakes are excellent sources of water. They serve as natural reservoirs for storing large amounts of water. Lakes are usually more constant in quality than the streams that feed them. Large lakes are preferable because the water is usually purer. Shallow lakes and small ponds are more likely to be polluted or contaminated. Lakes located in humid regions are generally fresh and permanent. Lakes in desert regions are rare but can occur in basins between mountains. These lakes could have a high percentage of dissolved salts and should not be considered as a permanent source of water.
SWAMPS
7-8. Swamps are likely to occur where wide, flat, poorly drained land and an abundant supply of water exist. A large quantity of water is usually available in swamps; but it may be poor in quality, brackish, or salty.
SPRINGS AND SEEPS
7-9. Water that naturally emerges at the surface is called a spring if there is a distinctive current, and a
seep if there is no current. Most springs and seeps consist of water that has slowly gravitated from nearby
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Movement of Water Through Soils
higher ground. The water’s underground course depends on the permeability and structure of the material through which it moves. Any spring that has a temperature higher than the yearly average temperature of a given region is termed a thermal spring and indicates a source of heat other than the surface climate.
GRAVITY SPRINGS AND SEEPS
7-10. In gravity springs and seeps, subsurface water flows by gravity, not by hydrostatic pressure, from a high point of intake to a lower point of issue. Water-table springs and seeps are normally found around the margin of depressions, along the slope of valleys, and at the foot of alluvial fans. Contact springs appear along slopes, and may be found at almost any elevation depending on the position of the rock formations.
ARTESIAN SPRINGS
7-11. When water is confined in a rock layer under hydrostatic pressure, an artesian condition is said to exist. A well drilled into an aquifer where this condition is present, is called an artesian well (figure 7b). If the water rises to the surface, it is called an artesian spring. Certain situations are necessary for an artesian condition to exist—
l There must be a permeable aquifer that has impervious layers above and below it to confine the water.
l There must be an intake area where water can enter the aquifer.
l A structural dip must exist so that hydrostatic pressure is produced in the water at the lower areas of the aquifer.
Figure 7b. Artesian groundwater
GROUNDWATER
7-12. Groundwater or subsurface water is any water that exists below the earth’s surface. Groundwater is located in two principle zones in the earth’s surface—aeration and saturation (figure 7c, page 7-4).
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Chapter 7
Figure 7c. Groundwater zones
7-13. The zone of aeration consists of three major belts—soil moisture, intermediate, and capillary fringe. As water starts infiltrating the ground surface, it encounters a layer of organic matter. The root systems of plants, decaying organic material, and the small pores found within the upper soil zone hold some of the water in suspension. This shallow layer is called the soil moisture belt. The water passes through this belt and continues downward through the intermediate belt. The pore spaces in this belt are generally larger than those in the soil moisture belt, and the amount of organic material is considerably reduced. The intermediate belt contains voids so it does not hold water, and the water gradually drains downward. The next belt is called the capillary fringe. Most deep-rooted plants sink roots into this area.
7-14. The water table is the contact between the zone of aeration and the zone of saturation. It fluctuates up and down, depending on the recharge rate and the rate of flow away from the area. The pores are filled with water in the zone of saturation.
FREE,ORGRAVITATIONAL, WATER
7-15. Water that percolates down from the surface eventually reaches a depth where there is some medium that restricts (to varying degrees) the further percolation of the moisture. This medium may be bedrock or a layer of soil, not wholly solid but with such small void spaces that the water which leaves this zone is not as great as the volume or supply of water added. In time, the accumulating water completely saturates the soil above the restricting medium and fills all voids with water. When this zone of saturation is under no pressure except from the atmosphere, the water it contains is called free, or gravitational, water. It will flow through the soil and be resisted only by the friction between the soil grains and the free water. This movement of free water through a soil mass frequently is termed see page. The upper limit of the saturated zone of free water is called the groundwater table, which varies with climatic conditions. During a wet winter, the groundwater table rises. However, a dry summer might remove the source of further accumulation of water. This results in a decreased height of the saturated zone, for the free water then flows downward, through, or along its restricting layer. The presence of impervious soil layers may result in an area of saturated soil above the normal groundwater table. This is called a “perched” water table.
HYGROSCOPIC MOISTURE
7-16. When wet soil is air-dried, moisture is removed by evaporation until the hygroscopic moisture in the soil is in equilibrium with the moisture vapor in the air. The amount of moisture in air-dried soil, expressed as a percentage of the weight of the dry soil, is called the hygroscopic moisture content. Hygroscopic moisture films may be driven off from air-dried soil by heating the material in an oven at 100 to 110
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Movement of Water Through Soils
degrees Centigrade (C) (210 to 230 degrees Fahrenheit (F)) for 24 hours or until constant weight is attained.
CAPILLARY MOISTURE
7-17. Another source of moisture in soils results from what might be termed the capillary potential of a soil. Dry soil grains attract moisture in a manner similar to the way clean glass does. Outward evidence of this attraction of water and glass is seen by observing the meniscus (curved upper surface of a water column). Where the meniscus is more confined (for example, as in a small glass tube), it will support a column of water to a considerable height. The diagram in figure 7-1 shows that the more the meniscus is confined, the greater the height of the capillary rise.
7-18. Capillary action in a soil results in the “capillary fringe” immediately above the groundwater table. The height of the capillary rise depends on numerous factors. One factor worth mentioning is the type of soil. Since the pore openings in a soil vary with the grain size, a fine-grained soil develops a higher capillary fringe area than a coarse-grained soil. This is because the fine-grained soil can act as many very small glass tubes, each having a greatly confined meniscus. In clays, capillary water rises sometimes as high as 30 feet, and in silts the rise is often as high as 10 feet. Capillary rise may vary from practically zero to a few inches in coarse sands and gravels.
Figure 7-1. Capillary rise of moisture
7-19. When the capillary fringe extends to the natural ground surface, winds and high temperatures help carry this moisture away and reduce its effects on the soil. Once a pavement of watertight surface is applied, however, the effect of the wind and temperature is reduced. This explains the accumulation of moisture often found directly beneath an impervious pavement.
7-20. Capillary moisture in soils located above the water table may be visualized as occurring in the following three zones:
7-21. In the zone of capillary saturation, the soil is essentially saturated. The height of this zone depends not only on the soil but also on the history of the water table, since the height will be greater if the soil mass has been saturated previously.
7-22. The height of the zone of partial capillary saturation is likely to be considered greater than that of the zone of capillary saturation; it also depends on the water-table history. Its existence is the result of a few large voids serving effectively to stop capillary rise in some parts of the soil mass. Capillary water in this zone is still interconnected or “continuous,” while the air voids may not be.
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Chapter 7
7-23. Above the zones of capillary and partial capillary saturation, water that percolates downward from the surface may be held in the soil by surface tension. It may fill the smaller voids or be present in the form of water films between the points of contact of the soil grains. Water may also be brought into this zone from the water table by evaporation and condensation. This moisture is termed “contact moisture.”
7-24. One effect of contact moisture is apparent cohesion. An example of this is the behavior of sand on certain beaches. On these beaches, the dry sand located back from the edge of the water and above the height of capillary rise is generally dry and very loose and has little supporting power when unconfined. Closer to the water’s edge, and particularly during periods of low tide, the sand is very firm and capable of supporting stationary or moving automobiles and other vehicles. This apparent strength is due primarily to the existence of contact moisture left in the voids of the soil when the tide went out. The surface soil may be within the zone of partial or complete capillary saturation very close to the edge of the water. Somewhat similarly, capillary forces may be used to consolidate loose cohesionless deposits of very fine sands or silts in which the water table is at or near the ground surface. This consolidation is accomplished by lowering the water table by means of drains or well points. If the operation is properly carried out within the limits of the height of capillary rise, the soil above the lowered water table remains saturated by capillary moisture. The effect is to place the soil structure under capillary forces (such as tension in the water) that compress it. The soil may be compressed as effectively as though an equivalent external load had been placed on the surface of the soil mass.
7-25. Methods commonly used to control the detrimental effects of capillarity, particularly concerning roads and airport pavements, are mentioned briefly here. Additional attention is given to this subject in section II, which is devoted to the closely allied subject of frost action.
7-26. As has been noted, if the water table is closer to the surface than the height of capillary rise, water will be brought up to the surface to replace water removed by evaporation. If evaporation is wholly or partially prevented, as by the construction of impervious pavement, water accumulates and may cause a reduction in shearing strength or cause swelling of the soil. This is true particularly when a fine-grained soil or a coarse soil that contains a detrimental amount of plastic fines is involved.
7-27. One obvious solution is to excavate the material that is subject to capillary action and replace it with a granular material. This is frequently quite expensive and usually may be justified only in areas where frost action is a factor.
7-28. Another approach is to include in the pavement structure a layer that is unaffected by capillary action. This is one of the functions of the base that is invariably used in flexible pavements. The base serves to interrupt the flow of capillary moisture, in addition to its other functions. Under certain circumstances, the base itself may have to be drained to ensure the removal of capillary water (see figure 7-2). This also is usually not justified unless other circumstances, such as frost action, are of importance.
7-29. Still another approach is to lower the water table, which may sometimes be accomplished by the use of side ditches. Subdrains may be installed for the same purpose (see figure 7-3, page 7-8). This approach is particularly effective in relatively pervious or free-draining soils. Some difficulty may be experienced in lowering the water table by this method in flat country because finding outlets for the drains is difficult. An alternative, used in many areas where the permanent water table is at or near the ground surface, is simply to build the highway or runway on a fill. Material that is not subject to detrimental capillarity is used to form a shallow fill. The bottom of the base is normally kept a minimum of 3 or 4 feet above the natural ground surface, depending on the soil used in the fill and other factors. A layer of sand, known as a sand blanket, or a geotextile fabric may be used to intercept capillary moisture, preventing its intrusion into the base course.
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Figure 7-2. Base drains in an airfield pavement
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Chapter 7
Figure 7-3. Typical subgrade drainage installation
LOCATING GROUNDWATER SOURCES
7-30. Consider exploring rock aquifers only when soil aquifers are not present or when the soil aquifer cannot provide the required water supply. Identifying suitable well sites in rock aquifers is much more difficult than in soil aquifers. Also, water development is usually more time-consuming and costly and has a higher risk of failure. However, in some areas, rock aquifers are the only potential source of groundwater.
Hydrogeologic Indicators
7-31. Indicators that help identify groundwater sources are referred to as hydrogeologic indicators (table 7a). They are divided into three major groups—reservoir, groundwater, and boundary. Groundwater indicators are those conditions or characteristics that directly or indirectly indicate groundwater occurrence. No indicator is 100 percent reliable, but the presence or absence of certain indicators or associations of indicators is fairly reliable.
Table 7a. Hydrogeologic indicators for groundwater exploration
|
Reservoir Indicators |
Groundwater Indicators |
Boundary Indicators |
|
Rock type/geometry |
Springs and seeps |
Location of recharge areas |
|
Stratigraphic sequence |
Soil moisture |
Location of discharge areas |
|
Degree of lithification |
Vegetation type |
Impermeable barriers |
|
Grain size |
Vegetation density |
Semipermeable barriers |
|
Fracture density |
Wetlands |
Surface-water divides |
|
Dissolution potential |
Playas |
|
|
Cumulative structure density |
Wells |
|
|
Drainage basin size |
Reservoirs |
|
|
Drainage basin elevation and relief |
Crop irrigation |
|
|
Drainage pattern |
Salt encrustations |
|
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Movement of Water Through Soils
Table 7a. Hydrogeologic indicators for groundwater exploration
|
Reservoir Indicators |
Groundwater Indicators |
Boundary Indicators |
|
Drainage density |
Population distribution |
|
|
Landforms |
Streams/rivers |
|
|
|
Snow-melt patterns |
|
|
|
Karst topography |
|
Geologic Indicators
7-32. The type of rock or soil present is an important indicator because it usually defines the types of aquifers present and their water-producing characteristics. For field reconnaissance, the engineer need only recognize igneous, metamorphic, and sedimentary rocks and alluvium soil.
PERMEABILITY
7-33. Permeability is the property of soil that permits water to flow through it. Water may move through the continuous voids of a soil in much the same way as it moves through pipes and other conduits. As has been indicated, this movement of water through soils is frequently termed seepage and may also be called percolation. Soils vary greatly in their resistance to the flow of water through them. Relatively coarse soils, such as clean sands and gravels, offer comparatively little resistance to the flow of water; these are said to be permeable or pervious soils. Fine-grained soils, particularly clays, offer great resistance to the movement of water through them and are said to be relatively impermeable or impervious. Some water does move through these soils, however. The permeability of a soil reflects the ease with which it can be drained; therefore, soils are sometimes classed as well-drained, poorly drained, or impervious. Permeability is closely related to frost action and to the settlement of soils under load.
7-34. The term k is called the coefficient of permeability. It has units of velocity and may be regarded as the discharge velocity under a unit hydraulic gradient. The coefficient of permeability depends on the properties of the fluid involved and on the soil. Since water is the fluid normally involved in soil problems, and since its properties do not vary enough to affect most practical problems, the coefficient of permeability is regarded as a property of the soil. Principal factors that determine the coefficient of permeability for a given soil include—
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Chapter 7
7-35. The relationships among these different variables for typical soils are quite complex and preclude the development of formulas for the coefficient of permeability, except for the simplest cases. For the usual soil, k is determined experimentally, either in the laboratory or in the field. These methods are discussed briefly in the next paragraph. Typical values of the coefficient of permeability for the soil groups of the USCS are given in column 8 of table 5-4, page 5-14.
DRAINAGE CHARACTERISTICS
7-36. The general drainage characteristics of soils classified under the USCS are given in column 12 of table 5-3, page 5-12. Soils may be divided into three general groups on the basis of their drainage characteristics. They are—
WELL-DRAINED SOILS
7-37. Clean sands and gravels, such as those included in the (GW), (GP), (SW), or (SP) groups, fall into the classification of well-drained soils. These soils may be drained readily by gravity systems. In road and airfield construction, for example, open ditches may be used in these soils to intercept and carry away water that comes in from surrounding areas. This approach is very effective when used in combination with the sealing of the surface to reduce infiltration into the base or subgrade. In general, if the groundwater table around the site of a construction project is controlled in these soils, then it will be controlled under the site also.
POORLY DRAINED SOILS
7-38. Poorly drained soils include inorganic and organic fine sands and silts, organic clays of low compressibility, and coarse-grained soils that contain an excess of nonplastic fines. Soils in the (ML), (OL), (MH), (GM), (GC), (SC), and (SM) groups, and many from the (Pt) group, generally fall into this category. Drainage by gravity alone is likely to be quite difficult for these soils.
IMPERVIOUS SOILS
7-39. Fine-grained, homogeneous, plastic soils and coarse-grained soils that contain plastic fines are considered impervious soils. This normally includes (CL) and (CH) soils and some in the (OH) groups. Subsurface drainage is so slow on these items that it is of little value in improving their condition. Any drainage process is apt to be difficult and expensive.
FILTER DESIGN
7-40. The selection of the proper filter material is of great importance since it largely determines the success or failure of the drainage system. A layer of filter material approximately 6 inches deep should be placed around all subsurface piping systems. The improper selection of a filter material can cause the drainage system to become inoperative in one of three ways:
7-41. To prevent these failures from occurring, criteria have been developed based on the soil’s gradation curve (see chapter 5).
7-42. To prevent the clogging of a pipe by filter material moving through the perforations or openings, the following limiting requirements must be satisfied (see Engineer Manual (EM) 1110-2-1901):
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Movement of Water Through Soils
7-43. To prevent the movement of particles from the protected soil into or through the filters, the following conditions must be satisfied:
fx
7-44. To permit free water to reach the pipe, the filter material must be many times more pervious than the protected soil. This condition is fulfilled when the following requirement is met:
15 𝑝𝑒𝑟𝑐𝑒𝑛𝑡 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑓𝑖𝑙𝑡𝑒𝑟 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 > 5
15 𝑝𝑒𝑟𝑐𝑒𝑛𝑡 𝑠��𝑧𝑒 𝑜𝑓 𝑝𝑟𝑜𝑡𝑒𝑐𝑡𝑒𝑑 𝑠𝑜𝑖𝑙
7-45. If it is not possible to obtain a mechanical analysis of available filter materials and protected soils, concrete sand with mechanical analysis limits as shown in figure 7-4, page 7-12, may be used. Experience indicates that a well-graded concrete sand is satisfactory as a filter material in most sandy, silty soils.
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Chapter 7
Figure 7-4. Mechanical analysis curves for filter material
POROSITY AND PERMEABILITY OF ROCKS
7-46. Porosity and permeability determine the water-bearing capability of a natural material.
POROSITY
7-47. The amount of water that rocks can contain depends on the open spaces in the rock. Porosity is the percentage of the total volume of the rock that is occupied by voids. Rock types vary greatly in size, number, and arrangement of their pore spaces and, consequently, in their ability to contain and yield water. The following list explains the porosity values of the types of rock displayed in figure 7-4a.
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Movement of Water Through Soils
Figure 7-4a. Porosity in rocks
PERMEABILITY
7-48. The permeability of rock is its capacity for transmitting a fluid. The amount of permeability depends on the degree of porosity, the size and the shape of interconnections between pores, and the extent of the pore system.
WATER TABLE
7-49. In most regions, the depth that rocks are saturated with water depends largely on the permeability of the rocks, the amount of precipitation, and the topography. In permeable rocks, the surface below where the rocks are saturated is called the water table (figure 7b, page 7-3). The water table is—
PERCHED WATER TABLE
7-50. If impermeable layers are present, descending water stops at their upper surfaces. If a water table lies well below the surface, a mass of impermeable rock may intercept the descending water and hold it suspended above the normal saturated zone. This isolated, saturated zone then has its own water table. Wells drilled into this zone are poor quality, because the well could quickly be drained of its water supply.
