Instability of loose sand under drained conditions

A type of prefailure instability which occurs under fully drained conditions is studied in this paper. It is observed experimentally that when a specimen is sheared along a drained stress path involving a decrease in effective mean stress, it becomes unstable after the stress path crosses an instability line. The instability occurring under drained conditions is different from the type observed under undrained conditions. The differences and similarities between the two types of instability are elaborated. The practical implication of the study in analyzing the failure mechanisms of granular soil slope is also discussed.