Modeling hysteresis of the soil-water characteristic curve

Soil suction is an essential parameter for describing the mechanical behavior of unsaturated soils. For analytical purposes, its value is usually inferred from the soil-water characteristic curve. However, the expressions developed up till now to describe this curve, do not prioperly account for the physical facts involved in the hysteresis phenomenon. It is generally accepted that hysteresis arises because pores of different sizes are interconnected generating the "ink-bottle" effect. In this paper, a twofold model that simulates the porous structure of soils is used and the mathematical expressions describing the hysteresis of the soil-water characteristic curves are obtained. This model considers that the voids in the soil mass are composed of two kinds of elements: the sites (cavities) and the bonds (throats), each kind showing its proper size distribution. Based on these premises and making use of the probability theory, the equations corresponding to the main hysteresis loop during wetting or drying are. established and it is shown that the extent of the hysteretic behavior depends on the parameters describing the size distributions. In addition, the equations describing the specific internal (scanning) paths for cyclic wetting and drying are obtained. Finally, these results are compared with experimental results that include the main hysteresis loop, some scanning curves and the pore size distribution.