Instability of unsaturated soil slopes during wet periods is common in many countries around the world. According to previous studies, rainfall leads to the development of a perched water table, a rise in the main groundwater level, surface erosion and an increase in unit weight with an increase in moisture content. In general, unsaturated residual soil slope failures occur most frequently during wet periods when there is an increase in moisture content and a decrease in matric suction, since the additional shear strength provided by the matric suction can be reduced enough to trigger the failure.
With an understanding of the mechanism of these slope failures, a reasonable procedure can be developed for analyzing soil slope stability pertaining to transient seepage due to rainfall.
This paper presents a procedure for calculating the safety factor for an unsaturated slope to examine the mechanism of slope failures within a soil mass during rainfall infiltration. A typical process of rainfall infiltration was illustrated through a two-dimensional finite element flow-deformation coupled analysis. Changes in wetting zones and distribution of pore pressures and stresses can be calculated by using the finite element method. The modified Mohr-Coulomb failure criterion was adopted for unsaturated soil strength to consider variations in strength due to the presence of matric suction.
A safety factor was calculated at various elapsed times after the commencement of rainfall, as in the following procedure: first, stresses are estimated at each Gaussian point from a coupled finite element analysis. Then, the global stress smoothing method was applied to obtain a continuous stress field. Based on this stress field, a safety factor was calculated for a specified slip surface by a stress integration scheme. Finally, a search strategy was used to find a critical slip surface associated with the minimum safety factor.
Some numerical examples are analyzed. The results illustrate a typical process of infiltration into unsaturated soil slopes. In particular, the effect of hydraulic heterogeneity on hydrologic response is examined, as the magnitude and spatial variation of hydraulic conductivity control the generation of pore water pressure.
Besides, shallow slope failures in residual soil during periods of prolonged infiltration are common and the failure surfaces are usually parallel to the slope surface. Therefore, this study examines an infinite slope analysis to estimate the influence of infiltration on surficial stability of slopes by the limit equilibrium method. An approximate method that accommodates the boundary condition of a uniform rainfall has been proposed to evaluate the likelihood of shallow slope failure that is induced by a particular rainfall event. The method based on the Mein and Larson model that provides an explicit solution for pre-ponding infiltration has been applied to various types of soil having measured unsaturated hydraulic properties. To compare the results with those obtained from the approximate method, a series of numerical analyses were carried out. According to the results, with the use of properly estimated input parameters, the approximate method was found to give results that compare reasonably well with those of the more rigorous finite element analyses.