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Effective Shoulder Design and Maintenance
tech transfer summary
June 2007
RESEARCH PROJECT TITLE
Effective Shoulder Design and Maintenance
SPONSORS
Iowa Highway Research Board (TR-531) Iowa Department of Transportation (CTRE Project 05-198)
PRINCIPAL INVESTIGATOR
David J. White
Associate Professor, Civil, Construction, and Environmental Engineering
Iowa State University 515-294-1463
CO-AUTHORS
Mohamed Mekkawy Charles Jahren Duane Smith Muhannad Suleiman
KEYWORDS
edge drop-off, geogrid, granular shoulders, rutting, shoulder stabilization
MORE INFORMATION
PGA
Iowa State University
2711 S. Loop Drive, Suite 4700
Ames, IA 50010-8664
515-294-8103
The Partnership for Geotechnical Advancement (PGA) is part of the Center for Transportation Research and Education (CTRE) at Iowa State University. The mission of the PGA is to increase highway performance in a cost-effective manner by developing and implementing methods, materials, and technologies to solve highway construction problems in a continuing and sustainable manner.
The sponsors of this research are not responsible for the accuracy of the information presented herein. The conclusions expressed in this publication are not necessarily those of the sponsors.
Objectives
Problem Statement
Shoulders are an important element of the highway system, providing space for emergency stops, a recovery zone for errant vehicles, struc- tural support for the pavement, drainage, improved sight distance, passage for bicyclists, and increased roadway width to accommodate agricultural vehicles.
Granular shoulders are a commonly used shoulder option. Although the construction of granular shoulders is initially less expensive than that of paved shoulders (by up to 70%), granular shoulders often add expense later because they require more frequent maintenance and have performance problems. Common granular shoulder problems include edge drop-off, shoulder rutting, erosion by water or wind, ir- regular slope, and settlement of the subgrade soil.
Current maintenance procedures for granular shoulders in Iowa typically involve shoulder regrading, placing additional material, and recompaction. These maintenance and repair problems are costly and require investigation to illuminate the factors that contribute to these problems.
Common granular shoulder problems
Change in granular material graduation by wind erosion
Field Observations of Granular Shoulder Sites
In field investigations across Iowa, various granular shoulders were inspected. These inspections revealed the following:
Edge drop-off along the outside shoulder caused by vehicle off-tracking
Severe shoulder rutting caused by bearing capacity failure of the soft underlying subgrade
Monitoring tire-aggregate interaction using stationary high-speed camera
Digital images captured by camera showing elevated aggregate particles
Observation of Vehicle Tire-Aggregate Interaction
Vehicle off-tracking is a prime contributor to the development of edge drop-off. An approach was therefore conceived to study vehicle tire-aggregate interaction for unpaved shoulders using high-speed cameras. To capture the vehicle tire-aggregate behavior, a pickup truck was driven on an unpaved shoulder at 40 mph while the high-speed camera captured the aggregate trajectories. The results show that aggregates are elevated upward and pushed in the opposite direction of vehicle travel.
Evaluation of Granular Shoulder Stabilization Techniques
To test different chemical and mechanical stabilization techniques, six problematic granular shoulder sections were selected. The test sections were either experiencing edge drop-off or severe rutting due to a soft subgrade layer.
Stabilization of Granular Layer
The granular layer at four of the six test sections was stabilized using either a polymer emulsion, foamed asphalt with fly ash, soybean oil, or portland cement.
Performance summary of chemically stabilized granular layer at four test sections
Test Section |
Location |
Stabilizer |
Application |
Observations |
1 |
Highway 122, |
Soil Sement polymer |
Topical |
Edge drop-off started to redevelop after 30 days |
|
Clear Lake, IA |
emulsion |
|
|
2 |
I-35 |
Foamed asphalt |
Full depth |
Failed along the pavement edge due to water |
|
|
|
reclamation |
migration and off-tracking vehicles |
3 |
Highway 18, |
Soybean oil |
Reclamation to a |
Separation of soy oil and emulsion obstructed |
|
Rudd, IA |
|
depth of 6 inches (0.7gal/yd2) |
the construction process |
4 |
16th St., |
Portland cement |
Reclamation to a |
Improved the strength of the section; however, |
|
Ames, IA |
|
depth of 6 inches |
signs of edge drop-off and washboarding was |
|
|
|
(10% cement) |
noted due to poor mixing and water control |
Stabilization of Subgrade Layer
Two of the six test sections that had soft subgrade layers were stabilized using class C fly ash and different geosynthetic prod- ucts.
