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TITLE: NEW MEXICO INSTITUTE OF TECHNOLOGY CHEMICAL SOIL STABILIZATION HANDOUT
(SECTIONS 2.3.4 TO 2.4 ONLY)
2.3.4 Soil Stabilization with Traditional and Non-Traditional Stabilizers
The artificial traditional admixtures in order of their usage are:
• Portland Cement (and Cement-Fly Ash)
• Lime (and Lime-Fly Ash)
• Fly Ash
• Fly Ash with Cement or Lime
• Bitumen and Tar
• Cement Kiln Dust (CKD)
In recent years an increasing number of non-traditional additives have been developed for soil stabilization purposes. These stabilizers are becoming popular due to their relatively low cost, ease of application, and short curing time. Since the chemical formulas of the products are modified often based on market tendency, it is rather difficult to evaluate the performance of a single product. Non-traditional stabilizers are:
• Polymers Based Products
• Copolymer Based Products
• Fiber Reinforcement
• Calcium Chloride
• Sodium Chloride
22.214.171.124 Cement Stabilization
Portland cement as an additive modifies and improves the quality of soil for the purpose of increasing strength and durability. Cement also has been used to control the erosion of inorganic soils (Oswell, and Joshi, 1986). Oswell, and Joshi (1986) found a good correlation between unconfined compressive strength and erosion resistance. As the compressive strength increases the erosion rate decreases.
Cement can be applied to stabilize any type of soil, except soils with organic content greater than 2% or having pH lower than 5.3 (ACI 230.1R-90, 1990). Kezdi (1979) reports that cement treatment slightly increases the maximum dry density of sand and highly plastic clays but it decreases the maximum dry density of silt. In contrast studies by Tabatabi (1997) shows that cement increases the optimum water content but decreases the maximum dry density of sandy soils. Cement increases plastic limit and reduces liquid limit, which mainly reduces plasticity index (Kezdi, 1979). The other significant effects of cement-soil stabilization is reduction in shrinkage and swell potential, increase in strength, elastic modulus, and resistance against the effect of moisture, freeze, and thaw. Cement treated soils show a brittle behavior compare of non-treated soils. Addition of cement can affect strength and durability of the treated soils as follows.
The effect of cement content and curing time on unconfined compressive strength are shown in Figures 4 and 5. Figure 4 shows that unconfined compressive strength for both fine-grained and coarse-grained soils increases with increasing cement content. The 28-day unconfined compressive strength is proportional to the cement content; it varies from 40 percent of cement content for fine-grained soils to 150 percent cement content for coarse-grained soils (Mitchell, 1976).
Figure 4: Relationship between unconfined compressive strength and cement content (Mitchell, 1976)
The unconfined compressive strength increases by increasing curing time (Figure 5). Improvement in unconfined compressive strength due to curing time for coarse-grained soils is more significant compared to fine-grained soils. Equation 1 shows the empirical relationship between unconfined compressive strength and curing time for a given soil and cement content (Mitchell, 1976):
Equation 1: Unconfined Compressive Strength (psi)
Figure 5: Effect of curing time on unconfined compressive strength of cement (Mitchell, 1976)
Lo and Wardani (2002) report that addition of stabilization agent increases the cohesion significantly. Figure 6 shows the effect of cement content on cohesion for coarse-grained and fine-grained soil. Equation 2 shows that cohesion is a function of unconfined compressive strength (Mitchell, 1976).
Equation 2: c = 7.0 + 0.225(UCS) where UCS is unconfined compressive strength (psi) and c is cohesion.
Cement increases both cohesion and internal friction angle of the soil (Uddin et al., 1997; Bragdo eral, 1996) though for some cement treated soils internal friction angle remains constant (Balmer, 1958).
Figure 6: Effect of cement content on cohesion for several coarse-grained and fine- grained soils (Mitchell, 1976)
Unconfined compressive strength increases with increasing relative compaction as well (White and Gnanendran, 2005). Figure 7 shows the relationship between the uniaxial compressive strength and relative compaction. Delay in mixing and compaction decreases the unconfined compressive strength.
