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Soil-water interaction at high soil suction
Khorshidi, Morteza
Khorshidi, Morteza
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2015
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A substantial amount of research has been done to understand the mechanisms and associated applications of the soil water retention curve (SWRC). However, the SWRC in the range of high matric suction (soil water sorption isotherm) has drawn less attention relative to laboratory and theoretical investigations of the SWRC at low matric suction. The primary goal of this research is to improve the understanding of the fundamental hydration mechanisms at high matric suctions, and to determine the effect of soil properties, such as clay-mineral composition, cation exchange capacity (CEC), and specific surface area (SSA) on the relation between water content and soil suction. A suite of soils ranging from expansive to nonexpansive was used in the experimental part of the work. In addition, different soils are also treated with various solutions to form different homoionic soils. Water sorption isotherms of soils are obtained using Vapor Sorption Analyzer. The CEC and SSA of soils are also measured independently by other methods. Soil water sorption isotherms are analyzed to develop conceptual and theoretical models describing various soil features, such as hysteresis. It is found that crystalline swelling is the main reason for hydraulic hysteresis of water sorption isotherms. Cation hydration and particle surface hydration are two mechanisms responsible for the adsorption of water on soil with the former occurs at matric potentials greater than that which the latter occurs at high matric suction range. As such, a unitary state is introduced that relates monolayer cation hydration water content to the type and hydration number of cations in soil. Two methods are developed to determine the boundaries between cation hydration and particle surface hydration during water sorption on soil, namely, specific moisture capacity and Brunauer-Emmett-Teller (BET) plot method. Using these two methods, SWRC-based methodologies are proposed for determination of specific surface area and swell potential of soil. The results obtained by these methodologies are much closer to the experimental measurements in comparison to the existing methods. A theoretical SWRC model is constructed to quantify matric potential and the corresponding retained water on soil by taking into account of all water sorption mechanisms on or within soil particles, particularly charge-dipole or cation-water molecule interaction. It is found that in clays at matric potentials less than 150 MPa, charge-dipole interaction energy contributes dominantly to matric potential. Furthermore, the predicted matric potentials at zero water content for various soils agree well with the experimental data. It is found that the lowest matric potential of a soil depends mainly on type of cations, thus is not a universal constant for all soils.
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