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Pore-water pressure in soil is caused by three physically distinguishable sources: ambient (environmental) pressure, surface tension–induced capillary pressure, and the soil's electromagnetic potential–induced adsorptive pressure. The former two form the conventional concept of pore-water pressure, which is considered a constant within a soil-water-air representative elementary volume and can be directly measured by piezometer (under saturated and compressive states) or tensiometer (under unsaturated and tensile states). The third one can be called adsorption-induced pore-water pressure and is localized within a certain distance to the particle surface of soil or intercrystalline surface of swelling clay. The adsorption-induced pore-water pressure is always compressive and dictates the water phase transition in soil by altering water's freezing point, density, and viscosity, among other physical properties. A framework of quantifying the adsorption-induced pore-water distribution via the measured soil water isotherm is presented for any soil type under any given water content. It is demonstrated that the adsorption-induced pore-water pressure can be up to 1.6 GPa in the first few layers of hydration, but will diminish to zero at a distance equivalent to the gravimetric water content >1% for sandy soil and greater than a few percent for silty soil. In clayey soil, the adsorption-induced pore-water pressure can sustain tens of megapascals even at much farther distance, equivalent to ∼30% water content. In expansive clay, the adsorption-induced pore-water pressure inside the crystalline lamellae can exceed 800 MPa. The soil water density functions of a silty soil and a bentonite clay predicted by the proposed framework matched well with that measured independently from the conventional consolidation testing, validating the framework to determine the spatial distribution of the adsorption-induced pore-water pressure in soil. [ABSTRACT FROM AUTHOR]

A theoretical expression for clay soil-water characteristic curves (SWCC) is developed for cases in which the osmotic potential is small relative to the matrix potential, for the range of water potential lower than the cavitation value, based on the following foundations. Soil structure consists of saturated soil pods containing parallel soil particles, and angular interpod macropores possibly containing both air and water. The pod water is adsorbed to the particle surfaces, and subject to adsorption while the macropore water is subject to capillary potential (commonly referred to as suction). It is suggested that the capillary potential cannot be lower than the cavitation potential of the capillary water; at lower potentials, the macropore water drains into the pods and the matrix potential is represented solely by the adsorption potential. The paper examines the hypothesis that the adsorption potential is dominated by van der Waal forces, and this forms the basis for the theoretical SWCC. While the theoretical curve reasonably fits experimental data from several soils, kaolin soils depart from it, and pure montmorillonites fit only over a range of low potential, indicating that van der Waal adsorption forces may control adsorption potential for some, but not for all soils. [ABSTRACT FROM AUTHOR]