Slope stability analysis under unsaturated conditions: Case studies of rainfall-induced failure of cut slopes

We present two case studies of rainfall-induced failure of engineered slopes. The traditional limit equilibrium and finite element methods are expanded to unsaturated conditions using a generalized effective stress framework. Because effective stress is represented by the suction stress characteristic curve (SSCC), and the SSCC and the soil water retention curve (SWRC) have been unified with the same set of hydromechanical parameters, the expanded framework requires only three hydromechanical parameters in addition to those used for saturated slope stability analysis. Using recorded rainfall, measured shear strength parameters and the SWRC, and site geology, transient slope stability analyses are conducted to reconstruct the failure events. We find that, despite differences in slope geometry, hydromechanical properties, shear strength, and rainfall history, the actual failure occurred when the simulated factor of safety approaches its minimum below 1.0. It is shown that the hydromechanical framework under the suction stress-based effective stress can reconcile the observed timing of failure.