Expanding collapse in partially submerged granular soil slopes

Abstract

The traditional approach to stability analysis of granular slopes leads to the safety factor that is associated with a planar failure surface approaching the slope face, whether the slope is “dry” or submerged. However, for partially submerged slopes, a more critical, nonplanar failure surface can be formed. A family of geometrically similar surfaces can be found that is characterized by the same safety factor. If the safety factor drops down to unity and the slope becomes unstable, then a mechanism of any size can form. Alternatively, the failure may start at some small region and then the volume of the mechanism of failure can expand, giving rise to a progressive failure of a different kind that is typically associated with slopes. This progression has the character of a “disturbance” or a shock-like kinematic discontinuity propagating into the soil at rest. A quantitative analysis is presented and it is demonstrated that the soil dilates while the mechanism expands, leaving the slope weakened and susceptible to a deep failure. This is a plausible mode of failure of partially submerged slopes, the type that is most likely responsible for large subaqueous landslides, and is similar to the well-documented instability propagation in “quick clay.”