Finite element analysis of flow liquefaction in a tailings dam using two advanced constitutive soil models

Abstract

Tailings dam failures are one of the most destructive phenomena in terms of the number of victims and the environmental impact generated. Over the years, different causes have been identified, with flow liquefaction being a prominent factor to consider when assessing the stability of tailings deposits. Due to the complex nature of these events, numerical models are considered an important alternative for the analysis and design phases of tailings dams. These models require the application of appropriate advanced soil constitutive models to reproduce the relevant features of the soil behaviour and, thus, obtain results that are a valid representation of the observations in reality. In this work, the capabilities and limitations of the Clay And Sand Model (CASM), originally proposed by Yu (1995, 1998) and NorSand model (Jefferies, 1993) are assessed using the finite element software PLAXIS in the analysis of flow liquefaction. The criteria considered in the assessment consist of the accuracy of the models to predict the response of the soil under different monotonic loading conditions, the efficiency of the models for their application and the consistency of results obtained from boundary value problems compared with reality.

Both CASM and NorSand are models developed within the critical state soil mechanics framework and use the concept of the state parameter (Been and Jefferies, 1985). However, they present differences in their formulations and assumptions. The performance of the model is explored in this study at three different levels: the material point level, simple boundary value problem level and complex boundary value problem. Different laboratory tests are simulated for the material point level, analysing a single stress point. For the boundary value problems, the construction of an embankment and the well-known Tar Island Dyke flow liquefaction event are simulated. Before applying the models, the implementation of CASM into PLAXIS is widely verified, and additional verification exercises for the NorSand implementation are also presented. Parametric analyses are performed to have a better insight into the effect of the parameters in the response of the models. Moreover, CASM and NorSand calibrated for the same tailings materials are used for the simulations.

Given that most of the model parameters can be estimated through very well established procedures using experimental data, in principle, the calibration of both models is relatively straightforward if the required experimental data is available and when the data is limited, correlations can be used to estimate the parameters of CASM assuming normally consolidated conditions, which is the state at which tailings are often found. The results of the thesis, at the material point level, show that CASM presents limitations to quantitatively predicting the response of dense materials. This is associated with the formulation and assumptions of the model. In the analysis of the boundary value problems, lower strengths and higher stiffness in terms of stress-strain response are obtained with CASM compared with NorSand. Despite these differences, the results of this thesis demonstrate that both CASM and NorSand can successfully reproduce flow liquefaction at the different study levels.