Stability analysis of unsaturated soil slopes under reservoir drawdown and rainfall conditions: steady and transient state analysis

A finite-element upper-bound procedure is presented to assess the stability of a soil slope under reservoir drawdown and rainfall conditions. In order to overcome the scientific challenge of generating a kinematically admissible velocity field, discretizing the problem of interest into finite elements is carried out for this specific purpose, and based on which the upper bound formulation of slope safety factor is derived. The slope stability analysis is transformed to the optimization of a linear programming model under four constraint conditions, by virtue of an interior-point algorithm. Steady-state and transient water flows are discussed. Based on the soil–water characteristic curve (SWCC), matric suction above the phreatic surface is considered so as to provide a reliable assessment for unsaturated slope stability. True cohesion of soils and apparent cohesion stemmed from matric suction are combined to describe the shear strength of unsaturated soils. After validating the robustness of the proposed procedure by limit equilibrium and FEM, it is further applied to evaluate the slope stability under (rapid, general, slow) reservoir drawdown and rainfall conditions.