Procedure for assessing the liquefaction vulnerability of tailings dams

Abstract

This paper presents a procedure to assess the vulnerability of tailings dams to failure due to flow liquefaction. Vulnerability is understood as the inability of a system to resist adverse actions. The procedure uses a numerical model to verify if a set of actions that induce undrained shear in the tailings would result in a progressive failure and instability of the system. The procedure is tested by applying it to the well-documented Fundão Dam failure. The undrained behavior of loose tailings, including the onset of liquefaction and post-peak loss of strength, are simulated using the state parameter-based constitutive model, Modified Pastor-Zienkiewicz (MPZ), implemented as a user-defined model in Plaxis. Three actions are analyzed that cover most of the stress paths leading to the failure of tailings dams as geotechnical structures: (i) a surface load applied on the crest of the dam; (ii) a horizontal deformation at the toe, and (iii) a rise in the phreatic surface within the tailings. These three actions were applied to the Fundão Dam case and used to evaluate the vulnerability of two relevant configurations in the dam's history. This example is intended to illustrate that the proposed procedure can be used to assess the vulnerability of tailings dam failure to flow liquefaction using currently available modelling tools and standard laboratory and field characterization. While not part of the proposed actions, an additional analysis was conducted to assess the Fundão Dam's response to the failure mechanism identified in the post-failure report.