Mechanics of Instability of Sand at High Pressures

High-pressure triaxial tests were performed to investigate instability of sand at high pressures. Instability is a liquefaction-type phenomenon that suddenly reduces the load carrying capacity of sand undergoing homogeneous deformations due to high rising pore pressures. Experiments were performed on Cambria sand with initial relative densities of 90% with confining pressures ranging from 6.4 to 68.9 MPa. Sands exhibit nonassociated plastic resulting in violation of the stability postulate by Drucker. Theoretically, the boundaries of the undrained instability region, which is the portion of stress space where granular materials may become unstable, was presented and verified with special high pressure experiments. The upper stress bound of the instability region is the effective stress failure surface and the lower bound is the instability line, which connects the top points of the effective stress paths of a series of undrained tests performed at different initial confining pressures. To demonstrate that the instability line is the lower bound, experiments were performed that loaded sand to stress states below the instability line. Then, specimens were allowed to creep across it under undrained conditions. Generally, experiments sheared to high levels will result in immediate instability upon crossing the instability line. However, this was not observed at lower stress ratios. To explain this phenomenon, it was found that another condition must be satisfied in addition to crossing the instability line. It was found that it was also necessary to be advancing the current yield surface before instability will occur. Constitutive model predictions of the yield surface locations appear to verify this observation.