Instability of granular materials with nonassociated flow

Materials exhibiting nonassociated flow should, according to Drucker's stability postulate, become unstable when exposed to certain stress paths inside the failure surface. Results of a series of triaxial tests designed to expose the type of behavior displayed by granular materials are presented and discussed. The sand tends to compress during shear, it exhibits nonassociated flow, and instability is observed along stress paths in the region of potential instability. In a previous study of similar type in which dilation developed during shear and the sand exhibited nonassociated flow, when the specimens were exposed to stress paths in the region of potential instability, none was observed. The type of volume change behavior displayed by the material is of great importance in the question of stability of granular materials. Stability conditions for materials with nonassociated flow are presented.