Upper-bound optimized solution of unsaturated soil slope stability under steady and unsteady flows

The increasing frequency of extreme weather events is leading to a rise in unsaturated soil slope instability near the earth's surface year by year. Establishing the method for analyzing slope stability under complicated hydrological conditions is imperative. This study proposes an approach that combines finite element (FE) seepage analysis with upper-bound limit analysis to evaluate the stability of unsaturated soil slopes. Two kinematically failure mechanisms are constructed based on the spatial discretization technique via point by point including the rotational and rotational-translational combinational failure modes, which can adapt to the complex pore water pressure distribution and varying soil unit weight. Additionally, a novel optimized solution scheme that integrates dichotomy and meta-heuristic optimization approaches is adopted to determine the factor of safety (FoS) and critical slip surface of slopes. The effectiveness and practicality of this method are validated by comparing with the published examples of slope stability evaluation under horizontal and inclined water table. Moreover, the variation of failure mode during the rainfall infiltration is illustrated by a case study of the slope under heavy rainfall. The methodological framework presented in this study can provide a basis for evaluating the stability of saturated-unsaturated soil slopes with 2D seepage conditions.