Probability-dependent failure modes of slopes and cuts in heterogeneous cohesive soils

Improbable slope failure is addressed in the framework of reliability analysis of slopes in heterogeneous cohesive soils, as the small subset of realisations that fail without additional measures to trigger failure. The mode of these slope failures, located at the weak tail of the reliability curve, is demonstrated to differ significantly from the deterministic solution as well as the stochastic average solution found when shear strength reduction is applied to trigger slope failure. Subset simulation is applied to compute the probability-dependent difference in failure mode for a range of slopes down to very low levels of probability, which is required to properly account for the actual failure in predominantly safe slopes. The results demonstrate possible differences in the mode of failure when properly accounting for the uncertainty in spatial variability in a full probabilistic slope stability analysis, and highlight that caution may be needed when using the strength reduction method.