Instability of granular materials

Experimental observations form the background for studies of the conditions for instability of granular materials that exhibit nonassociated plastic flow. Triaxial tests on sand were performed to study the regions of stable and unstable behavior. The variables were the sign of the second work increment (positive or negative), the volumetric strain behavior (compression or dilation), the drainage condition (drained or undrained) and the degree of saturation (fully or partly saturated). For saturated soil that tend to compress, undrained conditions may lead to instability if the state of stress is located in the region between the instability line and the failure surface. Thus, granular materials may become unstable inside the failure surface, but instability is not synonymous with failure. The proposed analysis method for static instability of slopes involves the state of stress in the ground and the region of potential instability. A trigger mechanism is required to initiate instability.