Numerical analysis of effective soil porosity and soil thickness effects on slope stability at a hillslope of weathered granitic soil formation

Abstract

Modeling rainwater infiltration in slopes is vital to the analysis of slope failure induced by heavy rainfall. Amongst the soil hydraulic properties, the hydraulic conductivity K has been frequently analyzed for its effects on slope stability. In contrast, few studies have been published on the effects of water retention characteristics on slope stability. In this study, a numerical model was developed to estimate the extent of rainwater infiltration into an unsaturated slope, the formation of a saturated zone, and the change in slope stability. This model is then used to analyze the effects of the soil porosity parameters (i.e., saturated soil water content θs and effective soil porosity (ESP)) and soil thickness on the occurrence of slope failure. Results showed that when the surface soil of a slope has a relatively large ESP value, it has a greater capacity for holding rainwater, and therefore delays rainwater infiltration into the subsurface layer. Consequently, the increase in pore water pressure in the subsurface layer is also delayed. In this manner, a relatively large surface layer ESP value contributes to delaying slope failure. Under weaker storm conditions, slope failure tends not to occur when the surface soil has a relatively large ESP value. In addition, the thickness of soil is also a significant parameter in slope stability analysis. A shallow soil depth resulted in greater discharge volume and a lower peak pore water pressure during the major rainfall event, and consequently the slope failure tends not to occur. However a deeper soil depth increased the weight of solids and the soil moisture conditions in the slope consequently increasing the pore water pressure causing slope failure.