On the effect of anisotropy on drained static liquefaction triggering

A number of recent tailings storage facility (TSF) failures has been indicated through post-failure investigations to have been triggered in tailings that were under drained conditions up to the initiation of instability. Such field-scale behaviour is consistent with many laboratory element studies on drained triggering of instability. However, essentially all available data on the drained initiation of instability has been carried out under conditions where the principal stress angle (α) is zero. This is despite the principal stress angle in zones of TSFs where instability is likely to be triggered being below-slope, where α is higher than zero. To assess the implications of anisotropy on the drained initiation of instability, the limited data available that enables assessment of the effects of α on instability in undrained testing is referenced. These data suggest that the likely range of principal stress angles below-slope areas of TSFs are such that anisotropy should be considered in estimating relevant triggering stress ratios. The need for more experimental data of drained triggering of instability at principal stress angles relevant to below-slope conditions is outlined.