Modes of hill-slope failure under overburden loads: Insights from physical and numerical models

Using sand model experiments this paper investigates failure mechanism of bed materials along hill slopes due to overburden loading. The analysis takes into account three factors: 1) surface slope (α), 2) loading pattern and 3) mechanical anisotropy of bed materials. In the experiments, external loading was simulated by moving down a rigid block extended either up to the brink of the slope (Type 1 loading) or beyond it (Type 2 loading). With progressive loading, the process of slope instability in sand models involves deformation localization in two modes: compaction (Mode 1) and shear failure (Mode 2). In isotropic models with Type 1 loading, Mode 1 deformation occurs in an elliptical region below the rigid block, which is flanked by a pair of shear bands of Mode 2. With increasing α (α > 45°), Mode 2 localization propagates rapidly towards the sloping surface leading to collapse on slopes. The failure zones also propagate in the vertical direction, and its depth of penetration appears to be inversely related to α. With Type 2 loading, Mode 2 localization becomes dominant, forming multiple shear bands, and Mode 1 deformation decreases with increasing α. In case of anisotropic bed materials, the orientation of the anisotropy plane relative to that of the surface slope determines the mode of deformation localization. With Type 2 loading, Mode 1 is the dominant process of deformation when the anisotropy plane dips same as the surface slope (α = 45°), whereas it is coupled with Mode 2 for the anisotropy planes dipping in the opposite directions. We complement the experimental findings with results from finite element models, and demonstrate the patterns of Mode 1 and Mode 2 localization considering the I1 (= σ1 + σ2 + σ3) stress invariant and Drucker-Prager yield criterion for elasto-plastic materials respectively.