Stability analysis of unsaturated slopes under rainfall and drainage using the vector-sum-based numerical manifold model

Rain-induced slope failure is a worldwide disaster in engineering practice. This paper presents a framework for stability analysis of unsaturated slopes. A fully coupled model of wetting and non-wetting fluid phases with deformable solid phase is used to model hydro-mechanics of unsaturated slopes under water infiltration and discharge. Plasticity of the solid phase is described using the classical non-smooth Mohr-Coulomb (MC) model, which is implemented using a novel subspace-tracking scheme by updating stresses in the principal stress space that is continuously dependent on stresses. The Vector Sum Method (VSM) and Pattern Search Method (PSM) are combined to transform the slope stability analysis into an optimization problem and to determine critical slip surfaces and safety factors simultaneously. Unlike the conventional definition of safety factor, vector characteristics of the sliding and resistance forces are considered. The present framework is implemented using the Numerical Manifold Method (NMM) and is advantageous in modeling complex boundary conditions and discontinuities in geomaterials. Through benchmark slope stability analyses, effectiveness and accuracy of the proposed model are validated. Numerical simulations show that there is a lag between the time instant rainfall ceases and the time instant fluid phases reach equilibrium and the safety factor assumes its minimum.