Figure 7: Unconfined compressive strength versus relative compaction for cement treated material (White and Gnanendran, 2005)
In most of soil stabilization projects achieving a maximum durability is desirable. Cement treated soils have a good reputation for having a good resistance against freeze-thaw and wet-dry cycling tests. Figure 8 shows the relationship between unconfined compressive strength and durability of cement treated soils. It is evident that resistance against freeze-thaw and wet-dry cycling increase with increasing unconfined compressive strength.
Figure 8: Relationship between unconfined compressive strength and durability of cement treated soils (ACI 230, IR- 90, 1990)
126.96.36.199 Lime Stabilization
Lime is one of the additives, which is widely used in stabilization of fine- grained soils. Various forms of lime such as hydrated high-calcium lime (Ca(OH)2), monohydrated dolomitic lime (Ca(OH)2 — MgO ), and dolomitic quicklime (CaO — MgO) have been successfully used as stabilizing agent for many years. Quick lime (calcium oxide) is delivered in the form of coarse-grained powder. It reacts quickly with water producing hydrated or slaked lime, generating heat and volume change (Equation 3):
Equation 3: CaO + H2O = Ca(OH)2 + 65.3 kJ / mol
Quick lime must be handled with care; it can burn the skin in the presence of moisture it also can cause corrosion of equipment (Kezdi, 1979). The main contribution of lime to the strength of soil is from its ability to create cementation between soil particles. The higher the surface area of the soil, the more effective this process of lime cementation is.
188.8.131.52.1 Chemical Reactions in Lime treated Soils
Several reactions occur when lime is added to clay in the presence of water. The reactions are cation exchange, flocculation-agglomeration, carbonation, and pozzolanic reaction (Mallela et al., 2004). Cation exchange and flocculation- agglomeration reaction occur immediately after mixing and these reactions cause immediate changes in strength, plasticity index, and workability of the soils (sections 184.108.40.206.2 and 220.127.116.11.3). Carbonation is reaction of carbon dioxide in the open air or voids in the ground with lime, which forms a relatively weak cementing agent. Cementation caused by carbonation on the clay surface results a rapid initial increase in strength (Hausmann, 1990). Pozzolanic reaction occurs between lime and silica and alumina of the clay mineral and produces cementing material including calcium- silicate-hydrates and calcium alumina hydrates. The long term result of pozzolanic reactions (Equation 4 and 5) is solidification of the soil (Hausmann, 1990). Rate of the pozzolanic reactions depends on time and temperature.
Equation 4: Ca(OH)2 + SiO2 = CaO — SiO2 — H2O
Equation 5: Ca(OH)2 + Al2O3 = CaO — Al2O3 — H2O
18.104.22.168.2 Stress Strength Behavior
Lime treatment leads to significant increase in strength. The immediate increase in strength results from flocculation-agglomeration reaction and leads to better workability, whereas long-term strength gain is due to pozzolanic reactions (Thompson, 1966). Figure 9 shows that, as lime content increases unconfined compressive strength increases (Giffen et al., 1978).
Figure 9: Relationship between unconfined compressive strength and lime content of the treated soils with lime (after Giffen et al., 1978)
22.214.171.124.3 Atterberg Limits
Lime changes the Atterberg limits of the soils. An increase in lime content decreases the liquid limit, increases plastic limit and that leads to a significant decrease in plasticity index (Figure10).
Figure 10: Relationship between Atterberg limits with lime content (after Giffen et al., 1978)
126.96.36.199.4 Compaction Characteristics
Several changes occur when lime is added to the soil. Addition of lime increases optimum water content but decreases maximum dry density (Figure 11).
Figure 11: Change in compaction curve of a lime treated soil (after Giffen et al., 1978)
188.8.131.52.5 Swell Potential
As lime content increases, swell potential decreases significantly (Figure 12). It is evident that reduction in plasticity index leads to a significant decrease in swell potential (Giffen et al., 1978).
