System Reliability Analysis of Unsaturated Soil Slopes under Rainfall: Rotational or Translational Failure?

Most of the rainfall-induced landslides reported in the literature were shallow translational, with depth usually less than two meters. But occasionally deep rotational failures also occurred. A deep failure is likely to be more destructive than a shallow failure due to its greater volume of failure mass. Conventional unsaturated slope stability analysis usually focuses on a particular failure mode. For example, limit equilibrium analysis of slices with a circular slip surface is most likely to capture the deep rotational failure, while infinite slope analysis implicitly assumes the failure mechanism is shallow translational. However, either failure mode is possible. Considering the uncertain geological parameters, this study performs system reliability analysis on unsaturated slopes under rainfall considering both deep rotational and shallow translational failure mechanisms. Through parametric studies the factors that could control the failure mechanism of an unsaturated soil slope under rainfall are identified and investigated. The results show that the system failure probability is obviously different from that of a single failure mode when the rainfall infiltration exceeds a certain depth (more than 1m in this study) and the slope angle is between 50° and 75°. The research finding is useful to guide the selection of the right method for a slope stability problem in engineering practice.