AQUIFER
7-51. An aquifer is a layer of rock below the water table. It is also called a water-bearing formation or a water-bearing stratum. Aquifers can be found in almost any area; however, they are difficult to locate in areas that do not have sedimentary rocks. Sands and sandstones usually constitute the best aquifers, but any rock with porosity and permeability can serve as a good water aquifer.
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Chapter 7
SALTWATER INTRUSION
7-52. There is always a danger of saltwater intrusion into groundwater sources along coastal areas and on islands. Because saltwater is unfit for most human needs, contamination can cause serious problems. When saltwater intrusion is discovered in the groundwater supply, determine the cause and mitigate it as soon as possible.
SECTION II – FROST ACTION
PROBLEMS
7-53. Frost action refers to any process that affects the ability of the soil to support a structure as a result of—
7-54. A difficult problem resulting from frost action is that pavements are frequently broken up or severely damaged as subgrades freeze during winter and thaw in the spring. In addition to the physical damage to pavements during freezing and thawing and the high cost of time, equipment, and personnel required in maintenance, the damage to communications routes or airfields may be great and, in some instances, intolerable strategically. In the spring or at other warm periods, thawing subgrades may become extremely unstable. In some severely affected areas, facilities have been closed to traffic until the subgrade recovered its stability.
7-55. The freezing index is a measure of the combined duration and magnitude of below-freezing temperature occurring during any given freezing season. Figure 7-5 shows the freezing index for a specific winter.
Figure 7-5. Determination of freezing index
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Movement of Water Through Soils
FREEZING
7-56. Early theories attributed frost heaves to the expansion of water contained in soil voids upon freezing. However, this expansion would only be about 9 percent of the thickness of a frozen layer if caused by the water in the soil changing from the liquid to the solid state. It is not uncommon to note heaves as great as 60 percent; under laboratory conditions, heaves of as much as 300 percent have been recorded. These facts clearly indicate that heaving is due to the freezing of additional water that is attracted from the nonfrozen soil layers. Later studies have shown that frost heaves are primarily due to the growth of ice lenses in the soil at the plane of freezing temperatures.
7-57. The process of ice segregation may be pictured as follows: the thin layers of water adhering to soil grains become supercooled, meaning that this water remains liquid below 32 degrees Fahrenheit. A strong attraction exists between this water and the ice crystals that form in larger void spaces. This supercooled water flows by capillary action toward the already-formed crystals and freezes on contact. Continued crystal growth leads to the formation of an ice lens, which continues to grow in thickness and width until the source of water is cut off or the temperature rises above the normal freezing point (see figure 7-6).
Figure 7-6. Formation of ice crystals on frost line
THAWING
7-58. The second phase of frost damage occurs toward the end of winter or in early spring when thawing begins. The frozen subgrade thaws both from the top and the bottom. For the latter case, if the air temperature remains barely below the freezing point for a sufficient length of time, deeply frozen soils gradually thaw from the bottom upward because of the outward conduction of heat from the earth’s interior. An insulating blanket of snow tends to encourage this type of thawing. From an engineering standpoint, this thawing condition is desirable, because it permits melted water from thawed ice lenses to seep back through the lower soil layers to the water table from which it was drawn during the freezing process. Such dissipation of the melted water places no load on the surface drainage system, and no
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Chapter 7
tendency exists to reduce subgrade stability by reason of saturation. Therefore, there is little difficulty in maintaining unpaved roads in a passable condition.
7-59. Thawing occurs from the top downward if the surface temperature rises from below the freezing point to well above that point and remains there for an appreciable time. This leaves a frozen layer beneath the thawed subgrade. The thawed soil between the pavement and this frozen layer contains an excessive amount of moisture resulting from the melting of the ice it contained. Since the frozen soil layer is impervious to the water, adequate drainage is almost impossible. The poor stability of the resulting supersaturated road or airfield subgrade accounts for many pavement failures. Unsurfaced earthen roads may become impassable when supersaturated.
7-60. Thawing from both the top and bottom occurs when the air temperature remains barely above the freezing point for a sufficient time. Such thawing results in reduced soil stability, the duration of which would be less than for soil where the thaw is only from the top downward.
CONDITIONS
7-61. Temperatures below 32 degrees Fahrenheit must penetrate the soil to cause freezing. In general, the thickness of ice layers (and the amount of consequent heaving) is inversely proportional to the rate of penetration of freezing temperature into the soil. Thus, winters with fluctuating air temperatures at the beginning of the freezing season produce more damaging heaves than extremely cold, harsh winters where the water is more likely to be frozen in place before ice segregation can take place.
7-62. A source of water must be available to promote the accumulation of ice lenses. Water may come from—
7-63. Ice segregation usually occurs in soils when a favorable source of water and freezing temperatures are present. The potential intensity of ice segregation in a soil depends largely on the size of the void space and may be expressed as an empirical function of grain size.
7-64. Inorganic soils containing 3 percent or more by weight of grains finer than 0.02 mm in diameter are generally considered frost susceptible. Although soils may have as high as 10 percent by weight of grains finer than 0.02 mm without being frost susceptible, the tendency of these soils to occur interbedded with other soils makes it impractical to consider them separately.
7-65. Frost-susceptible soils are classified in the following groups:
7-66. They are listed approximately in the order of increasing susceptibility to frost heaving or weakening as a result of frost melting (see table 7-1). The order of listing of subgroups under groups F-3 and F-4 does not necessarily indicate the order of susceptibility to frost heaving or weakening of these subgroups. There is some overlapping of frost susceptibility between groups. The soils in group F-4 are of especially high frost susceptibility. Soil names are defined in the USCS.
7-67. Varved clays consist of alternating layers of medium-gray inorganic silt and darker silty clay. The thickness of the layers rarely exceeds ½ inch, but occasionally much thicker varves are encountered. They are likely to combine the undesirable properties of both silts and soft clays. Varved clays are likely to soften more readily than homogeneous clays with equal water content. However, local experience and conditions should be taken into account since, under favorable conditions (as when insufficient moisture is
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Movement of Water Through Soils
available for significant ice segregation), little or no detrimental frost action may occur. Some evidence exists that pavements in the seasonal frost zone, constructed on varved clay subgrades in which the deposit and depth to groundwater are relatively uniform, have performed satisfactorily. Where subgrade conditions are uniform and local evidence indicates that the degree of heave is not exceptional, the varved clay subgrade soil should be assigned a group F-4 frost-susceptibility classification.
Table 7-1. Frost-susceptible soil groups
|
Frost Group |
Kind of Soil |
Percentage Finer Than 0.02 mm by Weight |
Typical Soil Types Under Unified Soil Classification System |
|
F-1 |
Gravelly Soils |
6 to 10 |
GM, GW-GM, GP-GM |
|
F-2 |
Gravelly Soils |
10 to 20 |
GM, GW-GM, GP-GM |
|
Sands |
6 to 15 |
SM, SW-SM, SP-SM |
|
|
F-3 |
Gravelly Soils |
Over 20 |
GM, GC |
|
Sands, except very fine silty sands |
Over 15 |
SM, SC |
|
|
Clays, PI >12 |
— |
CL, CH |
|
|
F-4 |
All silts |
— |
ML, MH |
|
Very fine silty sands |
Over 15 |
SM |
|
|
Clays, PI > 12 |
— |
CL, CL-ML |
|
|
Varved clays and other fine- grained, banded sediments |
— |
CL and ML; CL, ML; and SM; CL, CH, and ML; CL, CH, ML, and SM |
EFFECTS
7-68. Frost action can cause severe damage to roads and airfields. The problems include heaving and the resultant loss of pavement strength.
HEAVING
7-69. Frost heave, indicated by the raising of the pavement, is directly associated with ice segregation and is visible evidence on the surface that ice lenses have formed in the subgrade material. Heave may be uniform or nonuniform, depending on variations in the character of the soils and the groundwater conditions underlying the pavement.
7-70. The tendency of the ice layers to develop and grow increases rapidly with decreasing grain size. On the other hand, the rate at which the water flows in an open system toward the zone of freezing decreases with decreasing grain size. Therefore, it is reasonable to expect that the worst frost heave conditions would be encountered in soils having an intermediate grain size. Silt soils, silty sands, and silty gravels tend to exhibit the greatest frost heave.
7-71. Uniform heave is the raising of adjacent areas of pavement surface by approximately equal amounts. In this type of heave, the initial shape and smoothness of the surface remains substantially unchanged. When nonuniform heave occurs, the heave of adjacent areas is appreciably different, resulting in objectionable unevenness or abrupt changes in the grade at the pavement surface.
7-72. Conditions conducive to uniform heave may exist, for example, in a section of pavement constructed with a fairly uniform stripping or fill depth, uniform depth to groundwater table, and uniform soil characteristics. Conditions conducive to irregular heave occur typically at locations where subgrades vary between clean sand and silty soils or at abrupt transitions from cut to fill sections with groundwater close to the surface.
7-73. Lateral drains, culverts, or utility lines placed under pavements on frost-susceptible subgrades frequently cause abrupt differential heaving. Wherever possible, such facilities should not be placed
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Chapter 7
beneath these pavements, or transitions should be provided so as to moderate the roughening of the pavement during the period of heave.
LOSS OF PAVEMENT STRENGTH
7-74. When ice segregation occurs in a frost-susceptible soil, the soil’s strength is reduced as is the load- supporting capacity of the pavement during prolonged frost-melting periods. This often occurs during winter and spring thawing periods, because near-surface ice melts and water from melting snow or rain may infiltrate through the surface causing an excess of water. This water cannot drain through the still- frozen soil below, or through the shoulders, or redistribute itself readily. The soil is thus softened.
7-75. Supporting capacity may be reduced in clay subgrades even through significant heave has not occurred. This may occur because water for ice segregation is extracted from the clay lattice below, and the resulting shrinkage of the lattice largely balances the volume of the ice lenses formed.
7-76. Further, traffic may cause remolding or develop hydrostatic pressure within the pores of the soil during the period of weakening, thus resulting in further-reduced subgrade strength. The degree to which a soil loses strength during a frost-melting period and the length of the period during which the strength of the soil is reduced depend on—
Rigid Pavements (Concrete)
7-77. Concrete alone has only a little tensile strength, and a slab is designed to resist loads from above while receiving uniform support from the subgrade and base course. Therefore, slabs have a tendency to break up as a result of the upthrust from nonuniform heaving soils causing a point bearing. As a rule, if rigid pavements survive the ill effects of upheaval, they will generally not fail during thawing. Reinforced concrete will carry a load by beam action over a subgrade having either frozen or supersaturated areas. Rigid pavements will carry a load over subgrades that are both frozen and supersaturated. The capacity to bear the design load is reduced, however, when the rigid slab is supported entirely by supersaturated, semiliquid subgrades.
Flexible Bituminous Pavements
7-78. The ductility of flexible pavements helps them to deflect with heaving and later resume their original positions. While heaving may produce severe bumps and cracks, usually it is not too serious for flexible pavements. By contrast, a load applied to poorly supported pavements during the thawing period normally results in rapid failure.
Slopes
7-79. Exposed back slopes and side slopes of cuts and fills in fine-grained soil have a tendency to slough off during the thawing process. The additional weight of water plus the soil exceeds the shearing strength of the soil, and the hydrostatic head of water exerts the greatest pressure at the foot of the slope. This causes sloughing at the toe of the slope, which multiplies the failure by consecutive shear failures due to inadequate stability of the altered slopes. Flatter slopes reduce this problem. Sustained traffic over severely weakened areas afflicted with frost boils initiates a pumping action that results in complete pavement failure in the immediate vicinity.
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INVESTIGATIONAL PROCEDURES
Movement of Water Through Soils
7-80. The field and laboratory investigations conducted according to Chapter 5 of this manual usually provide sufficient information to determine whether a given combination of soil and water conditions beneath the pavement are conducive to frost action. This procedure for determining whether the conditions necessary for ice segregation are present at a proposed site are discussed in the following paragraphs. As stated earlier in this chapter, inorganic soils containing 3 percent or more by weight of grains finer than
0.02 mm are generally considered susceptible to ice segregation. Thus, examination of the fine portion of the gradation curve obtained from hydrometer analysis or the decantation process for these materials indicates whether they should be assumed frost susceptible. In borderline cases, or where unusual materials are involved, slow laboratory freezing tests may be performed to measure the relative frost susceptibility.
7-81. The freezing index value should be computed from actual daily air temperatures, if possible. Obtain the air temperatures from a weather station located as close as possible to the construction site. Differences in elevations, topographical positions, and nearness of cities, bodies of water, or other sources of heat may cause considerable variations in freezing indexes over short distances. These variations are of greater importance to the design in areas of a mean design freezing index of less than 100 (that is, a design freezing index of less than 500) than they are farther north.
7-82. The depth to which freezing temperatures penetrate below the surface of a pavement depends principally on the magnitude and duration of below-freezing air temperatures and on the amount of water present in the base, subbase, and subgrade.
7-83. A potentially troublesome water supply for ice segregation is present if the highest groundwater at any time of the year is within 5 feet of the proposed subgrade surface or the top of any frost-susceptible base materials. When the depth to the uppermost water table is in excess of 10 feet throughout the year, a source of water for substantial ice segregation is usually not present unless the soil contains a significant percentage of silt. In homogeneous clay soils, the water content that the clay subgrade will attain under a pavement is usually sufficient to provide water for some ice segregation even with a remote water table. Water may also enter a frost-susceptible subgrade by surface infiltration through pavement areas. Figure 7-7, page 7-20, illustrates sources of water that feed growing ice lenses, causing frost action.
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Chapter 7
Figure 7-7. Sources of water that feed growing ice lenses
CONTROL
7-84. An engineer cannot prevent the temperatures that cause frost action. If a road or runway is constructed in a climate where freezing temperatures occur in winter, in all probability the soil beneath the pavement will freeze unless the period of lowered temperatures is very short. However, several construction techniques may be applied to counteract the presence of water and frost-susceptible soils.
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Movement of Water Through Soils
7-85. Every effort should be made to lower the groundwater table in relation to the grade of the road or runway. This may be accomplished by installing subsurface drains or open side ditches, provided suitable outlets are available and that the subgrade soil is drainable. The same result may be achieved by raising the grade line in relation to the water table. Whatever means are employed for producing the condition, the distance from the top of the proposed subgrade surface (or any frost-susceptible base material used) to the highest probably elevation of the water table should not be less than 5 feet. Distances greater than this are very desirable if they can be obtained at a reasonable cost.
7-86. Where it is possible, upward water movement should be prevented. In many cases, lowering the water table may not be practical. An example is in swampy areas where an outlets for subsurface drains might not be present. One method of preventing the rise of water would be to place a 6-inch layer of pervious, coarse-grained soil 2 or 3 feet beneath the surface. This layer would be designed as a filter to prevent clogging the pores with finer material, which would defeat the original purpose. If the depth of frost penetration is not too great, it may be less expensive to backfill completely with granular material. Another successful method, though expensive, is to excavate to the frost line and backfill with granular material. In some cases, soil cement and asphalt-stabilized mixtures 6 inches thick have been used effectively to cut off the upward movement of water.
7-87. Even though the site selected may be on ideal soil, invariably on long stretches of roads or on wide expanses of runways, localized areas will be subject to frost action. These areas should be removed and replaced with select granular material. Unless this is meticulously carried out, differential heaving during freezing and severe strength loss upon thawing, may result.
7-88. The most generally accepted method of preventing subgrade failure due to frost action is to provide a suitable insulating cover to keep freezing temperatures from penetrating the subgrade to a significant depth. This insulating cover consists of a suitable thick pavement and a thick nonfrost-susceptible base course.
7-89. If the wearing surface is cleared of snow during freezing weather, the shoulders should also be kept free of snow. Where this is not the case, freezing will set in first beneath the wearing surface. This permits water to be drawn into and accumulate in the subgrade from the unfrozen shoulder area, which is protected by the insulating snow. If both areas are free of snow, then freezing will begin in the shoulder area because it is not protected by a pavement. Under this condition, water is drawn from the subgrade to the shoulder area. As freezing progresses to include the subgrade, there will be little frost action unless more water is available from groundwater or seepage.
BASE COMPOSITION REQUIREMENTS
7-90. All base and subbase course materials lying within design depth of frost penetration should be nonfrost-susceptible. Where the combined thickness of pavement and base or subbase over a frost- susceptible subgrade is less than the design depth of frost penetration, the following additional design requirements apply.
7-91. For both flexible and rigid pavements, the bottom 4 inches of base or subbase in contact with the subgrade, as a minimum, will consist of any nonfrost-susceptible gravel, sand, screening, or similar material. This bottom of the base or subbase will be designed as a filter between the subgrade soil and the overlying material to prevent mixing of the frost-susceptible subgrade with the nonfrost-susceptible base during and immediately after the frost-melting period. The gradation of this filter material shall be determined using these guidelines:
fx
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Chapter 7
fx
7-92. A major difficulty in the construction of the filter material is the tendency of the grain-size particles to segregate during placing; therefore, a Cu > 20 is usually not desirable. For the same reason, filter materials should not be skip- or gap-graded. Segregation of coarse particles results in the formation of voids through which fine particles may wash away from the subgrade soil. Segregation can best be prevented during placement by placing the material in the moist state. Using water while installing the filter blanket also aids in compaction and helps form satisfactory transition zones between the various materials. Experience indicates that nonfrost-susceptible sand is particularly suitable for use as filter course material. Also fine-grained subgraded soil may work up into an improperly graded overlying gravel or crushed stone base course. This will occur under the kneading action of traffic during the frost-melting period if a filter course is not provided between the subgrade and base course.
7-93. For rigid pavements, the 85-percent size of filter or regular base course material placed directly beneath pavement should be ≥ 2.00 mm in diameter (Number 10 US standard sieve size) for a minimum thickness of 4 inches. The purpose of this requirement is to prevent loss of support by pumping soil through the joints.
