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Suggested Citation:"SOIL CLASSIFICATION." National Academies of Sciences, Engineering, and Medicine. 2009. Recommended Practice for Stabilization of Subgrade Soils and Base Materials. Washington, DC: The National Academies Press. doi: 10.17226/22999.
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Page 7
Page 8
Suggested Citation:"SOIL CLASSIFICATION." National Academies of Sciences, Engineering, and Medicine. 2009. Recommended Practice for Stabilization of Subgrade Soils and Base Materials. Washington, DC: The National Academies Press. doi: 10.17226/22999.
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Page 8

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5 Again, it is incumbent upon the agency involved to determine acceptable levels of oxides and trace elements that comprise the by-product. As a general guide on the level of risk associated with the presence of oxides and trace elements in these by product stabilizers, the development of expansive mineral products may become intolerable when the S03 content exceeds about 3 percent or when the MgO content exceeds about 3 to 5 percent. The impact of organics can also be a problem as their presence can interfere with the availability of calcium to the soil or aggregate being treated. Several tests can be used to screen for the presence of organics. One quick test if loss on ignition (LOI). Although it does not identify the type of organic, which is definitely important, an LOI of greater than about 8 to 10 percent flags a potentially problematic quantity of organics. Non Traditional Stabilizers This standard practice is limited to traditional, chemical stabilizers like: Portland cement, lime and fly ash. However, it is important when considering treatment with these traditional products to broach the subject of non-traditional or alternative stabilizers. The mechanism of stabilization for non-traditional stabilizers varies greatly among the stabilizers. Asphalt may or may not be grouped as a traditional stabilizer depending on perspective. Asphalt is not a “chemical” stabilizer in the sense that it does not react chemically with the soil to produce a product that alters surface chemistry of the soil particles or that binds particles together. Instead asphalt waterproofs aggregate and soil particles by coating them and developing an adhesive bond among the particles and the asphalt binder (6). The process is dependent on the surface energies of the aggregate or soil and the asphalt binder. Consequently, since this mechanism is more physical than chemical, soils with very high surface areas are not amenable to asphalt stabilization and such stabilization is normally limited to granular materials such as gravels or sands, and perhaps some silty sands. As a visco-elastic, visco-plastic material, temperature and/or dilution methods are required to make asphalt stabilization effective in soils. Either lower viscosity liquid asphalts (normally developed by mixing bitumen with diluents) or emulsified asphalts are used in soil stabilization. Because the nature of asphalt stabilization is so mechanistically different from chemical stabilization, asphalt stabilization is not considered as a candidate in this standard practice. The mechanisms of stabilization of other non-traditional stabilizers including sulfonated oils, enzymes, ionic stabilizers, etc. are discussed in detail by Petry and Little (1). Such stabilizers may have a role in modification and/or stabilization, especially when high soluble sulfate contents in the soil limits the applicability of calcium-based, traditional stabilizers. SOIL CLASSIFICATION Soil is a broad term used in engineering applications which includes all deposits of loose material on the earth’s crust that are created by weathering and erosion of underlying rocks. Although weathering occurs on a geologic scale, the process is continuous and keeps the soil in constant transition. The physical, chemical, and biological processes that form soils vary widely with time, location and environmental conditions and result in a wide range of soil properties (7). Physical weathering occurs due to temperature changes, erosion, alternate freezing and thawing and due to plant and animal activities causing disintegration of underlying rock strata whereas chemical weathering decomposes rock minerals by oxidation, reduction, hydrolysis, chelation, and carbonation. These weathering processes, individually or in combination, can create residual

6 in-place soils or facilitate the transport of soil fractions away from the parent rocks by geologic agents like wind, water, ice or gravity. These transport processes often result in mixing of soil minerals or introducing salts or organic material of a variety of species and concentrations. Soil impacted by the presence of organics and salts, such as sulfates, can exist as remote outcrops or over large areas and often do not have clearly defined boundaries. The soil pedological profile also varies considerably with location and even within a specific soil series or association. The complexity of soils requires a disciplined yet efficient method to identify and classify them for their use as a construction material. Soil texture is defined, at least initially, by its appearance and is dependent on the size, shape and distribution of particles in the soil matrix. Soil particle sizes may vary from boulders or cobbles, roughly a meter in diameter, to very fine clay particles, roughly a few microns in diameter. Engineering properties of coarse fractions are dependent on physical interlocking of grains and vary with the size and shape of individual particles. Finer fractions in soil have a significantly higher specific surface area and their behavior is influenced more by electro-chemical and physio-chemical aspects than particle interaction. Among finer particles, clays exhibit varying levels of consistency and engineering behavior and demonstrate various levels of plasticity and cohesiveness in the presence of water. Silt fractions are also classified as fine-grained soils because more than 50 percent of the soil mass is smaller than 75 μm, which fits in the designation of fine-grained material according to the Unified Classification System (AASHTO M 145). However, the specific surface area of silt fines is several orders of magnitude larger than that of clay soil particles. This difference is part of the reason that clay particles are more reactive than silt particles. In addition, clay minerals have a unique sheet particle structure and a crystalline layer structure that is amenable to significant isomorphous substitution. As a result of the isomorphous substitution of lower valence cations for higher valence cations within the layer structure, clay mineral surfaces carry a significant negative surface charge that can attract positively charged ions and dipolar water molecules. The cumulative effect of high surface area and surface charge makes clay particles particularly reactive, especially with water, and is the root cause of the propensity of clay particles to shrink and swell depending on the availability of water. The AASHTO (M 145) soil classification system differentiates soils, first based on particle size and secondly based on Atterburg limits. If 35 percent or more of the mass of the soil is smaller than 75 µm in diameter, then the soil is considered either a silt or clay and if less than 35 percent of particles are smaller than 75 micron sieve, then the soil is considered to be coarse-grained, either a sand or gravel. For stabilization purposes, soils can be classified into subgrade and base materials based on fractions passing No. 200 sieve. If 25 percent or more passes through the no. 200 sieve the soil can be considered as a subgrade, and if not, they may be classified as a base material. However, more than simple gradation impacts the definition of a subgrade or base. In order to be termed a base material, the material in question must also be targeted for use as a base layer from a structural perspective. On the other hand, an in situ coarse-grained soil with less than 25 percent fines, may be, by definition a native subgrade even though it may achieve the required classification of a base. For stabilization purposes, the soils may be differentiated into subgrade (soil) stabilization and base stabilization (coarse-grained) on the basis on the fine content index.

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TRB’s National Cooperative Highway Research Program (NCHRP) Web-Only Document 144: Recommended Practice for Stabilization of Subgrade Soils and Base Materials explores a methodology to determine which stabilizers should be considered as candidates for stabilization for a specific soil, pavement, and environment.

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