Study on the steep decrease in the pressure-dependent critical state stress ratio of remolded deep clay

Previous studies show that with increasing pressure level, the critical state stress ratio, M of some clays decreases remarkably within a narrow range of Δe. In other words, “steep decrease” in M of clay occurs. However, stress path, end restraint and second saturation after did not been taken into account in these studies, which could lead to error test conclusions. For the further validation, taking measurements including using smooth end and second saturation by increasing back pressure after consolidation, triaxial undrained compression and extension tests on remolded clay from deep stratum in East China were performed under the stress level of 0.3~2.5MPa. It was shown that the above “steep decrease” in M exists for both triaxial compression and tension tests, and that compared to alleviating the end friction restraint, second saturation has a significant effect on the measured critical stress ratio of clays under high consolidation pressure. Then, comparative analysis indicates that the existing nonlinear strength theories of geomaterials cannot reasonably describe the observed pressure and stress path dependent nonlinear M of the studied clays. At last, it was preliminarily revealed that the “steep decrease” in M of clays is closely related to the predominant face-to-face contacts between parallel clay particles which has very small friction coefficient in the clay at the critical state under high pressure.