Theoretical Soil-Water Characteristic Curves Based on Adsorption, Cavitation, and a Double Porosity Model

Abstract

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.