PAVEMENT DESIGN
7-94. Pavement may be designed according to either of two basic concepts. The design may be based primarily on—
Control of Surface Deformation
7-95. In this method of pavement design, a sufficient combined thickness of pavement and nonfrost- susceptible base is provided to reduce subgrade frost penetration. Consequently, this reduces pavement heave and subgrade weakening to a low, acceptable level.
Provision of Adequate Bearing Capacity
7-96. In this method, the amount of heave that will result is neglected and the pavement is designed solely on the anticipated reduced subgrade strength during the frost-melting period.
7-97. Detailed design methods used in determining the required thickness of pavement, base, and subbase for given traffic and soil conditions where frost action is a factor are described in FM 5-430.
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Chapter 8
Soil Compaction
Soil compaction is one of the most critical components in the construction of roads, airfields, embankments, and foundations. The durability and stability of a structure are related to the achievement of proper soil compaction. Structural failure of roads and airfields and the damage caused by foundation settlement can often be traced back to the failure to achieve proper soil compaction.
Compaction is the process of mechanically densifying a soil. Densification is accomplished by pressing the soil particles together into a close state of contact with air being expelled from the soil mass in the process. Compaction, as used here, implies dynamic compaction or densification by the application of moving loads to the soil mass. This is in contrast to the consolidation process for fine-grained soil in which the soil is gradually made more dense as a result of the application of a static load. With relation to compaction, the density of a soil is normally expressed in terms of dry density or dry unit weight. The common unit of measurement is pcf. Occasionally, the wet density or wet unit weight is used.
SECTION II – SOIL PROPERTIES AFFECTED BY COMPACTION
ADVANTAGES OF SOIL COMPACTION
8-1. Certain advantages resulting from soil compaction have made it a standard procedure in the construction of earth structures, such as embankments, subgrades, and bases for road and airfield pavements. No other construction process that is applied to natural soils produces so marked a change in their physical properties at so low a cost as compaction (when it is properly controlled to produce the desired results). Principal soil properties affected by compaction include—
8-2. Compaction does not improve the desirable properties of all soils to the same degree. In certain cases, the engineer must carefully consider the effect of compaction on these properties. For example, with certain soils the desire to hold volume change to a minimum may be more important than just an increase in shearing resistance.
SETTLEMENT
8-3. A principal advantage resulting from the compaction of soils used in embankments is that it reduces settlement that might be caused by consolidation of the soil within the body of the embankment. This is true because compaction and consolidation both bring about a closer arrangement of soil particles.
8-4. Densification by compaction prevents later consolidation and settlement of an embankment. This does not necessarily mean that the embankment will be free of settlement; its weight may cause consolidation of compressible soil layers that form the embankment foundation.
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Chapter 8
SHEARING RESISTANCE
8-5. Increasing density by compaction usually increases shearing resistance. This effect is highly desirable in that it may allow the use of a thinner pavement structure over a compacted subgrade or the use of steeper side slopes for an embankment than would otherwise be possible. For the same density, the highest strengths are frequently obtained by using greater compactive efforts with water contents somewhat below OMC. Large-scale experiments have indicated that the unconfined compressive strength of a clayey sand could be doubled by compaction, within the range of practical field compaction procedures.
MOVEMENT OF WATER
8-6. When soil particles are forced together by compaction, both the number of voids contained in the soil mass and the size of the individual void spaces are reduced. This change in voids has an obvious effect on the movement of water through the soil. One effect is to reduce the permeability, thus reducing the seepage of water. Similarly, if the compaction is accomplished with proper moisture control, the movement of capillary water is minimized. This reduces the tendency for the soil to take up water and suffer later reductions in shearing resistance.
VOLUME CHANGE
8-7. Change in volume (shrinkage and swelling) is an important soil property, which is critical when soils are used as subgrades for roads and airfield pavements. Volume change is generally not a great concern in relation to compaction except for clay soils where compaction does have a marked influence. For these soils, the greater the density, the greater the potential volume change due to swelling, unless the soil is restrained. An expansive clay soil should be compacted at a moisture content at which swelling will not exceed 3 percent. Although the conditions corresponding to a minimum swell and minimum shrinkage may not be exactly the same, soils in which volume change is a factor generally may be compacted so that these effects are minimized. The effect of swelling on bearing capacity is important and is evaluated by the standard method used by the US Army Corps of Engineers in preparing samples for the CBR test.
SECTION II – DESIGN CONSIDERATIONS
MOISTURE-DENSITY RELATIONSHIPS
8-8. Nearly all soils exhibit a similar relationship between moisture content and dry density when subjected to a given compactive effort (see figure 8-1). For each soil, a maximum dry density develops at an OMC for the compactive effort used. The OMC at which maximum density is obtained is the moisture content at which the soil becomes sufficiently workable under a given compactive effort to cause the soil particles to become so closely packed that most of the air is expelled. For most soils (except cohesionless sands), when the moisture content is less than optimum, the soil is more difficult to compact. Beyond optimum, most soils are not as dense under a given effort because the water interferes with the close packing of the soil particles. Beyond optimum and for the stated conditions, the air content of most soils remains essentially the same, even though the moisture content is increased.
8-9. The moisture-density relationship shown in figure 8-1 is indicative of the workability of the soil over a range of water contents for the compactive effort used. The relationship is valid for laboratory and field compaction. The maximum dry density is frequently visualized as corresponding to 100 percent compaction for the given soil under the given compactive effort.
8-10. The curve on figure 8-1 is valid only for one compactive effort, as established in the laboratory. The standardized laboratory compactive effort is the compactive effort (CE) 55 compaction procedure, which has been adopted by the US Army Corp of Engineers. Detailed procedures for performing the CE 55 compaction test are given in TM 5-530. The maximum dry density (ydmax) at the 100 percent compaction mark is usually termed the CE 55 maximum dry density, and the corresponding moisture content is the optimum moisture content. Table 8-1 shows the relationship between the US Army Corps of Engineers
8-2 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Compaction
compaction tests and their civilian counterparts. Many times the names of these tests are used interchange- ably in publications.
Table 8-1. Compaction test comparisons
|
Test Designation |
Blow Per Layer |
No of Layers |
Hammer Weight lb |
Hammer Drop in |
Mold |
|
|
Volume cu ft |
Diameterin |
|||||
|
US Army Corps of Engineers |
|
|
|
|
|
|
|
(MIL-STD-631A) |
|
|
|
|
|
|
|
CE 55 |
55 |
5 |
10 |
18 |
0.07636 |
6 |
|
CE 12 |
12 |
5 |
10 |
18 |
0.07636 |
6 |
|
ASTM |
|
|
|
|
|
|
|
D-1557 Modified Proctor |
25 |
5 |
10 |
18 |
0.0333 |
4 |
|
|
56 |
5 |
10 |
18 |
0.0750 |
6 |
|
Standard Proctor |
25 |
3 |
5.5 |
12 |
0.0333 |
4 |
|
American Association |
|
|
|
|
|
|
|
of State Highway and |
|
|
|
|
|
|
|
Transportation Officials |
|
|
|
|
|
|
|
(AASHTO) |
|
|
|
|
|
|
|
T-180 Modified AASHTO |
25 |
5 |
10 |
18 |
0.0333 |
4 |
|
|
56 |
5 |
10 |
18 |
0.0750 |
6 |
|
T-99 Standard AASHTO |
25 |
3 |
5.5 |
12 |
0.0333 |
4 |
8-11. Figure 8-1 shows the zero air-voids curve for the soil involved. This curve is obtained by plotting the dry densities corresponding to complete saturation at different moisture contents. The zero air-voids curve represents theoretical maximum densities for given water contents. These densities are practically unattainable because removing all the air contained in the voids of the soil by compaction alone is not possible. Typically, at moisture contents beyond optimum for any compactive effort, the actual compaction curve closely parallels the zero air-voids curve. Any values of the dry density curve that plot to the right of the zero air-voids curve are in error. The specific calculation necessary to plot the zero air-voids curve are in TM 5-530.
Figure 8-1. Typical moisture-density relationship
25 September 2012 TM 3-34.64/MCRP 3-17.7G 8-3
Chapter 8
COMPACTION CHARACTERISTICS OF VARIOUS SOILS
8-12. The nature of a soil itself has a great effect on its response to a given compactive effort. Soils that are extremely light in weight, such as diatomaceous earths and some volcanic soils, may have maximum densities under a given compactive effort as low as 60 pcf. Under the same compactive effort, the maximum density of a clay may be in the range of 90 to 100 pcf, while that of a well-graded, coarse granular soil may be as high as 135 pcf. Moisture-density relationships for seven different soils are shown in figure 8-2. Compacted dry-unit weights of the soil groups of the Unified Soil Classification System are given in table 5-2, page 5-8. Dry-unit weights given in column 14 are based on compaction at OMC for the CE 55 compactive effort.
8-13. The curves of figure 8-2 indicate that soils with moisture contents somewhat less than optimum react differently to compaction. Moisture content is less critical for heavy clays (CH) than for the slightly plastic, clayey sands (SM) and silty sands (SC). Heavy clays may be compacted through a relatively wide range of moisture contents below optimum with comparatively small change in dry density. However, if heavy clays are compacted wetter than the OMC (plus 2 percent), the soil becomes similar in texture to peanut butter and nearly unworkable. The relatively clean, poorly graded sands also are relatively unaffected by changes in moisture. On the other hand, granular soils that have better grading and higher densities under the same compactive effort react sharply to slight changes in moisture, producing sizable changes in dry density.
8-14. There is no generally accepted and universally applicable relationship between the OMC under a given compactive effort and the Atterberg limit tests described in chapter 4. OMC varies from about 12 to 25 percent for fine-grained soils and from 7 to 12 percent for well-graded granular soils. For some clay soils, the OMC and the PL will be approximately the same.
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Soil Compaction
Figure 8-2. Moisture-density relationships of seven soils
OTHER FACTORS THAT INFLUENCE DENSITY
8-15. In addition to those factors previously discussed, several others influence soil density, to a smaller degree. For example, temperature is a factor in the compaction of soils that have a high clay content; both density and OMC may be altered by a great change in temperature. Some clay soils are sensitive to manipulation; that is, the more they are worked, the lower the density for a given compactive effort. Manipulation has little effect on the degree of compaction of silty or clean sands. Curing, or drying, of a soil following compaction may increase the strength of subgrade and base materials, particularly if cohesive soils are involved.
ADDITION OF WATER TO SOIL
8-16. Often water must be added to soils being incorporated in embankments, subgrades, and bases to obtain the desired degree of compaction and to achieve uniformity. The soil can be watered in the borrow pit or in place. After the water is added, it must be thoroughly and uniformly mixed with the soil. Even if additional water is not needed, mixing may still be desirable to ensure uniformity. In processing granular
25 September 2012 TM 3-34.64/MCRP 3-17.7G 8-5
Chapter 8
materials, the best results are generally obtained by sprinkling and mixing in place. Any good mixing equipment should be satisfactory. The more friable sandy and silty soils are easily mixed with water. They may be handled by sprinkling and mixing, either on the grade or in the pit. Mixing can be done with motor graders, rotary mixers, and commercial harrows to a depth of 8 inches or more without difficulty.
8-17. If time is available, water may also be added to these soils by diking or ponding the pit and flooding until the desired depth of penetration has taken place. This method usually requires several days to accomplish uniform moisture distribution. Medium clayey soils can be worked in the pit or in place as conditions dictate. The best results are obtained by sprinkling and mixing with cultivators and rotary mixers. These soils can be worked in lifts up to 8 inches or more without great difficulty. Heavy clay soils present many difficulties and should never be used as fill in an embankment foundation. They should be left alone without disturbance since usually no compactive effort or equipment is capable of increasing the in-place condition with reference to consolidation and shear strength.
8-18. The length of the section being rolled may have a great effect on densities in hot weather when water evaporates quickly. When this condition occurs, quick handling of the soil may mean the difference between obtaining adequate density with a few passes and requiring extra effort to add and mix water.
HANDLING OF WET SOILS
8-19. When the moisture content of the soil to be compacted greatly exceeds that necessary for the desired density, some water must be removed. In some cases, the use of excessively wet soils is possible without detrimental effects. These soils (coarse aggregates) are called free-draining soils, and their maximum dry density is unaffected by moisture content over a broad range of moisture. Most often, these soils must be dried; this can be a slow and costly process. The soil is usually dried by manipulating and exposing it to aeration and to the rays of the sun. Manipulation is most often done with cultivators, plows, graders, and rotary mixers. Rotary mixers, with the tail-hood section raised, permit good aeration and are very effective in drying excessively wet soils. An excellent method that may be useful when both wet and dry soils are available is simply to mix them together.
VARIATION OF COMPACTIVE EFFORT
8-20. For each compactive effort used in compacting a given soil, there is a corresponding OMC and maximum density. If the compactive effort is increased, the maximum density is increased and the OMC is decreased. This fact is illustrated in figure 8-3. It shows moisture-density relationships for two different soils, each of which was compacted using two different compactive efforts in the laboratory. When the same soil is compacted under several different compactive efforts, a relationship between density and compactive effort may be developed for that soil.
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Soil Compaction
Figure 8-3. Moisture-density relationships of two soils
8-21. This information is of particular interest to the engineer who is preparing specifications for compaction and to the inspector who must interpret the density test results made in the field during compaction. The relationship between compactive effort and density is not linear. A considerably greater increase in compactive effort will be required to increase the density of a clay soil from 90 to 95 percent of CE 55 maximum density than is required to effect the same changes in the density of a sand. The effect of variation in the compactive effort is as significant in the field rolling process as it is in the laboratory compaction procedure. In the field, the compactive effort is a function of the weight of the roller and the number of passes for the width and depth of the area of soil that is being rolled. Increasing the weight of the roller or the number of passes generally increases the compactive effort. Other factors that may be of consequence include—
8-22. To achieve the best results, laboratory and field compaction must be carefully correlated.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 8-7
Chapter 8
COMPACTION SPECIFICATIONS
8-23. To prevent detrimental settlement under traffic, a definite degree of compaction of the underlying soil is needed. The degree depends on the wheel load and the depth below the surface. For other airfield construction and most road construction in the theater of operations, greater settlement can be accepted, although the amount of maintenance will generally increase. In these cases, the minimum compaction requirements of table 8-2, should be met. However, strength can possibly decrease with increased compaction, particularly with cohesive materials. As a result, normally a 5 percent compaction range is established for density and a 4 percent range for moisture. Commonly, this “window” of density and moisture ranges is plotted directly on the CE 55 compaction curve and is referred to as the specifications block. Figure 8-4, shows a density range of 90 to 95 percent compaction and a moisture range of 12 to 16 percent.
Table 8-2. Minimum compaction requirements
|
Soil Placement |
Soil Definition |
Remarks |
|
Base |
Cohesionless, CBR > 80 |
100 percent compaction |
|
Subbase |
Cohesionless, CBR 20-50 |
100 percent compaction |
|
Select Subgrade |
CBR < 20, any in-place soil |
Cohesionless: 95 percent compaction Cohesive: 90 percent compaction Note. If subgrade CBR > 20, 100 percent compaction |
|
Embankment fill 50 ft > H |
|
Traffic areas Cohesionless: 95 percent compaction Cohesive: 90 percent compaction Nontraffic areas Cohesionless: 90 percent compaction Cohesive: 85 percent compaction |
|
Backfill for trenches |
|
Under pavement: Same requirement as base through subgrade Nontraffic area: Subtract 5 percentage points for each |
|
Top 6 inches of sidewalk |
|
Cohesionless: 90 percent compaction Cohesive: 85 percent compaction |
|
Small water-retaining structures |
|
Cohesionless: 95 percent compaction Cohesive: 90 percent compaction |
|
Note. Cohesionless: PI ≤, LL < 25 Cohesive: PI > 5 |
||
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Soil Compaction
Figure 8-4. Density, compaction, and moisture content
CBRDESIGNPROCEDURE
8-24. The concept of the CBR analysis was introduced in chapter 6. In the following procedures, the CBR analytical process will be applied to develop soil compaction specifications. Figure 8-5, page 8-10, outlines the CBR design process. The first step is to look at the CE 55 compaction curve on a DD Form 2463, page
1. If it is U-shaped, the soil is classified as “free draining” for CBR analysis and the left-hand column of the flowchart should be used through the design process. If it is bell-shaped, use the swell data graphically displayed on a DD Form 1211. Soils that, when saturated, increase in volume more than 3 percent at any initial moisture content are classified as swelling soils. If the percentage of swelling is 3 percent, the soil is considered nonswelling.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 8-9
Chapter 8
Figure 8-5. Density and moisture determination by CBR design method
8-25. Regardless of the CBR classification of the soil, the density value from the peak of the CE 55 moisture density curve is gdmax. The next step is to determine the design moisture content range. For nonswelling soils, the OMC is used. When the OMC is used, the design moisture content range is +2 percent. For swelling and free-draining soils, the minimum moisture content (MMC) is used. The MMC is determined differently for swelling soils than it is for free-draining soils. The MMC for swelling soils is
8-10 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Compaction
determined by finding the point at which the 3 percent swell occurs. The soil moisture content that corresponds to the 3 percent swell is the MMC. Free-draining soils exhibit an increase in density in response to increased soil moisture up to a certain moisture content, at which point no further increase in density is achieved by increasing moisture. The moisture content that corresponds to gdmax is the MMC. For both swelling and free-draining CBR soil classes, the design moisture-content range is MMC + 4 percent.
8-26. For swelling and free-draining soils, the final step in determining design compaction requirements is to determine the density range. Free-draining soils are compacted to 100-105 percent gdmax. Swelling soils are compacted to 90-95 percent gdmax.
8-27. Compaction requirement determinations for nonswelling soils require several additional steps. Once the OMC and design moisture content range have been determined, look at a DD Form 1207 for the PI of the soil. If PI > 5, the soil is cohesive and is compacted to 90-95 percent gdmax. If the PI 5, refer to the CBR Family of Curves on page 3 of DD Form 2463. If the CBR values are consistently above 20, compact the soil to 100-105 percent gdmax. If the CBR values are not above 20, compact the soil to 95-100 percent
gdmax.