Figure 12: The effect of lime on shrinkage and swelling properties of soils (after Giffen, 1978)
184.108.40.206.6 Fatigue and Durability
Fatigue strength is the number of load cycles that a metrical can carry at a given stress level. Studies show that the immediate strength of lime is an important factor in resisting to higher number of freeze-thaw cycles (Mallela et al., 2004).
220.127.116.11.7 Optimum Lime Content
The required amount of lime to be added to the soil depends on the application. For modification purposes 2% to 3% lime by dry weight of soil is sufficient (Maher et al. 2005). For stabilization purposes, normally 5% to 10% lime by dry weight of the soil is suitable. To determine the optimum lime content for soil stabilization several methods have been suggested. Hilt and Davidson (1960) suggest the following equation for the optimum lime content:
Equation 6: Optimum Lime Content by Weight
18.104.22.168 Fly Ash Stabilization
Fly ash is a by-product of coal combustion in power plants. Fly ash contains silica, alumina, and different oxides and alkalis in its composition (Das, 1990). Its general appearance is light to dark gray powder and the size is the same as silt. The specific gravity of fly ash ranges from 1.9 to 2.5. There are two types of fly ash: type “C” and type “F”. Type “C” fly ash has significant amount of free lime. This type of fly ash causes pozzolanic and cementitious reactions. Addition of fly ash to lime and cement can improve the engineering properties of soil like lime or cement. However, fly ash properties are highly variable and depend on chemical composition of coal and combustion technology.
22.214.171.124 Soil Modification with Fly Ash and Cement or Lime
Addition of mixtures of lime (L) or cement (C) and fly ash (F) to aggregates (A) results in LFA, CFA or LCFA. For cohesionless soils with low plasticity fly ash treatment with cement will be more effective than lime, and for plastic soils fly ash treatment either with cement or lime is more effective (Hausmann, 1990). Less permeable layer is created by stabilization of a sandy road base with fly ash-cement mixture rather than cement alone. It is also convenient that cement-flyash-sand or cement-flyash-gravel mixtures shrink less than soil-cement mixtures (Natt and Joshi, 1984). Lime and fly ash reduce the maximum dry density of clay; the corresponding optimum water content tends to increase (Hausmann, 1990). Results of the research by While and Genendran (2005) indicate that one hour delay between mixing and compaction lead to significant increase in unconfined compressive strength of lime- fly ash treated soils. Construction of runway 9-27 at Houston International Airport is an example of the applications of Lime-Cement-Fly ash stabilization (Little et al., 2000). The engineering properties of mixture of fly ash with cement or lime are summarized as follows:
126.96.36.199.1 Compaction and Strength Characteristics of Fly Ash with Cement or Lime
Figures 13, 14, and 15 show the compaction and strength characteristics of compacted fly ash with addition of cement or lime. Figure 13 shows that fly ash tends to improve the dry density of soil better when combined with cement compared with lime (Hausmann, 1990). Figure 14 illustrates the relationship between maximum dry density and fly ash content for different percentage of cement (Hausmann, 1990).
Figure 13: Compaction curves for stabilized soils with 5% fly ash (class F) (after Hausmann, 1990)
Figure 14: Correlation between maximum dry density of sand-fly-ash-cement mixes with fly ash content (after Giffen et al., 1978)
Figure 15 shows that fly ash improves the soil strength better when combined with cement compared with lime (Hausmann, 1990). Figure 16 illustrates the relationship between unconfined compressive strength of fly ash treated soils and water content with mixture of cement or lime (Giffen et al., 1978). Figure 17 shows by increasing the fly ash content, the uniaxial compressive strength of sand-fly ash-cement increases (Giffen et al., 1978).
