Application of a critical state model to the Merriespruit tailings dam failure

Abstract

The use of stress-deformation analyses to evaluate the characteristics of possible static liquefaction events is rapidly becoming the norm for tailings dams exposed to such risk. Ideally, those analyses should be based on constitutive models that incorporate the fundamental mechanics of the problem, that are able to perform robustly in commercial numerical platforms, and are simple to calibrate. This work proposes a model that aims to fulfil that specification. The model presented is a new implementation of CASM (Clay And Sand Model), a model based on critical state concepts able to represent static liquefaction. The model formulation is elaborated to show that all required inputs can be related to familiar concepts such as the critical state line, the state parameter, the normalised undrained peak strength, or the coefficient of earth pressure at rest. The methodology is then illustrated by analysing the failure of the Merriespruit tailings dam, which took place in South Africa in 1994 and was instrumental in the recognition of how widespread liquefaction risk might be in saturated tailings. Numerical analysis of the case evidences how a large static liquefaction failure might have been triggered by a relatively small slope erosion caused by overtopping.