8-28. Once you have determined the design density range and the moisture content range, you have the tools necessary to specify the requirements for and manage the compaction operations. However, placing a particular soil in a construction project is determined by its gradation, Atterberg limits, and design CBR value. Appendix A contains a discussion of the CBR design process.
8-29. A detailed discussion of placing soils and aggregates in an aggregate surface or a flexible pavement design is in FM 5-430 (for theater-of-operations construction), TM 5-822-2 (for permanent airfield design), and TM 5-822-5 (for permanent road design).
SUBGRADE COMPACTION
8-30. In fill sections, the subgrade is the top layer of the embankment, which is compacted to the required density and brought to the desired grade and section. For subgrades, plastic soils should be compacted at moisture contents that are close to optimum. Moisture contents cannot always be carefully con-trolled during military construction, but certain practical limits must be recognized. Generally, plastic soils cannot be compacted satisfactorily at moisture contents more than 10 percent above or below optimum. Much better results are obtained if the moisture content is controlled to within 2 percent of optimum. For cohesionless soils, moisture control is not as important, but some sands tend to bulk at low moisture content. Compaction should not be attempted until this situation is corrected. Normally, cohesionless soils are compacted at moisture contents that approach 100 percent saturation.
8-31. In cut sections, particularly when flexible pavements are being built to carry heavy wheel loads, subgrade soils that gain strength with compaction should be compacted to the general requirements given earlier. This may make it necessary to remove the soil, replace it, and compact it in layers to obtain the required densities at greater depths. In most construction in the theater of operations, subgrade soil in cut sections should be scarified to a depth of about 6 inches and recompacted. This is commonly referred to as a scarify/compact in-place (SCIP) operation. This procedure is generally desirable in the interest of uniformity.
Expansive Clays
8-32. As indicated previously, soils that have a high clay content (particularly (CH), (MH), and (OH)) may expand in detrimental amounts if compacted to a high density at a low moisture content and then exposed to water. Such soils are not desirable as subgrades and are difficult to compact. If they have to be used, they must be compacted to the maximum density obtainable using the MMC that will result in a minimum amount of swelling. Swelling soils, if placed at moisture contents less than the MMC, can be expected to swell more than 3 percent. Soil volume increases of up to 3 percent generally do not adversely affect theater-of-operations structures. This method requires detailed testing and careful control of compaction. In some cases, a base of sufficient thickness should be constructed to ensure against the harmful effects of expansion.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 8-11
Chapter 8
Clays and Organic Soils
8-33. Certain clay soils and organic soils lose strength when remolded. This is particularly true of some (CH) and (OH) soils. They have high strengths in their undisturbed condition, but scarifying, reworking, and compacting them in cut areas may reduce their shearing strengths, even though they are compacted to design densities. Because of these qualities, they should be removed from the construction site.
Silts
8-34. When some silts and very fine sands (predominantly (ML) and (SC) soils) are compacted in the presence of a high water table, they will pump water to the surface and become “quick”, resulting in a loss of shearing strength. These soils cannot be properly compacted unless they are dried. If they can be compacted at the proper moisture content, their shearing resistance is reasonably high. Every effort should be made to lower the water table to reduce the potential of having too much water present. If trouble occurs with these soils in localized areas, the soils can be removed and replaced with more suitable ones. If removal, or drainage and later drying, cannot be accomplished, these soils should not be disturbed by attempting to compact them. Instead, they should be left in their natural state and additional cover material used to prevent the subgrade from being overstressed.
8-35. When these soils are encountered, their sensitivity may be detected by performing unconfined compression tests on the un- disturbed soil and on the remolded soil compacted to the design density at the design moisture content. If the undisturbed value is higher, do not attempt to compact the soil; manage construction operations to produce the least possible disturbance of the soil. Base the pavement design on the bearing value of the undisturbed soil.
BASE COMPACTION
8-36. Selected soils that are used in base construction must be compacted to the general requirements given earlier. The thickness of layers must be within limits that will ensure proper compaction. This limit is generally from 4 to 8 inches, depending on the material and the method of construction.
8-37. Smooth-wheeled or vibratory rollers are recommended for compacting hard, angular materials with a limited amount of fines or stone screenings. Pneumatic-tired rollers are recommended for softer materials that may break down (degrade) under a steel roller.
MAINTENANCE OF SOIL DENSITY
8-38. Soil densities obtained by compaction during construction may be changed during the life of the structure. Such considerations are of great concern to the engineer engaged in the construction of semipermanent installations, although they should be kept in mind during the construction of any facility to ensure satisfactory performance. The two principal factors that tend to change the soil density are—
8-39. As far as embankments are concerned, normal embankments retain their degree of compaction unless subjected to unusual conditions and except in their outer portions, which are subjected to seasonal wetting and drying and frost action. Subgrades and bases are subject to more severe climatic changes and traffic than are embankments. Climatic changes may bring about seasonal or permanent changes in soil moisture and accompanying changes in density, which may distort the pavement surface. High-volume-change soils are particularly susceptible and should be compacted to meet conditions of minimum swelling and shrinkage. Granular soils retain much of their compaction under exposure to climatic conditions. Other soils may be somewhat affected, particularly in areas of severe seasonal changes, such as—
8-40. Frost action may change the density of a compacted soil, particularly if it is fine-grained. Heavy traffic, particularly for subgrades and bases of airfields, may bring about an increase in density over that
8-12 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Compaction
obtained during construction. This increase in density may cause the rutting of a flexible pavement or the subsidence of a rigid pavement. The protection that a subgrade soil receives after construction is complete has an important effect on the permanence of compaction. The use of good shoulders, the maintenance of tight joints in a concrete pavement, and adequate drainage all contribute toward maintaining the degree of compaction achieved during construction.
SECTION III – CONSTRUCTION PROCEDURES
GENERAL CONSIDERATIONS
8-41. The general construction process of a rolled-earth embankment requires that the fill be built in relatively thin layers or “lifts,” each of which is rolled until a satisfactory degree of compaction is obtained. The subgrade in a fill section is usually the top lift in the compacted fill, while the subgrade in a cut section is usually compacted in in-place soil. Soil bases are normally compacted to a high degree of density. Compaction requirements frequently stipulate a certain minimum density. For military construction, this is generally a specified minimum percentage of CE 55 maximum density for the soil concerned. The moisture content of the soil is maintained at or near optimum, within the practical limits of field construction operations (normally +2 percent of the OMC). Principal types of equipment used in field compaction are sheepsfoot, smooth steel-wheeled, vibratory, and pneumatic-tired rollers.
SELECTION OF MATERIALS
8-42. Soils used in fills generally come from cut sections of the road or airfield concerned, provided that this material is suitable. If the material excavated from cut sections is not suitable, or if there is not enough of it, then some material is obtained from other sources. Except for highly organic soils, nearly any soil can be used in fills. However, some soils are more difficult to compact than others and some require flatter side slopes for stability. Certain soils require elaborate protective devices to maintain the fill in its original condition. When time is available, these considerations and others may make it advantageous to thoroughly investigate construction efforts, compaction characteristics, and shear strengths of soils to be used in major fills. Under expedient conditions, the military engineer must simply make the best possible use of the soils at hand.
8-43. In general terms, the coarse-grained soils of the USCS are desirable for fill construction, ranging from excellent to fair. The fine-grained soils are less desirable, being more difficult to compact and requiring more careful control of the construction process. Tables 5-2 and 5-3, pages 5-7 and 5-8, respectively contain more specific information concerning the suitability of these soils.
DUMPING AND SPREADING
8-44. Since most fills are built up of thin lifts to the desired height, the soil for each lift must be spread in a uniform layer of the desired thickness. In typical operations, the soil is brought in, dumped, and spread by scraper units. The scrapers must be adjusted carefully to accomplish this objective. Materials may also be brought in by trucks or wagons and dumped at properly spaced locations so that a uniform layer may be easily spread by blade graders or bulldozers. Working alone, bulldozers may form very short and shallow fills. End dumping of soil material to form a fill without compaction is rarely permitted in modern embankment construction except when a fill is being built over very weak soils, as in a swamp. The bottom layers may then be end dumped until sufficient material has been placed to allow hauling and compacting equipment to operate satisfactorily. The best thickness of the layer to be used with a given soil and a given equipment cannot be determined exactly in advance. It is best determined by trial during the early stages of rolling on a project. No lift, however, will have a thickness less than twice the diameter of the largest size particle in the lift. As stated previously, compacted lifts will normally range from 4 to 8 inches in depth (see table 8-3, page 8-15).
25 September 2012 TM 3-34.64/MCRP 3-17.7G 8-13
Chapter 8
COMPACTION OF EMBANKMENTS
8-45. If the fill consists of cohesive or plastic soils, the embankment generally must be built up of uniform layers (usually 4 to 6 inches in compacted thickness), with the moisture content carefully controlled. Rolling should be done with the sheepsfoot or tamping-foot rollers. Bonding of a layer to the one placed on top of it is aided by the thin layer of loose material left on the surface of the rolled layer by the roller feet. Rubber-tired or smooth-wheeled rollers may be used to provide a smooth, dense, final surface. Rubber-tired construction equipment may provide supplemental compaction if it is properly routed over the area.
8-46. If the fill material is clean sand or sandy gravel, the moisture range at which compaction is possible is generally greater. Because of their rapid draining characteristics, these soils may be compacted effectively at or above OMC. Vibratory equipment may be used. Soils may be effectively compacted by combined saturation and the vibratory effects of crawler tractors, particularly when tractors are operated at fairly high speeds so that vibration is increased.
8-47. For adequate compaction, sands and gravels that have silt and clay fines require effective control of moisture. Certain soils of the (GM) and (SM) groups have especially great need for close control. Pneumatic-tired rollers are best for compacting these soils, although vibratory rollers may be used effectively.
8-48. Large rock is sometimes used in fills, particularly in the lower portion. In some cases, the entire fill may be composed of rock layers with the voids filled with smaller rocks or soil and only a cushion layer of soil for the subgrade. The thickness of such rock layers should not be more than 24 inches with the diameter of the largest rock fragment being not greater than 90 percent of the lift thickness. Compaction of this type of fill is difficult but may generally be done by vibration from the passage of tack-type equipment over the fill area or possibly 50-ton pneumatic-tired rollers.
8-49. Finishing in embankment construction includes all the operations necessary to complete the earthwork. Included among these operations are the trimming of the side and ditch slopes, where necessary, and the fine grading needed to bring the embankment section to final grade and cross section. Most of these are not separate operations performed after the completion of other operations but are carried along as the work progresses. The tool used most often in finishing operations is the motor grader, while scraper and dozer units may be used if the finish tolerances are not too strict. The provision of adequate drainage facilities is an essential part of the work at all stages of construction, temporary and final.
8-14 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Compaction
Table 8-3. Soil classification and compaction requirements (average)
25 September 2012 TM 3-34.64/MCRP 3-17.7G 8-15
Chapter 8
Table 8-3. Soil classification and compaction requirements (average)
DENSITY DETERMINATIONS
8-50. Density determinations are made in the field by measuring the wet weight of a known volume of compacted soil. The sample to be weighed is taken from a roughly cylindrical hole that is dug in the compacted layer. The volume of the hole may be determined by one of several methods, including the use of—
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Soil Compaction
8-51. When the wet weight and the volume are known, the unit wet weight may then be calculated, as described in FM 5-430.
8-52. In very arid regions, or when working with soils that lose strength when remolded, the adequacy of compaction should be judged by performing the in-place CBR test on the compacted soil of a subgrade or base. The CBR thus obtained can then be compared with the design CBR, provided that the design was based on CBR tests on unsoaked samples. If the design was based on soaked samples, the results of field in-place CBR tests must be correlated with the results of laboratory tests performed on undisturbed mold samples of the in-place soil subjected to soaking. Methods of determining the in-place CBR of a soil are described in TM 5-530.
FIELD CONTROL OF COMPACTION
8-53. As stated in previous paragraphs, specifications for adequate compaction of soil used in military construction generally require the attainment of a certain minimum density in field rolling. This requirement is most often stated in terms of a specified percentage range of CE 55 maximum density. With many soils, the close control of moisture content is necessary to achieve the stated density with the available equipment. Careful control of the entire compaction process is necessary if the required density is to be achieved with ease and economy. Control generally takes the form of field checks of moisture and density to—
8-54. The following discussion assumes that the laboratory compaction curve is available for the soil being compacted so that the maximum density and OMC are known. It is also assumed that laboratory-compacted soil and field-compacted soil are similar and that the required density can be achieved in the field with the equipment available.
DETERMINATION OF MOISTURE CONTENT
8-55. It may be necessary to check the moisture content of the soil during field rolling for two reasons. First, since the specified density is in terms of dry unit weight and the density measured directly in the field is generally the wet unit weight, the moisture content must be known so that the dry unit weight can be calculated. Second, the moisture content of some soils must be maintained close to optimum if satisfactory densities are to be obtained. Adjustment of the field moisture content can only be done if the moisture content is known. The determination of density and moisture content is often done in one overall test procedure; these determinations are described here separately for convenience.
Field Examination
8-56. Experienced engineers who have become familiar with the soils encountered on a particular project can frequently judge moisture content accurately by visual and manual examination. Friable or slightly plastic soils usually contain enough moisture at optimum to permit the forming of a strong cast by compressing it in the hand. As noted, some clay soils have OMCs that are close to their PLs; thus, a PL or “thread” test conducted in the field may be highly informative.
Field Drying
8-57. The moisture content of a soil is best and most accurately determined by drying the soil in an oven at a controlled temperature. Methods of determining the moisture content in this fashion are described in TM 5-530.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 8-17
Chapter 8
8-58. The moisture content of the soil may also be determined by air drying the soil in the sun. Frequent turning of the soil speeds up the drying process. From a practical standpoint, this method is generally too slow to be of much value in the control of field rolling.
8-59. Several quick methods may be used to determine approximate moisture contents under expedient conditions. For example, the sample may be placed in a frying pan and dried over a hot plate or a field stove. The temperature is difficult to control in this procedure, and organic materials may be burned, thus causing a slight to moderate error in the results. On large-scale projects where many samples are involved, this quick method may be used to speed up determinations by comparing the results obtained from this method with comparable results obtained by oven-drying.
8-60. Another quick method that may be useful is to mix the damp soil with enough denatured grain alcohol to form a slurry in a perforated metal cup, ignite the alcohol, and permit it to burn off. The alcohol method, if carefully done, produces results roughly equivalent to those obtained by careful laboratory drying. For best results, the process of saturating the soil with alcohol and burning it off completely should be repeated three times. This method is not reliable with clay soils. Safety measures must be observed when using this method. The burning must be done outside or in a well-ventilated room and at a safe distance from the alcohol supply and other flammable materials. The metal cup gets extremely hot, and it should be allowed to cool before handling.
“Speedy” Moisture-Content Test
8-61. The “speedy” moisture test kit provided with the soil test set provides a very rapid moisture- content determination and can be highly accurate if the test is performed properly. Care must be exercised to ensure that the reagent used has not lost its strength. The reagent must be very finely powdered (like portland cement) and must not have been exposed to water or high humidity before it is used. The specific test procedures are contained in the test set.
Nuclear Densimeter
8-62. This device provides real-time in-place moisture content and density of a soil. Accuracy is high if the test is performed properly and if the device has been calibrated with the specific material being tested. Operators must be certified, and proper safety precautions must be taken to ensure that the operator does not receive a medically significant dose of radiation during the operation of this device. There are stringent safety and monitoring procedures that must be followed. The method of determining the moisture content of a soil in this fashion is described in the operator’s manual.
DETERMINATION OF WATER TO BE ADDED
8-63. If the moisture content of the soil is less than optimum, the amount of water to be added for efficient compaction is generally computed in gallons per square yards. The computation is based on the dry weight of soil contained in a compacted layer. For example, assume that the soil is to be placed in 6-inch, compacted layers at a dry weight of 120 pcf. The moisture content of the soil is determined to be 5 percent while the OMC is 12 percent. Assume that the strip to be compacted is 40 feet wide. Compute the amount of water that must be added per 100-foot station to bring the soil to optimum moisture. The following formula applies:
fx
8.33 𝑙𝑏
8-64. Substituting in the above formula from the conditions given:
fx
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8-65. If either drying conditions or rain conditions exist at the time work is in progress, it may be advisable to either add to or reduce this quantity by up to 10 percent.
COMPACTION EQUIPMENT
8-66. Equipment normally available to the military engineer for the compaction of soils includes the following types of rollers:
PNEUMATIC-TIRED ROLLER
8-67. These heavy pneumatic-tired rollers are designed so that the weight can be varied to apply the desired compactive effort. Rollers with capacities up to 50 tons usually have two rows of wheels, each with four wheels and tires designed for 90 psi inflation. They can be obtained with tires designed for inflation pressures up to 150 psi. As a rule, the higher the tire pressure the greater the contact pressures and, consequently, the greater the compactive effort obtained. Information available from projects indicates that large rubber-tired compactors are capable of compacting clay layers effectively up to about 6 inches compacted depth and coarse granular or sand layers slightly deeper. Often it is used especially for final compaction (proof rolling) of the upper 6 inches of subgrade, for subbases, and for base courses. These rollers are very good for obtaining a high degree of compaction. When a large rubber-tired roller is to be used, care should be exercised to ensure that the moisture content of cohesive materials is low enough so that excessive pore pressures do not occur. Weaving or springing of the soil under the roller indicates that pore pressures are developing.
8-68. Since this roller does not aerate the soil as much as the sheepsfoot, the moisture content at the start of compaction should be approximately the optimum. In a soil that has the proper moisture content and lift thickness, tire contact pressure and the number of passes are the important variables affecting the degree of compaction obtained by rubber-tired rollers. Generally, the tire contact pressure can be assumed to be approximately equal to the inflation pressure.