Figure 15: Unconfined compressive strength of fly ash (class F) as a function of the additive content (after Hausmann, 1990)
Figure16: Unconfined compressive strength of fly ash (class F) as a function of the water content of compaction (after Hausmann, 1990)
Figure17: Relationship between the 7-day compressive strength of a medium sand fly-cement with fly ash content (after Giffen et al., 1978)
188.8.131.52 Bitumen and Tar
Bitumen is a by-product that remains after distillation or evaporation of crude petroleum. Tar is the result of destructive distillation of coal and other carbonaceous material. Asphalt consists of mineral particles impregnated or cemented by bitumen. Most suitable bitumen admixtures are used in sandy gravel, sands, silty sands, fine crashed rocks, and highly plastic clays. Bitumen is not as common as other stabilizers like lime and cement, mainly because of its relatively high cost. The effectiveness of bitumen on cohesion and waterproofing depends on the nature of the soil. The amount of fine soil particles is important in workability of bitumen. Too much fines could present problem with mixing, stability, and uniformity. Lack of fine could result an unstable mixture, causing loss of adhesion (Kezdi, 1979). Bitumen and Tar have several effects on soil engineering properties of soil as follows:
184.108.40.206.1 Compaction Characteristics
Kezdi (1979) and Ingles (1973) reported that maximum dry density with constant compactive effort decreases with increasing bitumen content. However results of studies by Giffen et al., (1978) show a different behavior in soils that are stabilized by tar (Figure18). Figure 19 shows that optimum amount of water necessary to reach the maximum dry density decreases with increasing tar content (Giffen et al., 1978).
Figure 18: Relationship between the maximum dry density and tar content (after Giffen et al., 1978)
Figure 19: Relationship between the optimum water content and tar content (after Giffen et al., 1978)
The strength of compacted stabilized soil with bitumen is measured in terms of unconfined compressive strength. Figure 20 shows that initially there is an increase in strength with quantity of binder added until maximum strength is reached and after the peak there is a slow drop in unconfined compressive strength of the soil (Giffen et al., 1978).
Figure 20: Unconfined Compressive strength of tar-stabilized clayey sand (after Giffen et al., 1978)
220.127.116.11 Cement Kiln Dust (CKD)
CKD is a by-product of Portland cement manufacturing process. CKD is a fine material that is carried by hot gasses in a cement kiln and collected by a filter system during the production of cement. CKD contains mostly dried raw materials like limestone, sand, shale, and iron ore. Table 1 shows the percentage of chemical compositions of CKD. Values in the first row are provided by Lafarge North America (2007) taken form their cement plants. The second row are the mean values of 63 different CKD types calculated from published data by Sreekrishnavilasam et al. (2006).
Table1: Cement kiln dust (CKD) chemical compositions
CKD has variety applications in agriculture, construction, and waste stabilization. In agriculture, the high concentration of soluble potassium in CKD is found to be a good source of potassium for growing plants in treated soils (Lafond and Simard, 1999). CKD can also be used for soil stabilization in road construction. For example it has been used as a stabilizer in road base in Oklahoma and also as a filler in asphalt pavements (Miller and Azad, 2000). CKD and asphalt binder can create low ductile asphalt that has been successfully used in Europe for bridge waterproofing and protection (Bghdadi, Fatani, and Sabban, 1995). Another important application of CKD is in soil stabilization and modification of waste materials. CKD has been used to stabilize the coal mine waste effluents (Haynes and Kramer, 1982). Nearly 4 million tons of CKD is disposed every year in the Unites States (Miller and Zaman, 2000). The cement industry loses money with CKD disposal because of the raw material and energy that was wasted to produce CKD. Therefore using CKD would be much more cost effective than just throwing it away as a waste (Kessler, 1995). The effects of CKD on geotechnical properties of soils are discussed in sections 18.104.22.168.1 through 22.214.171.124.4.
Addition of CKD to the soil increases the unconfined compressive strength (Miller and Azad, 2000). Also by increasing the curing time uniaxial compressive strength of CKD treated soils increases (Miller and Azad, 2000). Other studies such as those conducted by Baghdadi (1995) show that after 28-days, unconfined compressive strength of kaolinite samples mixed with 16% (by weight) CKD increased from 210 kPa to 1115 kPa. Based on the experimental studies given by Miller and Azad (2000) the stiffness of CKD treated soils increases and the failure occurs at a smaller axial strain compared to untreated soils (Figure 21).
