Mechanism of failure of the Mount Polley Tailings Storage Facility

On 4 August 2014, a breach developed in a portion of the embankment at the Mount Polley Tailings Storage Facility fol-lowing foundation failure. This paper explores the mechanism of this progressive failure in view of pronounced three-dimensional stability effects identified in this slope. The findings of a three-dimensional deformation analysis of this failure are reported. The results demonstrate that its initiation can be explained in terms of minor strain-weakening within a thin shear band in the glaciolacustrine unit at the base of this slide, combined with the delayed mobilization of shear resistance in the shell zone owing to its relative ductility, along with the reduction in the levels of mobilized shear resistance in the upstream region of the slide resulting from a loss of confinement and (or) failure in extension caused by the translational displacements the base of the side. While recognizing the limitations of analyses alone, simulation suggests that strain localization in a shear band with a thickness not exceeding 12-13 cm is requisite for the initiation of this failure.