For the fly ash stabilization techniques, shoulder sections located on Highway 34 near Batavia, Iowa, were undergoing severe rutting due to soft subgrade layers. The upper 12 in. of the subgrade were stabilized with 15% to 20% class C fly ash. The section was monitored for 19 months. Fly ash stabilization was successful in eliminating shoulder rutting and improving the overall long-term performance of the shoulder section.
For the geogrid stabilization techniques, the inside granular shoulder on Highway 218 near Nashua, Iowa, was experiencing severe rutting due to soft subgrade conditions. The section was stabilized by placing various types of geogrid at the interface between the granular and subgrade layers, and the test section was monitored for 10 months. Four different geogrids were evaluated, and all considerably improved the performance of the shoulder test section and eliminated shoulder rutting.
Performance of the fly ash–stabilized shoulder section after seven months: stabilized section (top) and control section (bottom)
Rolling the geogrid over the soft subgrade layer near Nashua, Iowa
Inside shoulder near Nashua, Iowa: before stabilization (top) and two months after stabilization (bottom)
Dynamic cone penetrometer test results with time for the fly ash–stabilized sections:
(a) 3.0 ft and (b) 6.0 ft from the pavement edge
Plate load test results 10 months after construction, geogrid-stabilized sections
Laboratory Evaluation of Stabilizers
To develop the findings from the field study, different stabilizers were selected to conduct a laboratory study of stabilization methods for shoulder granular material. The stabilizers selected for this study were Soil Sement polymer emulsion, Soiltac polymer emulsion, portland cement, and Dustlock soybean soil. The results of the study show that soybean oil can be a good candidate in stabilizing the shoulder granular layer.
Summary of compressive strength results for the different stabilizers tested
Laboratory Box Study
To simulate a shoulder section over- lying a soft subgrade, a laboratory study was conducted. Cyclic loading with three loading stages was used to study the performance of the labo- ratory model under different mechanical and chemical stabilization techniques. At each stage, a pressure was applied and sustained for 5,000 cycles at a frequency of 1 Hz. These pressures were 40, 80, and 120 psi. The soil properties before and after each test and the cumulative soil dis- placement over an increasing number of cycles were recorded.
Summary of cumulative measured soil displacement for all laboratory box tests
Schematic of the laboratory apparatus setup: Steel frame and hydraulic actuator for loading the stabilized soil (left) and steel box used to contain the soil (right)
Shoulder Design Charts
Using the results of these field and laboratory data, shoulder design charts were developed to help mitigate shoulder rutting caused by the bearing capacity failure of the subgrade layer. The design charts, validated by the field and laboratory data, were based on the semi-empirical method proposed by Giroud and Han (2004) and an equation developed by the U.S. Army Corps of Engineers in 1989 for predicting surface rutting for low-volume roads (Bolander et al. 1995). The charts can be a rapid tool for designing new granular shoulders and predicting the behavior of existing ones.
Shoulder design chart showing the relationship between subgrade CBR and expected granular shoulder rut depth for different load cycles
Economic Analysis of Shoulder Construction, Maintenance, and Repair
To compare the cost-benefit scenarios for possible shoulder improvements and repairs, a cost estimate analysis was conducted. The analysis demonstrated that the monetary benefits of the reduced granular shoulder maintenance costs are small in comparison to the required investment (construction costs), though granular shoulders do result in a small reduction in the present value of the necessary cash outflow. However, it should be noted that the investment in shoulder improvements is often made to garner an improved level of service, greater safety, and other benefits that are difficult to quantify.
Calculations for present worth of granular shoulder expenditures
Recommendations
Shoulder Construction
Subgrade Repair
Edge Drop-Off
Copyright Soilworks, LLC 2003-. All Rights Reserved. Soilworks®, Soiltac®, Gorilla-Snot®, and Durasoil®are registered trademarks of Soilworks, LCC.
Copyright Soilworks, LLC 2003-. All Rights Reserved. Soilworks®, Soiltac®, Gorilla-Snot®, and Durasoil® are registered trademarks of Soilworks, LCC.