8-69. Variants of the pneumatic-tired roller include the pneumatic roller and the self- propelled pneumatic- tired roller.
PNEUMATIC ROLLER
8-70. As used in this manual, the term “pneumatic roller” applies to a small rubber-tired roller, usually a “wobble wheel.” The pneumatic roller is suitable for granular materials; however, it is not recommended for fine-grained clay soils except as necessary for sealing the surface after a sheepsfoot roller has “walked out.” It compacts from the top down and is used for finishing all types of materials, following immediately behind the blade and water truck.
SELF-PROPELLED,PNEUMATIC-TIRED ROLLER
8-71. The self-propelled, pneumatic-tired roller has nine wheels (see figure 8-6, page 8-20). It is very maneuverable, making it excellent for use in confined spaces. It compacts from the top down. Like the towed models, the self-propelled, pneumatic-tired roller can be used for compaction of most soil materials. It is also suitable for the initial compaction of bituminous pavement.
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Figure 8-6. Self-propelled, pneumatic-tired roller
8-72. For a given number of passes of a rubber-tired roller, higher densities are obtained with the higher tire pressures. However, caution and good judgment must be used and the tire pressure adjusted in the field depending on the nature of the soil being compacted. For compaction to occur under a rubber-tired roller, permanent deformation has to occur. If more than slight pumping or spring occurs under the tires, the roller weight and tire pressure are too high and should be lowered immediately. Continued rolling under these conditions causes a decrease in strength even though a slight increase in density may occur. For any given tire pressure, the degree of compaction increases with additional passes, although the increase may be negligible after six to eight passes.
SHEEPSFOOT ROLLER
8-73. This roller compacts all fine-grained materials, including materials that will break down or degrade under the roller feet, but it will not compact cohesionless granular materials. The number of passes necessary for this type of roller to obtain the required densities must be determined for each type of soil encountered. The roller compacts from the bottom up and is used especially for plastic materials. The lift thickness for sheepsfoot rollers is limited to 6 inches in compacted depth. Penetration of the roller feet must be obtained at the start of rolling operations. This roller “walks out” as it completes its compactive effort, leaving the top 1 to 2 inches uncompacted.
8-74. The roller may tend to “walk out” before proper compaction is obtained. To prevent this, the soil may be scarified lightly behind the roller during the first two or three passes, and additional weight may be added to the roller.
8-75. A uniform density can usually be obtained throughout the full depth of the lift if the material is loose and workable enough to allow the roller feet to penetrate the layer on the initial passes. This produces compaction from the bottom up; therefore, material that becomes compacted by the wheels of equipment during pulverizing, wetting, blending, and mixing should be thoroughly loosened before compaction operations are begun. This also ensures uniformity of the mixture. The same amount of rolling generally produces increased densities as the depth of the lift is decreased. If the required densities are not being obtained, it is often necessary to change to a thinner lift to ensure that the specified density is obtained.
8-76. In a soil that has the proper moisture content and lift thickness, foot contact pressure and the number of passes are the important variables affecting the degree of compaction obtained by sheepsfoot rollers. The minimum foot contact pressure for proper compaction is 250 psi. Most available sheepsfoot rollers are equipped with feet having a contact area of 5 to 8 square inches. The foot pressure can be changed by varying the weight of the roller (varying the amount of ballast in the drum), or in special cases, by welding larger plates onto the faces of the feet. For the most efficient operation of the roller, the contact pressure should be close to the maximum at which the roller will “walk out” satisfactorily, as indicated in figure 8-7.
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Figure 8-7. Compaction by a sheepsfoot roller
8-77. The desirable foot contact pressure varies for different soils, depending on the bearing capacity of the soil; therefore, the proper adjustments have to be made in the field based on observations of the roller. If the feet of the roller tend to “walk out” too quickly (for example, after two passes), then bridging may occur and the bottom of the lift does not get sufficient compaction. This indicates that the roller is too light or the feet too large, and the weight should be increased. However, if the roller shows no tendency to “walk out” within the required number of passes, then the indications are that the roller is too heavy and the pressure on the roller feet is exceeding the bearing capacity of the soil. After making the proper adjustments in foot pressure (by changing roller size), the only other variable is the repetition of passes. Tests have shown that density increases progressively with an increase in the number of passes.
TAMPING-FOOT ROLLER
8-78. A tamping-foot roller is a modification of the sheepsfoot roller. The tamping feet are trapezoidal pads attached to a drum. Tamping-foot rollers are normally self-propelled, and the drum may be capable of vibrating. The tamping-foot roller is suitable for use with a wide range of soil types.
STEEL-WHEELED ROLLER
8-79. The steel-wheeled roller is much less versatile than the pneumatic roller. Although extensively used, it is normally operated in conjunction with one of the other three types of compaction rollers. It is used for compacting granular materials in thin lifts. Probably its most effective use in subgrade work is in the final finish of a surface, following immediately behind the blade, forming a dense and watertight surface. Figure 8-8, page 8-22, shows a two-axle tandem (5- to 8-ton) roller.
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Figure 8-8. Two axle, tandem steel-wheeled roller
SELF-PROPELLED,SMOOTH-DRUM VIBRATORY ROLLER
8-80. The self-propelled, smooth-drum vibratory roller compacts with a vibratory action that rearranges the soil particles into a denser mass (see figure 8-9). The best results are obtained on cohesionless sands and gravels. Vibratory rollers are relatively light but develop high dynamic force through an eccentric weight arrangement. Compaction efficiency is impacted by the ground speed of the roller and the frequency and amplitude of the vibrating drum.
Figure 8-9. Self-propelled, smooth-drum vibratory roller
OTHER EQUIPMENT
8-81. Other construction equipment may be useful in certain instances, particularly crawler-type tractor units and loaded hauling units, including rubber-tired scrapers. Crawler tractors are practical compacting units, especially for rock and cohesionless gravels and sands. The material should be spread in thin layers (about 3 or 4 inches thick) and is usually compacted by vibration.
COMPACTOR SELECTION
8-82. Table 8-3, page 8-15, gives information concerning compaction equipment and compactive efforts recommended for use with each of the groups of the USCS.
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8-83. Normally, there is more than one type of compactor suitable for use on a project’s type(s) of soil. When selecting a compactor, use the following criteria:
AVAILABILITY
8-84. Ascertain the types of compactors that are available and operationally ready. On major construction projects or when deployed, it may be necessary to lease compaction equipment. The rationale for leasing compaction equipment is based on the role it plays in determining overall project duration and construction quality. Uncompacted lifts cannot be built on until they are compacted. Substituting less efficient types of compaction equipment decreases productivity and may reduce project quality if desired dry densities are not achieved.
EFFICIENCY
8-85. Decide how many passes of each type of compactor are required to achieve the specified desired dry density. Determining the most efficient compactor is best done on a test strip. A test strip is an area that is located adjacent to the project and used to evaluate compactors and construction procedures. The compactive effort of each type of compactor can be determined on the test strip and plotted graphically. Figure 8-10 compares the following types of compactors:
Figure 8-10. Use of test strip data to determine compactor efficiency
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8-86. In this example, a dry density of 129 to 137 pcf is desired. The vibrating roller was the most efficient, achieving densities within the specified density range in three passes. The tamping foot compactor also compacted the soil to the desired density in three passes. However, the density achieved (130 pcf) is so close to the lower limit of the desired density range that any variation in the soil may cause the achieved density to drop below 129 pcf. The pneumatic-tired roller was the least efficient and did not densify the soil material to densities within the specified density range.
8-87. Once the type(s) of compactor is selected, optimum lift thicknesses can be determined. Table 8-3, page 8-15, provides information on average optimum lift thicknesses, but this information must be verified. Again, the test strip is a way to determine optimum lift thickness without interfering with other operations occurring on the actual project.
8-88. In actual operation, it is likely that more than one type of compactor will be operating on the project to maintain peak productivity and to continue operations when the primary compactors require maintenance or repair. Test-strip data helps to maintain control of project quality while providing the flexibility to allow construction at maximum productivity.
SECTION IV – QUALITY CONTROL
PURPOSE
8-89. Poor construction procedures can invalidate a good pavement or embankment design. Therefore, quality control of construction procedures is as important to the final product as is proper design. The purpose of quality control is to ensure that the soil is being placed at the proper density and moisture content to provide adequate bearing strength (CBR) in the fill. This is accomplished by taking samples or testing at each stage of construction. The test results are compared to limiting values or specifications, and the compaction should be accepted or reworked based on the results of the density and moisture content tests. A quality- control plan should be developed for each project to ensure that high standards are achieved. For permanent construction, statistical quality-control plans provide the most reliable check on the quality of compaction.
QUALITY-CONTROL PLAN
8-90. Generally, a quality-control plan consists of breaking the total job down into lots with each lot consisting of “X” units of work. Each lot is considered a separate job, and each job will be accepted or rejected depending on the test results representing this lot. By handling the control procedure in this way, the project engineer is able to determine the quality of the job on a lot-by-lot basis. This benefits the engineering construction unit and project engineer by identifying the lots that will be accepted and the lots that will be rejected. As this type of information is accumulated from lot to lot, a better picture of the quality of the entire project is obtained.
8-91. The following essential items should be considered in a quality-control plan:
LOT SIZE
8-92. There are two methods of defining a lot size (unit of work). A lot size may be defined as an operational time period or as a quantity of production. One advantage that the quantity-of-production method has over the operational-time-period method is that the engineering construction unit will probably have plant and equipment breakdowns and other problems that would require that production be stopped for certain periods of time. This halt in production could cause difficulties in recording production time. On the other hand, there are always records that would show the amount of materials that have been produced.
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Therefore, the better way to describe a lot is to specify that a lot will be expressed in units of quantity of production. By using this method, each lot will contain the same amount of materials, establishing each one with the same relative importance. Factors such as the size of the job and the operational capacity usually govern the size of a production lot. Typical lot sizes are 2,000 square yards for subbase construction and 1,200 square yards for stabilized subgrade construction. To statistically evaluate a lot, at least four samples should be obtained and tested properly.
RANDOM SAMPLING
8-93. For a statistical analysis to be acceptable, the data used for this analysis must be obtained from random sampling. Random sampling means that every sample within the lot has an equal chance of being selected. There are two common types of random sampling. One type consists of dividing the lot into a number of equal size sublots; one random sample is then taken from each of the sublots. The second method consists of taking the random samples from the entire lot. The sublot method has one big advantage, especially when testing during production, in that the time between testing is spaced somewhat; when taking random samples from the lot, all tests might occur within a short time. The sublot method is recommended when taking random samples. It is also recommended that all tests be conducted on samples obtained from in-place material. By conducting tests in this manner, obtaining additional samples for testing would not be a problem.
TEST TOLERANCE
8-94. A specification tolerance for test results should be developed for various tests with consideration given to a tolerance that could be met in the field and a tolerance narrow enough so that the quality of the finished product is satisfactory. For instance, the specifications for a base course would usually state that the material must be compacted to at least 100 percent CE 55 maximum density. However, because of natural variation in material, the 100 percent requirement cannot always be met. Field data indicates that the average density is 95 percent and the standard deviation is 3.5. Therefore, it appears that the specification should require 95 percent density and a standard deviation of 3.5, although there is a good possibility that the material will further densify under traffic.
PENALTY SYSTEM
8-95. After the project is completed, the job should be rated based on the results of the statistical quality- control plan for that project. A satisfactory job, meeting all of the specification tolerances, should be considered 100 percent satisfactory. On the other hand, those jobs that are not 100 percent satisfactory should be rated as such. Any job that is completely unsatisfactory should be removed and reconstructed satisfactorily.
THEATER-OF-OPERATIONS QUALITY CONTROL
8-96. In the theater of operations, quality control is usually simplified to a set pattern. This is not as reliable as statistical testing but is adequate for the temporary nature of theater-of-operations construction. There is no way to ensure that all areas of a project are checked; however, guidelines for planning quality control are as follows:
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CORRECTIVE ACTIONS
8-97. When the density and/or moisture of a soil does not meet specifications, corrective action must be taken. The appropriate corrective action depends on the specific problem situation. There are four fundamental problem situations:
8-98. It is possible to have a situation where one or more of these problems occur at the same time, such as when the soil is too dry and also undercompacted. The specification block that was plotted on the moisture density curve (CE 55) is an excellent tool for determining if a problem exists and what the problem is.
OVERCOMPACTION
8-99. Overcompaction occurs when the material is densified in excess of the specified density range. An overcompacted material may be stronger than required, which indicates—
8-100.
8-101. In the latter case, scarify the overcompacted lift and recompact to the specified density. Laboratory analysis of overcompacted soils (to include CBR analysis) is required before a corrective action decision can be made.
UNDERCOMPACTION
8-102. Undercompaction may indicate—
8-103. Corrective action is based on a sequential approach. Initially, apply additional compactive effort to the problem area. If undercompacting is a frequent problem or develops a frequent pattern, look beyond a missed roller pass as the cause of the problem.
TOO WET
8-104. Soils that are too wet when compacted are susceptible to shearing and strength loss. Corrective action for a soil compacted too wet is to—
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TOO DRY
8-105. Soils that are too dry when compacted do not achieve the specified degree of densification as do properly moistened soils. Corrective action for a soil compacted too dry is to—
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Chapter 9
Soil Stabilization for Roads and Airfields
Soil stabilization is the alteration of one or more soil properties, by mechanical or chemical means, to create an improved soil material possessing the desired engineering properties. Soils may be stabilized to increase strength and durability or to prevent erosion and dust generation. Regardless of the purpose for stabilization, the desired result is the creation of a soil material or soil system that will remain in place under the design use conditions for the design life of the project.
Engineers are responsible for selecting or specifying the correct stabilizing method, technique, and quantity of material required. This chapter is aimed at helping to make the correct decisions. Many of the procedures outlined are not precise, but they will “get you in the ball park.” Soils vary throughout the world, and the engineering properties of soils are equally variable. The key to success in soil stabilization is soil testing. The method of soil stabilization selected should be verified in the laboratory before construction and preferably before specifying or ordering materials.
SECTION I – METHODS OF STABILIZATION
BASIC CONSIDERATIONS
9-1. Deciding to stabilize existing soil material in the theater of operations requires an assessment of the mission, enemy, terrain, troops (and equipment), and time available (METT-T).
9-2. There are numerous methods by which soils can be stabilized; however, all methods fall into two broad categories. They are—
9-3. Some stabilization techniques use a combination of these two methods. Mechanical stabilization relies on physical processes to stabilize the soil, either altering the physical composition of the soil (soil blending) or placing a barrier in or on the soil to obtain the desired effect (such as establishing a sod cover to prevent dust generation). Chemical stabilization relies on the use of an admixture to alter the chemical properties of the soil to achieve the desired effect (such as using lime to reduce a soil’s plasticity).
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9-4. Classify the soil material using the USCS. When a soil testing kit is unavailable, classify the soil using the field identification methodology. Mechanical stabilization through soil blending is the most economical and expedient method of altering the existing material. When soil blending is not feasible or does not produce a satisfactory soil material, geotextiles or chemical admixture stabilization should be considered. If chemical admixture stabilization is being considered, determine what chemical admixtures are available for use and any special equipment or training required to successfully incorporate the admixture.
MECHANICAL STABILIZATION
9-5. Mechanical stabilization produces by compaction an interlocking of soil-aggregate particles. The grading of the soil-aggregate mixture must be such that a dense mass is produced when it is compacted. Mechanical stabilization can be accomplished by uniformly mixing the material and then compacting the mixture. As an alternative, additional fines or aggregates may be blended before compaction to form a uniform, well-graded, dense soil-aggregate mixture after compaction. The choice of methods should be based on the gradation of the material. In some instances, geotextiles can be used to improve a soil’s engineering characteristics (see chapter 11).
9-6. The three essentials for obtaining a properly stabilized soil mixture are—
9-7. To obtain uniform bearing capacity, uniform mixture and blending of all materials is essential. The mixture will normally be compacted at or near OMC to obtain satisfactory densities.
9-8. The primary function of the portion of a mechanically stabilized soil mixture that is retained on a Number 200 sieve is to contribute internal friction. Practically all materials of a granular nature that do not soften when wet or pulverize under traffic can be used; however, the best aggregates are those that are made up of hard, durable, angular particles. The gradation of this portion of the mixture is important, as the most suitable aggregates generally are well-graded from coarse to fine. Well-graded mixtures are preferred because of their greater stability when compacted and because they can be compacted more easily. They also have greater increases in stability with corresponding increases in density. Satisfactory materials for this use include—
9-9. Many other locally available materials have been successfully used, including disintegrated granite, talus rock, mine tailings, caliche, coral, limerock, tuff, shell, slinkers, cinders, and iron ore. When local materials are used, proper gradation requirements cannot always be met.
Note. If conditions are encountered in which the gradation obtained by blending local materials is either finer or coarser than the specified gradation, the size requirements of the finer fractions should be satisfied and the gradation of the coarser sizes should be neglected.
9-10. The portion of the soil that passes a Number 200 sieve functions as filler for the rest of the mixture and supplies cohesion. This aids in the retention of stability during dry weather. The swelling of clay material serves somewhat to retard the penetration of moisture during wet weather. Clay or dust from rock- crushing operations are commonly used as binders. The nature and amount of this finer material must be carefully controlled, since too much of it results in an unacceptable change in volume with change in moisture content and other undesirable properties. The properties of the soil binder are usually controlled by controlling the plasticity characteristics, as evidenced by the LL and PI. These tests are performed on the portion of the material that passes a Number 40 sieve. The amount of fines is controlled by limiting the
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amount of material that may pass a Number 200 sieve. When the stabilized soil is to be subjected to frost action, this factor must be kept in mind when designing the soil mixture.