Figure 21: Stress strain results from unconfined compressive strength tests on CKD treated soils after 28 days of curing (Miller and Azad, 2000)
126.96.36.199.2 Atterberg Limits
Miller and Azad (2000) investigated the change in Atterberg limits in different soil samples treated with CKD. The results indicate that an increase in CKD increases the plastic limit, decreases the liquid limit thus, significant PI reduction occurs with CKD treatment, especially for soils with high PI.
188.8.131.52.3 Compaction Characteristics
Compaction characteristics of the soils are affected by adding CKD. Miller and Azad (2000) report that addition in CKD content increases the optimum water content and decreases the maximum dry density of the treated soils.
184.108.40.206.4 Effect of CKD on pH
Experimental studies by Miller and Azad (2000) show that addition of CKD increases soil pH, so the soil becomes more alkaline. The higher the pH, the higher is the solubility of silica and alumina, which reacts with calcium ions released during cement hydration to form secondary cementitious products and this is called pozzolanic activity. CKD is a pozzolanic activator in low-strength materials.
220.127.116.11 Polymers Based Products
There are different types of polymers for the purpose of soil stabilization and erosion control such as Soil-Sement®, Curlex Net Free, Antiwash/Geojute, and Slopetame2. Soil-Sement® is an environmentally safe, advanced powerful polymer in dust control, erosion control and soil stabilization. Results of the research by Little et al. (2000) show the benefit of the polymer Soil-Sement® on stabilizing Eolian and Fluvial soils. Both types of soils are classified as poorly graded sand, based on Unified Soil Classification System. Addition of this polymer to dry Eolian and Fluvial soils increases their CBR values. The unconfined compressive strength of silty sand treated with Soil-Sement® also show a significant increase.
Curlex Net Free erosion control blankets are made with softly barbed, interlocking, curled wood fibers stitched together with thread (www.Curlex.com). Antiwash/Geojute is a woven gird pattern, made of natural fiber, suitable for erosion control of steep slopes (www.beltonindustries.com). Slopetame2 is a plastic grid product designed for immediate erosion control of eroding slopes (www.invisiblestructures.com).
18.104.22.168 Copolymer Based Products
There are different types of copolymer products for the purpose of soil stabilization and erosion control such as Soiltac®, Gorilla Snot®, and Durasoil®. Soiltac is a copolymer non-toxic soil stabilizer, and dust control product used for dust suppression, road base stabilization and soil stabilization worldwide. Powdered Soiltac® can be used by broadcasting the dry powder topically or mixing it in to the treatment area and adding water to the site. Also Powdered Soiltac can be pre- diluted into a liquid and applied in similar manner (www.soilworks.com).
Gorilla-Snot is a copolymer which forms bonds between soil or aggregate particles and is used as a soil stabilizer and dust control agent. Gorilla-Snot is a biodegradable product and environmentally safe to use. Durasoil is ultra-pure, synthetic organic fluid which is distinctively crystal clear, odorless and is applied neat and simple without dilution in water. Any equipment capable of spraying water can be safely used to apply Gorilla-Snot without any damage to the equipment, even in freezing and wet conditions (www.soilworks.com).
22.214.171.124 Fiber Reinforcement
The use of hair-sized polypropylene fibers in soil stabilization applications has been popular in soil stabilization projects for its low cost compared with other stabilization agents. These materials have a high resistance towards chemical and biological degradation and do not cause leaching in the soil (Puppala and, Musenda 2000). Puppala and Musenda (2000) have conducted a series of tests to study the engineering properties of clayey materials reinforced with randomly oriented fibers. The study used polypropylene fibers of nominal size of one inch and two inches in length. The physical and chemical properties of the fibers are shown in Table 2. These fibers have high resistance to chemical reaction and can be applied in high temperature conditions.
Table 2: Properties of polypropylene fibers (after Puppala and Musenda, 2000)
The results show that mixing soils with fibers increase uniaxial compressive strength. The results also indicate that swelling and shrinkage are reduced (Puppala and Musenda, 2000). Length and amount of fibers have an important effect on the level of improvement. One of the advantages of fiber technology is that it can be applied on the variety of soil types; it also does not need any special equipment or skills.
