Recommended Practice for Stabilization of Subgrade Soils and Base Materials (2009)

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28 SUMMARY The process of chemical modification or stabilization with calcium-based chemicals requires a basic understanding of the mechanisms of reaction. Each calcium-based stabilizer contains some amount of free lime (CaO or Ca(OH)2) that reacts pozzolanically with the fine particles (clay and some silt). Normally, lime is 90 percent or more CaO or Ca(OH)2 and therefore provides the most available free lime over the longest period of time in the treatment process of any of the traditional stabilizers: lime, Portland cement, and fly ash. However, Portland cement and class C fly ash also release lime during the hydration of calcium silicates and calcium aluminates. This lime participates in the pozzolanic reaction mechanism with soil fines (primarily clay). As against lime, Portland cement and class C fly ash possess calcium-silicate and calcium- aluminate compounds that hydrate upon the addition of water. The resulting cementitious products bond soil particles together and develop strength. The kinetics of these cementitious reactions is rapid. This can be a draw back in the treatment of plastic clay soils because these soils require time for diffusion of calcium into the soil matrix and mechanical manipulation in order to properly modify or stabilize them. The rapid cementitious reactions cut down on the time available to manipulate the soil-stabilizer blend. This available “mellowing” period makes lime a preferred additive, by many, when dealing with moderately and highly plastic clay soils. The standard of practice that will result from this study defines the following steps in modification or stabilization: selection of stabilizer based on soil index properties (primarily percent fines, smaller than 75 μm, and plasticity index), method of soil exploration and investigation required to support stabilizer selection and mixture design, and verification of stabilizer selection. However, the fundamentals of the mechanisms of stabilization should also be considered to recognize the variability among soils during treatment with the traditional additives. Soil exploration steps identify how readily available resources such as the National Resource Conservation Service (NRCS) soil surveys and U. S. Geological Survey reports can be used to plan a soil exploration program to support the stabilization process and to help deal with potentially problematic conditions such as high moisture contents due to fluctuating water levels and problematic composition such as high sulfate contents and high organic contents. If high sulfate contents are encountered, then the Standard on Stabilization of Sulfate-Bearing soils, also a product of this study, should be used. The verification process varies depending whether the goal of treatment is modification or stabilization. Modification refers to treatment of the soil with the selected stabilizer in order to reduce plasticity and improve workability. The associated reactions for modification must be relatively rapid. The verification process is then simply to add the amount of the selected additive that will achieve the properties desired. Stabilization, on the other hand, requires not only immediate improvement but also long term strength and durability. Stabilization requires a more detailed and sophisticated verification protocol for which a structured mixture design protocol is included. The mixture design protocol for each stabilizer includes an initial approximation of the appropriate stabilizer content either based on an empirical database or a screening test, such as the Eades and Grimm pH test. This is followed by strength testing where the critical conditions expected in the field are simulated in the testing laboratory. Since it is normally beyond the scope of stabilizer selection and testing to mimic moisture and environmental variations over the year, a critical condition is normally simulated by partially saturating the sample. The method and degree of this “moisture conditioning” process is based

29 on experience and varies among design agencies. A capillary suction diffusion process or “capillary soak” is favored by the authors for lime-soil mixtures as recommended by the National Lime Association. The normal moisture conditioning protocol for Portland cement stabilization includes moist curing and a 4-hour soak before strength testing. A similar soaking protocol is suggested by the American Coal Ash Association for fly ash stabilization of aggregates. However, when lime and fly ash are used in combination to treat soils, the authors recommend capillary soak is required for lime-soil-fly ash mixtures. The authors recommend strength testing after moisture conditioning as the appropriate metric of determining strength and durability for all stabilization processes.