USES
9-11. Mechanical soil stabilization may be used in preparing soils to function as—
9-12. Several commonly encountered situations may be visualized to indicate the usefulness of this method. One of these situations occurs when the surface soil is a loose sand that is incapable of providing support for wheeled vehicles, particularly in dry weather. If suitable binder soil is available in the area, it may be brought in and mixed in the proper proportions with the existing sand to provide an expedient all- weather surface for light traffic. This would be a sand-clay road. This also may be done in some cases to provide a “working platform” during construction operations. A somewhat similar situation may occur in areas where natural gravels suitable for the production of a well-graded sand-aggregate material are not readily available. Crushed stone, slag, or other materials may then be stabilized by the addition of suitable clay binder to produce a satisfactory base or surface. A common method of mechanically stabilizing an existing clay soil is to add gravel, sand, or other granular materials. The objectives here are to—
OBJECTIVE
9-13. The objective of mechanical stabilization is to blend available soils so that, when properly compacted, they give the desired stability. In certain areas, for example, the natural soil at a selected location may have low load-bearing strength because of an excess of clay, silt, or fine sand. Within a reasonable distance, suitable granular materials may occur that may be blended with the existing soils to markedly improve the soil at a much lower cost in manpower and materials than is involved in applying imported surfacing.
9-14. The mechanical stabilization of soils in military construction is very important. The engineer needs to be aware of the possibilities of this type of construction and to understand the principles of soil action previously presented. The engineer must fully investigate the possibilities of using locally available materials.
LIMITATIONS
9-15. Without minimizing the importance of mechanical stabilization, the limitations of this method should also be realized. The principles of mechanical stabilization have frequently been misused, particularly in areas where frost action is a factor in the design. For example, clay has been added to “stabilize” soils, when in reality all that was needed was adequate compaction to provide a strong, easily drained base that would not be susceptible to detrimental frost action. An understanding of the densification that can be achieved by modern compaction equipment should prevent a mistake of this sort. Somewhat similarly, poor trafficability of a soil during construction because of lack of fines should not necessarily provide an excuse for mixing in clay binder. The problem may possibly be solved by applying a thin surface treatment or using some other expedient method.
SOIL BASE REQUIREMENTS
9-16. Grading requirements relative to mechanically stabilized soil mixtures that serve as base courses are given in Table 7-3 of TM 5-330/Air Force Manual (AFM) 86-3, Volume II. Experience in civil highway
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construction indicates that best results are obtained with this type of mixture if the fraction passing the Number 200 sieve is not greater than two-thirds of the fraction passing the Number 40 sieve. The size of the largest particles should not exceed two-thirds of the thickness of the layer in which they are incorporated. The mixture should be well-graded from coarse to fine.
9-17. A basic requirement of soil mixtures that are to be used as base courses is that the PI should not exceed 5. Under certain circumstances, this requirement may be relaxed if a satisfactory bearing ratio is developed. Experience also indicates that under ideal circumstances the LL should not exceed 25. These requirements may be relaxed in theater-of-operations construction. The requirements may be lowered to a LL of 35 and a PI of 10 for fully operational airfields. For emergency and minimally operational airfields, the requirements may be lowered to a LL of 45 and a PI of 15, when drainage is good.
SOIL SURFACE REQUIREMENTS
9-18. Grading requirements for mechanically stabilized soils that are to be used directly as surfaces, usually under emergency conditions, are generally the same as those indicated in table 7-3 of TM 5- 330/AFM 86-3, Volume II. Preference should be given to mixtures that have a minimum aggregate size equal to 1 inch or perhaps 1½ inches. Experience indicates that particles larger than this tend to work themselves to the surface over a period of time under traffic. Somewhat more fine soil is desirable in a mixture that is to serve as a surface, as compared with one for a base. This allows the surface to be more resistant to the abrasive effects of traffic and penetration of precipitation. To some extent, moisture lost by evaporation can be replaced by capillarity.
9-19. Emergency airfields that have surfaces of this type require a mixture with a PI between 5 and 10. Experience indicates that road surfaces of this type should be between 4 and 9. The surface should be made as tight as possible, and good surface drainage should be provided. For best results, the PI of a stabilized soil that is to function first as a wearing surface and then as a base, with a bituminous surface being provided at a later date, should be held within very narrow limits. Consideration relative to compaction, bearing value, and frost action are as important for surfaces of this type as for bases.
PROPORTIONING
9-20. Mixtures of this type are difficult to design and build satisfactorily without laboratory control. A rough estimate of the proper proportions of available soils in the field is possible and depends on manual and visual inspection. For example, suppose that a loose sand is the existing subgrade soil and it is desired to add silty clay from a nearby borrow source to achieve a stabilized mixture. Each soil should be moistened to the point where it is moist, but not wet; in a wet soil, the moisture can be seen as a shiny film on the surface. What is desired is a mixture that feels gritty and in which the sand grains can be seen. Also, when the soils are combined in the proper proportion, a cast formed by squeezing the moist soil mixture in the hand will not be either too strong or too weak; it should just be able to withstand normal handling without breaking. Several trial mixtures should be made until this consistency is obtained. The proportion of each of the two soils should be carefully noted. If gravel is available, this may be added, although there is no real rule of thumb to tell how much should be added. It is better to have too much gravel than too little.
Use of Local Materials
9-21. The essence of mechanical soil stabilization is the use of locally available materials. Desirable requirements for bases and surfaces of this type were given previously. It is possible, especially under emergency conditions, that mixtures of local materials will give satisfactory service, even though they do not meet the stated requirements. Many stabilized mixtures have been made using shell, coral, soft limestone, cinders, marl, and other materials listed earlier. Reliance must be placed on—
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Soil Stabilization for Roads and Airfields
Blending
9-22. It is assumed in this discussion that an existing subgrade soil is to be stabilized by adding a suitable borrow soil to produce a base course mixture that meets the specified requirements. The mechanical analysis and limits of the existing soil will usually be available for the results of the subgrade soil survey (see chapter 3). Similar information is necessary concerning the borrow soil. The problem is to determine the proportions of these two materials that should be used to produce a satisfactory mixture. In some cases, more than two soils must be blended to produce a suitable mixture. However, this situation is to be avoided when possible because of the difficulties frequently encountered in getting a uniform blend of more than two local materials. Trial combinations are usually made on the basis of the mechanical analysis of the soil concerned. In other words, calculations are made to determine the gradation of the combined materials and the proportion of each component adjusted so that the gradation of the combination falls within specified limits. The PI of the selected combination is then determined and compared with the specification. If this value is satisfactory, then the blend may be assumed to be satisfactory, provided that the desired bearing value is attained. If the plasticity characteristics of the first combination are not within the specified limits, additional trials must be made. The proportions finally selected then may be used in the field construction process.
Numerical Proportioning
9-23. The process of proportioning will now be illustrated by a numerical example (see table 9-1, page 9-6). Two materials are available, material B in the roadbed and material A from a nearby borrow source. The mechanical analysis of each of these materials is given, together with the LL and PI of each. The desired grading of the combination is also shown, together with the desired plasticity characteristics.
Specified Gradation
9-24. Proportioning of trial combinations may be done arithmetically or graphically. The first step in using either the graphical or arithmetical method is to determine the gradation requirements. Gradation requirements for base course, subcourse, and select material are found in table 7-1, page 7-17, TM 5-330/AFM 86-3, Volume II. In the examples in figure 9-1, page 9-6, and figure 9-2, page 9-8, a base course material with a maximum aggregate size of 1 inch has been specified. In the graphical method, the gradation requirements are plotted to the outside of the right axis. In the arithmetical method, they are plotted in the column labelled “Specs.” Then the gradations of the soils to be blended are recorded. The graphical method has the limitation of only being capable of blending two soils, whereas the arithmetical method can be expanded to blend as many soils as required. At this point, the proportioning methods are distinctive enough to require separate discussion.
Graphical Proportioning
9-25. The actual gradations of soil materials A and B are plotted along the left and right axes of the graph, respectively. As shown in figure 9-1, material A has 92 percent passing the ¾-inch sieve while material B has 72 percent passing the same sieve. Once plotted, a line is drawn across the graph, connecting the percent passing of material A with the percent passing of material B for each sieve size.
Note. Since both materials A and B had 100 percent passing the 1-inch sieve, it was omitted from the graph and will not affect the results.
9-26. Mark the point where the upper and lower limits of the gradation requirements intersect the line for each sieve size. In figure 9-1, the allowable percent passing the Number 4 sieve ranges from 35 to 65 percent passing. The point along the Number 4 line at which 65 percent passing intersects represents 82 percent material A and 18 percent material B. The 35 percent passing intersects the Number 4 line at 19 percent material A and 81 percent material B. The acceptable ranges of material A to be blended with material B is the widest range that meets the gradation requirements for all sieve sizes. The shaded area of the chart represents the combinations of the two materials that will meet the specified gradation requirements. The boundary on the left represents the combination of 44 percent material A and 56 percent
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-5
Chapter 9
material B. The position of this line is fixed by the upper limit of the requirement relating to the material passing the Number 200 sieve (15 percent). The boundary on the right represents the combination of 21 percent material A and 79 percent material B. This line is established by the lower limit of the requirement relative to the fraction passing the Number 40 sieve (15 percent). Any mixture falling within these limits satisfies the gradation requirements. For purposes of illustration, assume that a combination of 30 percent material A and 70 percent material B is selected for a trial mixture. A similar diagram can be prepared for any two soils.
Table 9-1. Numerical example of proportioning
|
Mechanical Analysis |
|||
|
Sieve Designation |
Percent Passing, by Weight |
||
|
Material A |
Material B |
Desired |
|
|
1-inch |
100 |
100 |
100 |
|
¾ inch |
92 |
72 |
70-100 |
|
⅜ inch |
83 |
45 |
50-80 |
|
Number 4 |
75 |
27 |
35-65 |
|
Number 10 |
67 |
15 |
20-50 |
|
Number 40 |
52 |
— |
15-30 |
|
Number 200 |
33 |
1 |
5-15 |
|
Plasticity Characteristics |
|||
|
Liquid limit |
32 |
12 |
≤28 |
|
Plasticity index |
9 |
0 |
≤6 |
Figure 9-1. Graphical method of proportioning two soils to meet gradation requirements
9-6 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
Arithmetical Proportioning
9-27. Record the actual gradation of soils A and B in their respective columns (Columns 1 and 2, figure 9-2, page 9-8). Average the gradation limits and record in the column labelled “S”. For example, the allowable range for percent passing a 3/8-inch sieve in a 1-inch minus base course is 50 to 80 percent. The average, 50+80/2, is 65 percent. As shown in figure 9-2, S for 3/8 inch is 65. Next, determine the absolute value of S-A and S-B for each sieve size and record in the columns labelled “|S-A|” and “|S-B|”, respectively. Sum columns |S-A| and |S-B|. To determine the percent of soil A in the final mix, use the formula—
In the example in figure 9-2: fx
9-28. The percent of soil B in the final mix can be determined by the formula:
fx
Note. If three or more soils are to be blended, the formula would be—
fx
9-30. Multiply the percent passing each sieve for soil A by the percentage of soil A in the final mix; record the information in column 4 (see figure 9-2). Repeat the procedure for soil B and record the information in column 5 (see figure 9-2). Complete the arithmetical procedure by adding columns 4 and 5 to obtain the percent passing each sieve in the blended soil.
9-31. Both the graphical and arithmetical methods have advantages and disadvantages. The graphical method eliminates the need for precise blending under field conditions and the methodology requires less effort to use. Its drawback becomes very complex when blending more than two soils. The arithmetical method allows for more precise blending, such as mixing at a batch plant, and it can be readily expanded to accommodate the blending of three or more soils. It has the drawback in that precise blending is often unattainable under field conditions. This reduces the quality assurance of the performance of the blended soil material.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-7
Chapter 9
Figure 9-2. Arithmetical method of proportioning soils to meet gradation requirements
Plasticity Requirements
9-32. A method of determining the PI and LL of the combined soils serves as a method to indicate if the proposed trial mixture is satisfactory, pending the performance of laboratory tests. This may be done either arithmetically or graphically. A graphical method of obtaining these approximate values is shown in figure 9-3. The values shown in figure 9-3 require additional explanation, as follows. Consider 500 pounds of the mixture tentatively selected (30 percent as material A and 70 percent as material B). Of this 500 pounds, 150 pounds are material A and 350 pounds material B. Within the 150 pounds of material A, there are 150 (0.52) = 78 pounds of material passing the Number 40 sieve. Within the 350 pounds of material B, there are 150 (0.05) = 17.5 pounds of material passing the Number 40 sieve. The total amount of material passing the Number 40 sieve in the 500 pounds of blend = 78 + 17.5 = 95.5 pounds. The percentage of this material that has a PI of 9 (material A) is (78/95.5) 100 = 82. As shown in figure 9-3, the approximate PI of the mixture of 30 percent material A and 70 percent material B is 7.4 percent. By similar reasoning, the approximate LL of the blend is 28.4 percent. These values are somewhat higher than permissible under the specification. An increase in the amount of material B will somewhat reduce the PI and LL of the combination.
9-8 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
Figure 9-3. Graphical method of estimating plasticity characteristics of a combination two soils
Field Proportioning
9-33. In the field, the materials used in a mechanically stabilized soil mixture probably will be proportioned by loose volume. Assume that a mixture incorporates 75 percent of the existing subgrade soil, while 25 percent will be brought in from a nearby borrow source. The goal is to construct a layer that has a compacted thickness of 6 inches. It is estimated that a loose thickness of 8 inches will be required to form the 6-inch compacted layer. A more exact relationship can be established in the field as construction proceeds. Of the 8 inches loose thickness, 75 percent (or 0.75(8) = 6 inches) will be the existing soil. The remainder of the mix will be mixed thoroughly to a depth of 8 inches and compacted by rolling. The proportions may be more accurately controlled by weight, if weight measurements can be made under existing conditions.
WATERPROOFING
9-34. The ability of an airfield or road to sustain operations depends on the bearing strength of the soil. Although an unsurfaced facility may possess the required strength when initially constructed, exposure to water can result in a loss of strength due to the detrimental effect of traffic operations. Fine-grained soils or granular materials that contain an excessive amount of fines generally are more sensitive to water changes than coarse-grained soils. Surface water also may contribute to the development of dust by eroding or loosening material from the ground surface that can become dust during dry weather conditions.
Sources of Water
9-35. Water may enter a soil either by the percolation of precipitation or ponded surface water, by capillary action of underlying groundwater, by a rise in the water-table level, or by condensation of water vapor and accumulation of moisture under a vapor-impermeable surface. As a general rule, an existing groundwater table at shallow depths creates a low load-bearing strength and must be avoided wherever possible. Methods to protect against moisture ingress from sources other than the ground surface will not be
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-9
Chapter 9
considered here. In most instances, the problem of surface water can be lessened considerably by following the proper procedures for—
Objectives of Waterproofers
9-36. The objective of a soil-surface waterproofer is to protect a soil against attack by water and thus preserve its in-place or as-constructed strength during wet-weather operations. The use of soil waterproofers generally is limited to traffic areas. In some instances, soil waterproofers may be used to prevent excessive softening of areas, such as shoulders or overruns, normally considered nontraffic or limited traffic areas.
9-37. Also, soil waterproofers may prevent soil erosion resulting from surface water runoff. As in the case of dust palliatives, a thin or shallow-depth soil waterproofing treatment loses its effectiveness when damaged by excessive rutting and thus can be used efficiently only in areas that are initially firm. Many soil waterproofers also function well as dust palliatives; therefore, a single material might be considered as a treatment in areas where the climate results in both wet and dry soil surface conditions. Geotextiles are the primary means of waterproofing soils when grading, compaction, and drainage practices are insufficient. Use of geotextiles is discussed in detail in chapter 11.
CHEMICAL ADMIXTURE STABILIZATION
9-38. Chemical admixtures are often used to stabilize soils when mechanical methods of stabilization are inadequate and replacing an undesirable soil with a desirable soil is not possible or is too costly. Over 90 percent of all chemical admixture stabilization projects use—
9-39. Other stabilizing chemical admixtures are available, but they are not discussed in this manual because they are unlikely to be available in the theater of operations.
WARNING
Chemical admixtures may contain hazardous materials. Consult appendix C to determine the necessary safety precautions for the selected admixture.
9-40. When selecting a stabilizer additive, the factors that must be considered are the—
9-41. Table 9-2 lists stabilization methods most suitable for specific applications. To determine the stabilizing agent(s) most suited to a particular soil, use the gradation triangle in figure 9-4, page 9-12, to find the area that corresponds to the gravel, sand, and fine content of the soil. For example, soil D has the following characteristics:
9-10 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
9-42. Therefore the soil is 5 percent gravel, 81 percent sand, and 14 percent fines. Figure 9-4 shows this soil in Area 1C.
