Interpretation of the Failure of the Aznalcóllar (Spain) Tailings Dam

Abstract

The mechanisms and causes of the sudden failure of the Aznalcollar tailings pond were investigated. The dam underwent displacements of up to 55 m along a 700 m length, releasing large quantities of acidic waters and 5.5 million m3 of pyrite and pyroclastic tailings. It was a progressive type of failure, allowed by the brittle response of the pre-consolidated and cemented Guadalquivir blue clays and the high pore pressures left from the incomplete consolidation of the dam’s foundation. Special modelling difficulties were posed by the need to incorporate the strain softening behaviour of the clays. The results show that although the timing of the failure could have been approximated using equivalent ductile properties, predicting the shallow, planar geometry of the failure surface observed would have required a precise representation of the brittle response. The failure triggered liquefaction of the tailings, which accelerated at more than 0.1 g. This transition was modelled by migrating from a coupled effective stress approach employing implicit integration to a total stress formulation using an explicit solver.