Modes of improbable slope failure in spatially variable cohesive soils

Accounting for spatial variability in probabilistic slope stability analysis using the random finite element method (RFEM) typically leads to a distribution of calculated factors of safety as well as a distribution of resulting depths of the sliding body. Factor of safety (or its corresponding probability of failure) and sliding depth are weakly correlated when looking at the results of classical Monte Carlo simulations, but the mean (i.e. the expected) depth of the sliding surface as a function of the factor of safety shows a clear trend. This trend becomes more prominent when looking at the weak tail of the factor of safety distribution in more detail. In this paper, subset simulation is combined with RFEM to address the weak tail in an efficient manner and to simulate slope failure events at low probability levels, thereby demonstrating the tendency to shallow modes of failure in such cases. The mechanism behind the tendency to shallow modes of failure is then further investigated by simplification of the problem to limit equilibrium. Slide-specific variance reduction through spatial averaging is demonstrated as the mechanism behind the tendency to a shallow sliding surface as the mode of improbable slope failure.