Table 9-2. Stabilization methods most suitable for specific applications
|
Purpose |
Soil Type |
Method |
|
Subgrade Stabilization Improves load-carrying and stress- distribution characteristics |
Fine-grained |
SA, SC, MB, C |
|
Coarse-grained |
SA, SC, MB, C |
|
|
Clays of low PI |
C, SC, CMS, LMS, SL |
|
|
Clays of low PI |
SL, LMS |
|
|
Reduces frost susceptibility |
Fine-grained |
CMS, SA, SC, LF |
|
Clays of low PI |
CMS, SC, SL, LMS |
|
|
Improves waterproofing and runoff |
Clays of low PI |
CMS, SA, LMS, SL |
|
Controls shrinkage and swell |
Clays of low PI |
CMS, SC, C, LMS, SL |
|
Clays of low PI |
SL |
|
|
Reduces resiliency |
Clays of low PI |
SL, LMS |
|
Elastic silts or clays |
SC, CMS |
|
|
Base Course Stabilization Improves substandard materials |
Fine-grained |
SC, SA, LF, MB |
|
Clays of low PI |
SC, SL |
|
|
Improves load-carrying and stress- distribution characteristics |
Coarse-grained |
SA, SC, MB, LF |
|
Fine-grained |
SC, SA, LF, MB |
|
|
Reduces pumping |
Fine-grained |
SC, SA, LF, MB, membranes |
|
Dust Palliative |
Fine-grained |
CMS, SA, oil or bituminous surface spray, APSB |
|
Plastic soils |
CMS, SL, LMS, APSB, DCA 70 |
|
|
Legend: The methods of treatment are— APSB = Asphalt penetration surface binder LMS = Lime-modified soil C = Compaction MB = Mechanical blending CMS = Cement-modified soil SA = Soil-asphalt DCA 70 = Polyvinyl acetate emulsion SC = Soil-cement LF = Lime-fly ash SL = Soil-lime |
||
9-43. Table 9-3, page 9-12, shows that the stabilizing agents recommended for Area 1C soils include bituminous material, portland cement, lime, and lime-cement-fly ash. In this example, bituminous agents cannot be used because of the restriction on PI, but any of the other agents can be used if available.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-11
Chapter 9
Figure 9-4. Gradation triangle for use in selecting a stabilizing additive Table 9-3. Guide for selecting a stabilizing additive.
|
Area |
Soils Class |
Type of Stabilizing Additive Recommended |
Restriction on LL and PI of Soil |
Restriction on Percent Passing No 200 Sieve |
Remarks |
|
1A |
SW or SP |
(1) Bituminous (2) Portland cement (3) Lime-cement-fly ash |
PI not to exceed 25 |
|
|
|
1B |
SW-SM or SP-SM or SW-SC or SP-SC |
(1} Bituminous (2) Portland cement (3) Lime (4) Lime-cement-fly ash |
PI not to exceed 10 PI not to exceed 30 PI not to exceed 12 PI not to exceed 25 |
PI 30 or less PI 12 or greater |
|
|
1C |
SM or SC or SM-SC |
(1) Bituminous
(2) Portland cement (3) Lime (4) Lime-cement-fly ash |
PI not to exceed 10
—* PI not less than 12 PI not to exceed 25 |
Not to exceed 30 percent by weight |
|
9-12 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
Table 9-3. Guide for selecting a stabilizing additive.
|
Area |
Soils Class |
Type of Stabilizing Additive Recommended |
Restriction on LL and PI of Soil |
Restriction on Percent Passing No 200 Sieve |
Remarks |
|
2A |
GW or GP |
(1) Bituminous
(2) Portland cement |
|
|
Well-graded material only Material should contain at least 45 percent by weight of material passing No 4 sieve |
|
|
|
(3) Lime-cement-fly ash |
PI not to exceed 25 |
|
|
|
2B |
GW-GM or |
(1) Bituminous |
PI not to exceed 10 |
|
Well-graded material only |
|
|
GP-GM or
GW-GC or CP-GC |
(2) Portland cement
(3) Lime (4) Lime-cement-fly ash |
PI not to exceed 30
PI not less than 12 PI not to exceed 25 |
|
Material should contain at least 45 percent by weight of material passing No 4 sieve |
|
2C |
GH or GC or GM-GC |
(1) Bituminous
(2) Portland cement
(3) Lime (4) Lime-cement-fly ash |
PI not to exceed 10
—*
PI not less than 12 PI not to exceed 25 |
Not to exceed 30 percent by weight |
Well-graded material only
Material should contain at least 45 percent by weight of material passing No 4 sieve |
|
3 |
CH or CL
or MH or ML or OH or OL or HL-CL |
(1) Portland cement
(2) Lime |
LL less than 40 and PI less than 20
PI not less than 12 |
|
Organic and strongly acid soils falling within this area are not susceptible to stabilization by ordinary means |
|
50-percent passing No 200 sieve * 𝑃𝐼 ≤ 20 + .4 |
|||||
CEMENT
9-44. Cement can be used as an effective stabilizer for a wide range of materials. In general, however, the soil should have a PI less than 30. For coarse-grained soils, the percent passing the Number 4 sieve should be greater than 45 percent.
9-45. If the soil temperature is less than 40 degrees Fahrenheit and is not expected to increase for one month, chemical reactions will not occur rapidly. The strength gain of the cement-soil mixture will be minimal. If these environmental conditions are anticipated, the cement may be expected to act as a soil modifier, and another stabilizer might be considered for use. Soil-cement mixtures should be scheduled for construction so that sufficient durability will be gained to resist any freeze-thaw cycles expected.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-13
Chapter 9
9-46. Portland cement can be used either to modify and improve the quality of the soil or to transform the soil into a cemented mass, which significantly increases its strength and durability. The amount of cement additive depends on whether the soil is to be modified or stabilized. The only limitation to the amount of cement to be used to stabilize or modify a soil pertains to the treatment of the base courses to be used in flexible pavement systems. When a cement-treated base course for Air Force pavements is to be surfaced with asphaltic concrete, the percent of cement by weight is limited to 4 percent.
Modification
9-47. The amount of cement required to improve the quality of the soil through modification is determined by the trial-and-error approach. To reduce the PI of the soil, successive samples of soil-cement mixtures must be prepared at different treatment levels and the PI of each mixture determined.
9-48. The minimum cement content that yields the desired PI is selected, but since it was determined based on the minus 40 fraction of the material, this value must be adjusted to find the design cement content based on total sample weight expressed as—
A = 100Bc
where—
A = design cement content, percent of total weight of soil
B = percent passing Number 40 sieve, expressed as a decimal
c = percent of cement required to obtain the desired PI of minus Number 40 material, expressed as a decimal
9-49. If the objective of modification is to improve the gradation of granular soil through the addition of fines, the analysis should be conducted on samples at various treatment levels to determine the minimum acceptable cement content. To determine the cement content to reduce the swell potential of fine-grained plastic soils, mold several samples at various cement contents and soak the specimens along with untreated specimens for four days. The lowest cement content that eliminates the swell potential or reduces the swell characteristics to the minimum becomes the design cement content. The cement content determined to accomplish soil modification should be checked to see if it provides an unconfined compressive strength great enough to qualify for a reduced thickness design according to criteria established for soil stabilization (see tables 9-4 and 9-5).
9-50. Cement-modified soil may be used in frost areas also. In addition to the procedures for the mixture design described above, cured specimens should be subjected to the 12 freeze-thaw cycles test (omit wire brush portion) or other applicable freeze-thaw procedures. This should be followed by a frost-susceptibility test, determined after freeze- thaw cycling, and should meet the requirements set forth for the base course. If cement- modified soil is used as the subgrade, its frost susceptibility (determined after freeze-thaw cycling) should be used as the basis of the pavement thickness design if the reduced subgrade-strength design method is applied.
Table 9-4. Minimum unconfined compressive strengths for cement, lime, and combined lime-cement-fly ash stabilized soils
|
Stabilized Soil Layer |
Minimum Unconfined Compressive Strength, psia |
||
|
Flexible Pavement |
Rigid Pavement, All |
||
|
Army and Air Force |
Navy |
||
|
Base coarse, |
7.50 |
750 |
500 |
|
Subbase coarse, select material or subgrade |
250 |
300 (cement) 150 (lime) |
200 |
|
a Unconfined compressive strength determined at seven days for cement stabilization and 28 days for lime or lime-cement-fly ash stabilization. |
|||
9-14 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
Table 9-5. Durability requirements
|
|
Maximum Allowable Weight Loss After 12 Wet-Dry Freeze-Thaw Cycles, Percent of Initial Specimen Weight |
|
|
Type of Soil Stabilized |
Army and Air Force |
Navy |
|
Granular, PI <10 |
11 |
14 |
|
Granular, PI >10 |
8 |
14 |
|
Silt |
8 |
14 |
|
Clays |
6 |
14 |
Stabilization
9-51. The following procedure is recommended for determining the design cement content for cement- stabilized soils:
Step 1. Determine the classification and gradation of the untreated soil. The soil must meet the gradation requirements shown in table 9-6, page 9-16, before it can be used in a reduced thickness design (multilayer design).
Step 2. Select an estimated cement content from table 9-7, page 9-16, using the soil classification.
Step 3. Using the estimated cement content, determine the compaction curve of the soil-cement mixture.
Step 4. If the estimated cement content from step 2 varies by more than +2 percent from the value in tables 9-8 or 9-9, page 9-17, conduct additional compaction tests, varying the cement content, until the value from table 9-8 or 9-9 is within 2 percent of that used for the moisture-density test.
Note. Figure 9-5, page 9-18, is used in conjunction with table 9-9. The group index is obtained from figure 9-5 and used to enter table 9-9.
Step 5. Prepare samples of the soil- cement mixture for unconfined compression and durability tests at the dry density and at the cement content determined in step 4. Also prepare samples at cement contents 2 percent above and 2 percent below that determined in step 4. The samples should be prepared according to TM 5-530 except that when more than 35 percent of the material is retained on the Number 4 sieve, a CBR mold should be used to prepare the specimens. Cure the specimens for seven days in a humid room before testing. Test three specimens using the unconfined compression test and subject three specimens to durability tests. These tests should be either wet-dry tests for pavements located in nonfrost areas or freeze-thaw tests for pavements located in frost areas.
Step 6. Compare the results of the unconfined compressive strength and durability tests with the requirements shown in tables 9-4 and 9-5. The lowest cement content that meets the required unconfined compressive strength requirement and demonstrates the required durability is the design content. If the mixture should meet the durability requirements but not the strength requirements, the mixture is considered to be a modified soil.
9-52. Theater-of-operations construction requires that the engineer make maximum use of the locally available construction materials. However, locally available materials may not lend themselves to classification under the USCS method. The average cement requirements of common locally available construction materials is shown in table 9-10, page 9-19.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-15
Chapter 9
Table 9-6. Gradation requirements
|
Type Course |
Sieve Size |
Percent Passing |
|
|
Army and Air Force |
Navy |
||
|
|
22 |
100 |
— |
|
|
1 1/2 in |
70-100 |
— |
|
|
1 in |
45-100 |
100 |
|
|
3/4 in |
— |
90-100 |
|
Base |
1/2 in Number 4 |
30-90 20-70 |
— 40-70 |
|
|
Number 10 |
15-60 |
— |
|
|
Number 30 |
— |
12-40 |
|
|
Number 40 |
5-40 |
— |
|
|
Number 200 |
0-20 |
3-15 |
|
|
3 in |
100 |
100 |
|
|
Number 4 |
— |
45-100 |
|
Subbase |
Number 10 |
— |
36-60 |
|
|
Number 1000 |
— |
3-20 |
|
|
Number 2000 |
0-25 |
0-3 |
Table 9-7. Estimated cement requirements for various soil types
|
Soil Classification |
Initial Estimated Cement Requirement, Percent Dry Weight |
|
GW, SW |
5 |
|
GP, SW-SM, SW-SC |
6 |
|
GW-GM, GW-GC |
|
|
GM, SM, GC, SC, SP-SM, SP-SC, GP-GM, GP-GC, SM-SC, GM-GC |
7 |
|
SP, CL, ML, ML-CL |
10 |
|
MH-OH |
11 |
|
CH |
10 |
9-16 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
Table 9-8. Average cement requirements for granular and sandy soils
|
Material Retained on No 4 Sieve, Percent |
MaterialSmaller Than 0.05 mm, Per cent |
Cement Content, Percent by Weight Maximum Dry Density, lb/cu ft (Treated Material) |
|||||
|
116-120 |
121-126 |
127-131 |
132-137 |
138-142 |
143 or more |
||
|
|
0-19 |
10 |
9 |
8 |
7 |
6 |
5 |
|
0-14 |
20-39 |
9 |
8 |
7 |
7 |
5 |
5 |
|
|
40-50 |
11 |
10 |
9 |
8 |
6 |
5 |
|
|
0-19 |
10 |
9 |
8 |
6 |
5 |
5 |
|
15-29 |
20-39 |
9 |
8 |
7 |
6 |
6 |
5 |
|
|
40-50 |
12 |
10 |
9 |
8 |
7 |
6 |
|
|
0-19 |
10 |
8 |
7 |
6 |
5 |
5 |
|
30-45 |
20-39 |
11 |
9 |
8 |
7 |
6 |
5 |
|
|
40-50 |
12 |
11 |
10 |
9 |
8 |
6 |
|
Note. Base course goes to 70 percent retained on the No 4 sieve. |
|||||||
Table 9-9. Average cement requirements for silty and clayey soils
|
Group Index |
Material Between 0.05 and 0.005 mm, Percent |
Cement Content, Percent by Weight Maximum Dry density, lb/cu ft (Treated Material) |
||||||
|
99-104 |
105-109 |
110-115 |
116-120 |
121-126 |
127-131 |
132 or more |
||
|
|
0-19 |
12 |
11 |
10 |
8 |
8 |
7 |
7 |
|
0-3 |
20-39 40-59 |
12 13 |
11 12 |
10 11 |
9 9 |
8 9 |
8 8 |
7 8 |
|
|
60 or more |
— |
— |
— |
— |
— |
— |
— |
|
|
0-19 |
13 |
12 |
11 |
9 |
8 |
7 |
7 |
|
3-7 |
20-39 40-59 |
13 14 |
12 13 |
11 12 |
10 10 |
0 10 |
8 9 |
8 8 |
|
|
60 or more |
15 |
14 |
12 |
11 |
10 |
9 |
9 |
|
|
0-19 |
14 |
13 |
11 |
10 |
9 |
8 |
8 |
|
7 11 |
20-39 40-59 |
15 16 |
14 14 |
11 12 |
10 11 |
9 10 |
9 10 |
9 9 |
|
|
60 or more |
17 |
15 |
13 |
11 |
10 |
10 |
10 |
|
|
0-19 |
15 |
14 |
13 |
12 |
11 |
9 |
9 |
|
11-15 |
20-39 40-59 |
16 17 |
15 16 |
13 14 |
12 12 |
11 12 |
10 11 |
10 10 |
|
|
60 or more |
18 |
16 |
14 |
13 |
12 |
11 |
11 |
|
|
0-19 |
17 |
16 |
14 |
13 |
12 |
11 |
10 |
|
15-20 |
20-39 40-59 |
18 19 |
17 18 |
15 15 |
14 14 |
13 14 |
11 12 |
11 12 |
|
|
60 or more |
20 |
19 |
16 |
15 |
14 |
13 |
12 |
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-17
Chapter 9
Figure 9-5. Group index for determining average cement requirements
9-18 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
Table 9-10. Average cement requirements of miscellaneous materials
|
Type of Miscellaneous Material |
Estimated Cement Content and That Used in Moisture-Density Test |
Cement Contents for Wet-Dry and Freeze- Thaw Tests, Percent by Weight |
|
|
Percent by Volume |
Percent by Volume |
||
|
Shell soils |
8 |
7 |
5-7-9 |
|
Limestone screenings |
75 |
3-5-7 |
|
|
Red dog |
9 |
8 |
6-8-10 |
|
Shale or disintegrated shale |
11 |
10 |
8-10-12 |
|
Caliche |
8 |
7 |
5-7-9 |
|
Cinders |
8 |
8 |
6-8-10 |
|
Chert |
9 |
8 |
6-8-10 |
|
Chat |
8 |
7 |
5-7-9 |
|
Marl |
11 |
11 |
9-11-13 |
|
Scoria (containing material retained on the No 4 sieve) |
12 |
11 |
9-11-13 |
|
Scoria (not containing |
|
|
|
|
material retained on the No |
8 |
7 |
5-7-9 |
|
4 sieve) |
|
|
|
|
Air-cooled slag |
9 |
7 |
5-7-9 |
|
Water-cooled slag |
10 |
12 |
10-12-14 |
LIME
9-53. Experience has shown that lime reacts with medium-, moderately fine-, and fine-grained soils to produce decreased plasticity, increased workability and strength, and reduced swell. Soils classified according to the USCS as (CH), (CL), (MH), (ML), (SC), (SM), (GC), (GM), (SW-SC), (SP-SC), (SM-
SC), (GW-GC), (GP-GC), and (GM-GC) should be considered as potentially capable of being stabilized with lime.
9-54. If the soil temperature is less than 60 degrees Fahrenheit and is not expected to increase for one month, chemical reactions will not occur rapidly. Thus, the strength gain of the lime-soil mixture will be minimal. If these environmental conditions are expected, the lime may be expected to act as a soil modifier. A possible alternative stabilizer might be considered for use. Lime-soil mixtures should be scheduled for construction so that sufficient durability is gained to resist any freeze-thaw cycles expected.
9-55. If heavy vehicles are allowed on the lime-stabilized soil before a 10- to 14-day curing period, pavement damage can be expected. Lime gains strength slowly and requires about 14 days in hot weather and 28 days in cool weather to gain significant strength. Unsurfaced lime-stabilized soils abrade rapidly under traffic, so bituminous surface treatment is recommended to prevent surface deterioration.
9-56. Lime can be used either to modify some of the physical properties and thereby improve the quality of a soil or to transform the soil into a stabilized mass, which increases its strength and durability. The amount of lime additive depends on whether the soil is to be modified or stabilized. The lime to be used may be either hydrated or quicklime, although most stabilization is done using hydrated lime. The reason is that quicklime is highly caustic and dangerous to use. The design lime contents determined from the criteria presented herein are for hydrated lime. As a guide, the lime contents determined herein for hydrated lime should be reduced by 25 percent to determine a design content for quicklime.
Modification
9-57. The amount of lime required to improve the quality of a soil is determined through the same trial- and-error process used for cement-modified soils.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-19
Chapter 9
Stabilization
9-58. To take advantage of the thickness reduction criteria, the lime-stabilized soil must meet the unconfined compressive strengths and durability requirements shown in tables 9-4 and 9-5, pages 9-14 and 9-15, respectively.
