CREEP EFFECTS ON STATIC AND CYCLIC INSTABILITY OF GRANULAR SOILS

Stability postulates by Drucker and by Hill are applicable to solid metal. When applied to granular material exhibiting nonassociated flow, these concepts of stability imply that the material may be unstable when exposed to certain stress paths inside the failure surface. Series of triaxial tests on fully saturated and partly saturated specimens of sand have previously been performed under drained and undrained conditions to study the regions of stable and unstable behavior. The stability postulates by Drucker and by Hill do not capture the conditions for stable and unstable behavior of granular materials. Granular materials may become unstable if the state of stress is located above the instability line for the material. Presented here is a series of experiments conducted on loose sand that compresses during shear. In these tests the sand is allowed to creep over different periods of time resulting in movement of the yield surface. The influence of the new location of the yield surface on the static and cyclic instability of the sand is studied and discussed.