Effects of hysteresis on hydro-mechanical behavior of unsaturated soil

Hysteresis is a common feature in the hydraulic properties of unsaturated soils. At a given matric suction, the volumetric water content on a wetting curve is always lower than that on a drying curve. The hysteresis may affect the mechanical behavior and transient process in unsaturated soils. This study focuses on the hysteresis observed in soil-water retention curve (SWRC) tests in the laboratory and inferred from a semi-empirical equation for numerical implementations. Coupled hydro-mechanical finite element analyses, employing an initial drying, main drying, and main wetting curve as hydraulic properties of the soil, were performed to numerically study the effect of hysteresis on the hydro-mechanical behavior of unsaturated soil subjected to water infiltration. Numerical results show that the unsaturated soil exhibits lower soil strength and matric suction distributions when applying the main wetting curve as compared with other SWRCs. The hysteresis appears to affect the stress-strain behavior of the unsaturated soil during prolonged rainfall. Therefore, to predict the instability of the rainfall-induced soil slope more precisely, it is necessary to apply the wetting curve of the SWRC, which shows the hydraulic characteristics of the slope surface.