9-59. When lime is added to a soil, a combination of reactions begins to take place immediately. These reactions are nearly complete within one hour, although substantial strength gain is not reflected for some time. The reactions result in a change in both the chemical composition and the physical properties. Most lime has a pH of about 12.4 when placed in a water solution. Therefore, the pH is a good indicator of the desirable lime content of a soil-lime mixture. The reaction that takes place when lime is introduced to a soil generally causes a significant change in the plasticity of the soil, so the changes in the PL and the LL also become indicators of the desired lime content. Two methods for determination of the initial design lime content are presented in the following steps:
Step 1. The preferred method is to prepare several mixtures at different lime-treatment levels and determine the pH of each mixture after one hour. The lowest lime content producing the highest pH of the soil-lime mixtures is the initial design lime content. Procedures for conducting a pH test on lime-soil mixtures are presented in TM 5-530. In frost areas, specimens must be subjected to the freeze-thaw test as discussed in step 2 below. An alternate method of deter- mining an initial design lime content is shown in figure 9-6. Specific values required to use this figure are the PI and the percent of material passing the Number 40 sieve. These properties are determined from the PL and the gradation test on the untreated soil for expedient construction; use the amount of stabilizer determined from the pH test or figure 9-6.
Step 2. After estimating the initial lime content, conduct a compaction test with the lime-soil mixture. The test should follow the same procedures for soil-cement except the mixture should cure no less than one hour and no more than two hours in a sealed container before molding. Compaction will be accomplished in five layers using 55 blows of a 10-pound hammer having an 18-inch drop (CF 55). The moisture density should be determined at lime con- tents equal to design plus 2 percent and design plus 4 percent for the preferred method at design + 2 percent for the alternate method. In frost areas, cured specimens should be subjected to the 12 freeze-thaw cycles (omit wire brush portion) or other applicable freeze- thaw procedures, followed by frost susceptibility determinations in standard laboratory freezing tests. For lime-stabilized or lime-modified soil used in lower layers of the base course, the frost susceptibility (deter- mined after freeze-thaw cycling) should meet the requirements for the base course. If lime-stabilized or lime- modified soil is used as the subgrade, its frost susceptibility (determined after freeze-thaw cycling) should be the basis of the pavement thickness design if the reduced subgrade strength design method is applied.
Step 3. Uniformed compression tests should be performed at the design percent of maximum density on three specimens for each lime content tested. The design value would then be the minimum lime content yielding the required strength. Procedures for the preparation of lime-soil specimens are similar to those used for cement-stabilized soils with two exceptions: after mixing, the lime-soil mixture should be allowed to mellow for not less than one hour nor more than two hours; after compaction, each specimen should be wrapped securely to prevent moisture loss and should be cured in a constant-temperature chamber at
73 degrees Fahrenheit +2 degrees Fahrenheit for 28 days. Procedures for conducting unconfined compression tests are similar to those used for soil-cement specimens except that in lieu of moist curing, the lime-soil specimens should remain securely wrapped until testing.
Step 4. Compare the results of the unconfined compressive tests with the criteria in table 9-4. The design lime content must be the lowest lime content of specimens meeting the strength criteria indicated.
9-20 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
Figure 9-6. Alternate method of determining initial design lime content
Other Additives
9-60. Lime may be used as a preliminary additive to reduce the PI or alter gradation of a soil before adding the primary stabilizing agent (such as bitumen or cement). If this is the case, then the design lime content is the minimum treatment level that will achieve the desired results. For nonplastic and low-PI materials in
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-21
Chapter 9
which lime alone generally is not satisfactory for stabilization, fly ash may be added to produce the necessary reaction.
FLY ASH
9-61. Fly ash is a pozzolanic material that consists mainly of silicon and aluminum compounds that, when mixed with lime and water, forms a hardened cementitious mass capable of obtaining high compression strengths. Fly ash is a by-product of coal-fired, electric power-generation facilities. The liming quality of fly ash is highly dependent on the type of coal used in power generation. Fly ash is categorized into two broad classes by its calcium oxide (CaO) content. They are—
Class C
9-62. This class of fly ash has a high CaO content (12 percent or more) and originates from subbituminous and lignite (soft) coal. Fly ash from lignite has the highest CaO content, often exceeding 30 percent. This type can be used as a stand-alone stabilizing agent. The strength characteristics of Class C fly ash having a CaO less than 25 percent can be improved by adding lime. Further discussion of fly ash properties and a listing of geographic locations where fly ash is likely to be found are in appendix B.
Class F
9-63. This class of fly ash has a low CaO content (less than 10 percent) and originates from anthracite and bituminous coal. Class F fly ash has an insufficient CaO content for the pozzolanic reaction to occur. It is not effective as a stabilizing agent by itself; however, when mixed with either lime or lime and cement, the fly ash mixture becomes an effective stabilizing agent.
Lime Fly Ash Mixtures
9-64. LF mixtures can contain either Class C or Class F fly ash. The LF design process is a four-part process that requires laboratory analysis to determine the optimum fines content and lime-to-fly-ash ratio.
Step 1. Determine the optimum fines content. This is the percentage of fly ash that results in the maximum density of the soil mix. Do this by conducting a series of moisture-density tests using different percentages of fly ash and then determining the mix level that yields maximum density. The initial fly ash content should be about 10 percent based on the weight of the total mix. Prepare test samples at increasing increments (2 percent) of fly ash, up to 20 percent. The design fines content should be 2 percent above the optimum fines content. For example, if 14 percent fly ash yields the maximum density, the design fines content would be 16 percent. The moisture density relation would be based on the 16 percent mixture.
Step 2. Determine the rates of lime to fly ash. Using the design fines con- tent and the OMC determined in step 1, prepare triplicate test samples at LF ratios of 1:3, 1:4, and 1:5. Cure all test samples in sealed containers for seven days at 100 degrees Fahrenheit.
Step 3. Evaluate the test samples for unconfined compressive strength. If frost is a consideration, subject a set of test samples to 12 cycles of freeze-thaw durability tests (refer to FM 5-530 for actual test procedures).
Step 4. Determine the design LF ratio. Compare the results of the unconfined strength test and freeze-thaw durability tests with the minimum requirements found in tables 9-4 and 9-5, pages 9-14 and 9-15, respectively. The LF ratio with the lowest lime content that meets the required unconfined compressive strength and demonstrates the required durability is the design LF content. The treated material must also meet frost susceptibility requirements as indicated in Special Report 83-27. If the mixture meets the durability requirements but not the strength requirements, it is considered to be a modified soil. If neither strength nor
9-22 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
durability criteria are met, a different LF content may be selected and the testing procedure repeated.
Lime-Cement-Fly Ash (LCF) Mixtures
9-65. The design methodology for determining the LCF ratio for deliberate construction is the same as for LF except cement is added in step 2 at the ratio of 1 to 2 percent of the design fines content. Cement may be used in place of or in addition to lime; however, the design fines content should be maintained.
9-66. When expedient construction is required, use an initial mix proportion of 1 percent portland cement, 4 percent lime, 16 per-cent fly ash, and 79 percent soil. Minimum unconfined strength requirements (see table 9-4) must be met. If test specimens do not meet strength requirements, add cement in ½ percent increments until strength is adequate. In frost-susceptible areas, durability requirements must also be satisfied (see table 9-5).
9-67. As with cement-stabilized base course materials, LCF mixtures containing more than 4 percent cement cannot be used as base course material under Air Force airfield pavements.
BITUMINOUS MATERIALS
9-68. Types of bituminous-stabilized soils are—
Soil Gradation
9-69. The recommended soil gradations for subgrade materials and base or subbase course materials are shown in table 9-11 and table 9-12, page 9-24, respectively. Mechanical stabilization may be required to bring soil to proper gradation.
Table 9-11. Recommended gradations for bituminous-stabilized subgrade materials
|
Sieve Size |
Percent Passing |
|
3-in |
100 |
|
Number 4 |
50 – 100 |
|
Number 30 |
38 – 100 |
|
Number 200 |
2 – 30 |
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-23
Chapter 9
Table 9-12. Recommended gradations for bituminous- stabilized base and subbase materials
|
Sieve Size |
1½ in Max |
1 in Max |
¾ in Max |
½ in Max |
|
1 ½-in |
100 |
— |
— |
— |
|
1-in |
84 ± 9 |
100 |
— |
— |
|
¾-in |
76 ± 9 |
83 ± 9 |
100 |
— |
|
½-in |
66 ± 9 |
73 ± 9 |
82 ± 9 |
100 |
|
⅜-in |
59 ± 9 |
64 ± 9 |
72 ± 9 |
83 ± 9 |
|
No 4 |
45 ± 9 |
48 ± 9 |
54± 9 |
62 ± 9 |
|
No 8 |
35 ± 9 |
37 ± 9 |
41 ± 9 |
47 ± 9 |
|
No 16 |
27 ± 9 |
28 ± 9 |
32 ± 9 |
36 ± 9 |
|
No 30 |
20 ± 9 |
21 ± 9 |
24 ± 9 |
28 ± 9 |
|
No 50 |
14 ± 7 |
16 ± 7 |
17 ± 7 |
20 ± 7 |
|
No 100 |
9 ± 5 |
11 ± 5 |
12 ± 5 |
14 ± 5 |
|
No 200 |
5 ± 2 |
5 ± 2 |
5 ± 2 |
5 ± 2 |
Types of Bitumen
9-70. Bituminous stabilization is generally accomplished using—
9-71. The type of bitumen to be used depends on the type of soil to be stabilized, the method of construction, and the weather conditions. In frost areas, the use of tar as a binder should be avoided because of its high-temperature susceptibility. Asphalts are affected to a lesser extent by temperature changes, but a grade of asphalt suitable to the prevailing climate should be selected. Generally the most satisfactory results are obtained when the most viscous liquid asphalt that can be readily mixed into the soil is used. For higher quality mixes in which a central plant is used, viscosity-grade asphalt cements should be used. Much bituminous stabilization is performed in place with the bitumen being applied directly on the soil or soil- aggregate system. The mixing and compaction operations are conducted immediately thereafter. For this type of construction, liquid asphalts (cutbacks and emulsions) are used. Emulsions are preferred over cutbacks because of energy constraints and pollution control effects. The specific type and grade of bitumen depends on the characteristics of the aggregate, the type of construction equipment, and the climatic conditions. Table 9-13 lists the types of bituminous materials for use with soils having different gradations.
9-24 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
Table 9-13. Bituminous materials for use with soils of different gradations
|
Material |
Grade |
|
Open-Graded Aggregate |
|
|
Rapid- and medium-curing liquid asphalts |
RC-250, RC-800, and MC-3000 |
|
Medium-setting asphalt emulsion |
MS-2 and CMS-2 |
|
Well-Graded Aggregate With Little or No Material Passing the No 200 Sieve |
|
|
Rapid- and medium-curing liquid asphalts |
RC-250, RC-800, MC-250, and MC-800 |
|
Slow-curing liquid asphalts |
SC-250 and SC-800 |
|
Medium-setting and slow-setting asphalt emulsions |
MS-2, CMS-2, S-1, and CSS-1 |
|
Aggregate With a Considerable Percentage of Fine Aggregate and Material Passing the No 200 Sieve |
|
|
Medium-curing liquid asphalts |
MC-250 and MC-800 |
|
Slow-curing liquid asphalts |
SC-250 and SC-800 |
|
Slow-setting asphalt emulsions |
SS-1, SS-1h, CSS-1, and CSS-1h |
|
Medium-setting asphalt emulsions |
MS-2 and CMS-2 |
|
The simplest type of bituminous stabilization is the application of liquid asphalt to the surface of an unbound aggregate road. For this type of operation, the slow- and medium- curing liquid asphalts SC-70, SC-250, MC-70, and MC-250 are used. |
|
|
Legend: RC rapid curing SS slow setting MC medium curing CMS cement-modified soil MS medium setting CSS cationic slow setting SC slow curing |
|
Mix Design
9-72. Guidance for the design of bituminous-stabilized base and subbase courses is contained in TM 5-822-8. For subgrade stabilization, the following equation may be used for estimating the preliminary quantity of cutback asphalt to be selected:
𝑃 = 0.2(a) + 0.07(b) + 1.5c + 0.20(d) 𝑋 100 (100 − 𝑆)
where—
P = percent of cutback asphalt by weight of dry aggregate
a = percent of mineral aggregate retained on Number 50 sieve
b = percent of mineral aggregate passing Number 50 and retained on Number 100 sieve c = percent of mineral aggregate passing Number 100 and retained on Number 200 sieve d = percent of mineral aggregate passing Number 200 sieve
S = percent solvent
9-73. The preliminary quantity of emulsified asphalt to be used in stabilizing subgrades can be determined from table 9-14, page 9-26. Either cationic or anionic emulsions can be used. To ascertain which type of emulsion is preferred, first determine the general type of aggregate. If the aggregate contains a high content of silica, as shown in figure 9-7, page 9-26, a cationic emulsion should be used (see figure 9-8, page 9-27). If the aggregate is a carbonate rock (limestone, for example), an anionic emulsion should be used.
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-25
Chapter 9
Table 9-14. Emulsified asphalt requirements
|
Percent Passing No 200 Sieve |
Pounds of Emulsified Asphalt Per 100 lbs of Dry Aggregate at Percent Passing No10 Sieve |
|||||
|
|
≤50 |
60 |
70 |
80 |
90 |
100 |
|
0 |
6.0 |
6.3 |
6.5 |
6.7 |
7.0 |
7.2 |
|
2 |
6.3 |
6.5 |
6.7 |
7.0 |
7.2 |
7.5 |
|
4 |
6.5 |
6.7 |
7.0 |
7.2 |
7.5 |
7.7 |
|
6 |
6.7 |
7.0 |
7.2 |
7.5 |
7.7 |
7.9 |
|
8 |
7.0 |
7.2 |
7.5 |
7.7 |
7.9 |
8.2 |
|
10 |
7.2 |
7.5 |
7.7 |
7.9 |
8.2 |
8.4 |
|
12 |
7.5 |
7.7 |
7.9 |
8.2 |
8.4 |
8.6 |
|
14 |
7.2 |
7.5 |
7.7 |
7.9 |
8.2 |
8.4 |
|
16 |
7.0 |
7.2 |
7.5 |
7.7 |
7.9 |
8.2 |
|
18 |
6.7 |
7.0 |
7.2 |
7.5 |
7.7 |
7.9 |
|
20 |
6.5 |
6.7 |
7.0 |
7.2 |
7.5 |
7.7 |
|
22 |
6.3 |
6.5 |
6.7 |
7.0 |
7.2 |
7.5 |
|
24 |
6.0 |
6.3 |
6.5 |
6.7 |
7.0 |
7.2 |
|
25 |
6.2 |
6.4 |
6.6 |
6.9 |
7.1 |
7.3 |
Figure 9-7. Classification of aggregate
9-26 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
Figure 9-8. Approximate effective range of cationic and aniomic emulsion of various types of asphalt
9-74. Figure 9-9 and figure 9-10, page 9-28, can be used to find the mix design for asphalt cement. These preliminary quantities are used for expedient construction. The final design content of asphalt should be selected based on the results of the Marshall stability test procedure. The minimum Marshall stability recommended for subgrades is 500 pounds; for base courses, 750 pounds is recommended. If a soil does not show increased stability when reasonable amounts of bituminous materials are added, the gradation of the soil should be modified or another type of bituminous material should be used. Poorly graded materials may be improved by adding suitable fines containing considerable material passing a Number 200 sieve. The amount of bitumen required for a given soil increases with an increase in percentage of the finer sizes.
Figure 9-9. Determination of asphalt grade for expedient construction
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-27
Chapter 9
Figure 9-10. Selection of asphalt cement content
SECTION II – DESIGN CONCEPTS
STRUCTURAL CATEGORIES
9-75. Procedures are presented for determining design thicknesses for two structural categories of pavement. They are—
9-76. Typical examples of these pavements are indicated in figure 9-11.
9-77. A typical single-layer pavement is a stabilized soil structure on a natural subgrade. The stabilized layer may be mixed in place or premixed and later placed over the existing subgrade. A waterproofing surface such as membrane or a single bituminous surface (SBST) or a double bituminous surface treatment (DBST) may also be provided. A multilayer structure typically consists of at least two layers, such as a base and a wearing course, or three layers, such as a subbase, a base, and a wearing course. A thin waterproofing course may also be used on these structures. Single-layer and multilayer pavement design procedures are presented for all categories of roads and for certain categories of airfields as indicated in table 9-15, page 9-30.
9-78. Both single-layer and multilayer pavement structures may be constructed under either the expedient or nonexpedient concept. Different structural designs are provided to allow the design engineer wider latitude of choice. However, single-layer structures are often associated with expedient construction rather than nonexpedient construction, and multilayers are nonexpedient and permanent. Certain considerations should be studied to determine whether to use a single-layer or multilayer design under either concept.
9-79. The overall concept of design as described herein can be defined in four basic determinations as indicated in table 9-16, page 9-30.
9-28 TM 3-34.64/MCRP 3-17.7G 25 September 2012
Soil Stabilization for Roads and Airfields
Figure 9-11. Typical sections for single-layer and multilayer design
25 September 2012 TM 3-34.64/MCRP 3-17.7G 9-29
Chapter 9
Table 9-15. Thickness design procedures by airfield category
|
Airfield Category |
Thickness Design Procedures |
|||
|
Area |
Length |
Usage |
Single-Layer |
Multilayer |
|
|
2,000′ |
Load |
Yes |
No |
|
|
2,500′ |
Liaison |
Yes |
No |
|
Close battle |
|
Surveillance Load |
Yes Yes |
No No |
|
|
3,000′ |
Load |
Yes |
No |
|
Rear |
3,500′
6,000′ |
Liaison Surveillance Load Load Tactical |
Yes Yes Yes Yes Yes |
No Yes Yes Yes Yes |
|
|
6,000′ |
Load |
Yes |
Yes |
|
COMMZ |
10,000′ |
Load |
Yes |
Yes |
|
|
8,000′ |
Tactical |
Yes | |