Static Instability and Liquefaction of Loose Fine Sandy Slopes

TLDR

Soils with nonassociated flow can become unstable under certain stress paths per Drucker and Hill, and instability—distinct from failure—can also lead to catastrophic events. The study investigates the location of the instability line. Conventional triaxial tests on fully saturated and partly saturated specimens were performed under drained and undrained conditions to delineate stable and unstable regions. Undrained, highly saturated specimens follow stress paths into the instability region, causing instability when the yield surface opens outward, confirming that instability occurs inside the failure surface and that slopes considered stable by conventional analyses can become unstable and fail catastrophically under small disturbances.

Abstract

Soils that exhibit nonassociated flow may, according to stability postulates by Drucker and by Hill, become unstable when exposed to certain stress paths inside the failure surface. Series of conventional triaxial tests on fully saturated and on partly saturated specimens were performed under drained and undrained conditions to study the regions of stable and unstable behavior. For specimens that compress and have degrees of saturation higher than critical, undrained conditions lead to effective stress paths directed within the region of potential instability, and instability was observed provided the yield surface opens up in the outward direction of the hydrostatic axis. Thus, instability occurs inside the failure surface. Instability is not synonymous with failure, although both may lead to catastrophic events. The location of the instability line is discussed. Examples of a shallow submarine slope and a nearly fully saturated steeper slope representing a tailings dam, which both should remain stable according to conventional stability analyses, are presented to show that they could become unstable due to small disturbances and proceed to fail catastrophically.