Instability of gold mine tailings subjected to undrained and drained unloading stress paths

The drained instability of two gold mine tailings under lateral stress relief is investigated in this study. Constant deviator stress (CDS) unloading tests were performed using a triaxial apparatus to examine instability imposed by unloading in a drained condition. Instability was induced by enforcing a constant deviator stress while simultaneously reducing the mean effective stress. Stress paths and shear strengths of CDS tests were compared with undrained triaxial tests on comparable specimens which were anisotropically consolidated to the same initial stress ratios of the CDS tests. Several techniques were employed to determine the onset of instability in the CDS tests. The effects of unloading rate and initial stress ratio on the behaviour and the triggering of instability in the CDS tests were further investigated. It was observed that specimens consolidated to the same void ratio and initial stress ratio undergo instability at similar stress ratios or friction angles in both CDS and undrained shear tests. This suggests that the instability characteristic of tailings prone to stress relief can be predicted using undrained triaxial tests on anisotropically consolidated specimens. Critical states and state parameters of specimens subjected to CDS and unloading stress paths were also compared and analysed.