Instability of sand under a lateral extrusion stress path with different drainage conditions

A series of lateral extrusion (LE) tests were carried out on silica fine sand using the triaxial device to study the initiation of instability under various drainage conditions on this stress path. All specimens were reconstituted using the moist tamping technique, having achieved state parameter (ψ₀) values between −0.020 and +0.086 after anisotropic consolidation. The LE stress path consists in a decrease of mean effective stress and increase of the deviator stress at a constant rate, maintaining the vertical stress constant. The LE tests were performed either with the drainage valves open or with undrained increments followed by drainage. Different magnitudes of undrained increments were tested. For the same initial state parameter, the results indicated that the instability stress ratio ($\eta$_IL) decreases as drainage conditions changes from drained to undrained/drained increments. As the magnitude of the undrained increment increases, $\eta$_IL decreases. Further, consistent with previous studies, the test data shows that the static liquefaction can be triggered in tests where the drainage valves are open, generating high values of excess of pore pressure only after the onset of instability. The results were also compared to results from CSD and CAU tests.