126.96.36.199 Calcium Chloride
Calcium chloride is an inorganic salt, which is a by-product of sodium carbonates. It is mainly used in highway constructions, dust control, and maintenance. Calcium chloride has hygroscopic property. This means calcium chloride attracts and absorbs water. This is a function of relative humidity and temperature. It can easily liquefy in moisture of its own absorption. Calcium chloride is highly soluble and can be dissolved easily so it can be easily washed away by rain and may require more than one treatment in a single season to maintain its effectiveness (Sleeser, 1943). For the same humidity and temperature the vapor pressure of calcium chloride is lower than water (Ros, 1988; Shepard, 1991). Calcium chloride has a higher surface tension and a lower freezing point compared to water (Shepard, 1991). In calcium chloride treated pavement roads this property minimizes frost, heave, and reduces freeze-thaw cycles, thus reducing maintenance cost (Wood, 1990; Ingles, 1973). Calcium chloride is used as a dust palliative on unpaved roads as well as haul roads in mining and on the earth-moving project. It is also used as a secondary additive to increase the strength of the soils treated with cement or lime (Hausmann, 1990).
Addition of calcium chloride affects engineering properties of the treated soils. Calcium chloride, depending on the soil type, may decrease the soil strength (Kezdi, 1979) or increase it (Thornburn and Mura, 1969). Addition of calcium chloride has major effect on compaction characteristics of the treated soils. The results will lead to an increase in dry density and a decrease in optimum water content. Figure 22 shows the result of compaction tests on a gravely clay with and without calcium chloride (Pacific Chemical Industries Pty.Ltd., 1983).
Figure 22: Compaction curves of gravelly clay with and without calcium chloride (after Pacific Chemical Industries Pty. Ltd., 1983)
188.8.131.52 Sodium Chloride
Sodium chloride has the similar properties to calcium chloride. Singh and Das (1999) have reported a major improvement in California Bearing Ration (CBR), unconfined compressive strength, and indirect tensile strength of salt treated material. The main application of sodium chloride is in long-term highway pavement subgrade (Singh and Das, 1999).
2.4 ENVIRONMENTAL ISSUES OF CHEMICAL STABILIZERS USED FOR EROSION CONTROL
When chemicals and reagents are used as means of soil stabilization or erosion control, their chemical stability and environmental impacts must be evaluated and well understood. If the treated area with chemical additives is not adequately protected from surface runoff, the stabilized material can be washed onto surrounding areas and damage the adjacent vegetation. Cement appears to have the least environmental issues compared with lime or fly ash. Most of the fly ash products have heavy metals in their compositions. Therefore, fly ash treated materials have the potential to leach and contaminate water bodies. In case of lime treated soils there is a potential for increasing pH on the surrounding areas.
CKD is not considered to be hazardous by Environmental Protection Agency’s RCRA (Resource Conservation and Recovery Act) regulations. However CKD is not necessarily free of any environmental issues. CKD must be handled properly to prevent environmental contamination and the toxicity of CKD must be determined on a case-by-case basis (Haynes and Kramer, 1982). The finer particles contain higher concentration of sulfates and alkalis, while coarser particles that are collected closer to kiln have higher concentration of free lime.
The Environmental Protection Agency (EPA) has reviewed and studied the impact of CKD on human’s health and environment. It is concluded that the health and environmental risks associated with CKD are low. However, there is a potential danger to human’s health and environment under particular circumstances. The data collected by EPA shows that using CKD in different applications have caused and may continue to cause, contamination of air and nearby surface water and ground water. That leads to potentially risks to human’s health and environment. Material safety data sheet of Roanoke cement corporation also reveals that CKD in contact with moist in eyes or skin when mixed with water becomes caustic (pH>11) and may damage or burn the skin (third degree burn), It also can cause irritation to the moist mucous membrane of the nose, throat, and can lead to some respiratory problems (www.titanamerica.com). In addition Eckert and Qizhong (1998) report that substantial leaching from cement and CKD of specific metals, especially Cr and Ba are below limits for hazardous waste defined in the Resource Conservation and Recovery Act (RCRA).