Importance of Spatial Variability in Probabilistic Stability Analysis of Relatively Steep Undrained Slopes with a Foundation Layer

ABSTRACT The paper investigates the influence of a foundation layer on relatively steep undrained slopes with spatially variable soil strength. The definition of a relatively steep slope in this context is a slope angle that is steeper than that established by Taylor for the transition point between toe failures and base failures in uniform slopes which occurs at around 53°. It is shown that when the soil strength is spatially variable, critical failure mechanisms can pass into the foundation layer even in relatively steep slopes, which could never happen in a uniform soil. Although the worst‐case correlation length is a well‐established phenomenon in geotechnical reliability, it has usually been associated with slopes with relatively low factors of safety based on the mean. The paper demonstrates for the first time that even slopes with high factors of safety based on mean strength, can exhibit a striking worst‐case correlation length, confirming that failure to account for spatial variability can lead to unsafe predictions of the probability of failure.