Análise computacional de liquefação estática da barragem B1 na mina Córrego do Feijão em Brumadinho

Abstract

The mining industry generates a significant amount of ore waste that is discarded during the mining and beneficiation process. While minerals have significant economic importance, tailings dams present the disadvantage of rupture risk, which, in the event of occurrence, causes major negative socioeconomic and environmental impacts. The rupture of Dam I at Vale mining company in Brumadinho, Minas Gerais, in 2019 exemplifies the potential risk involved in the operation of these structures. Liquefaction was identified as the most probable cause of the rupture, which occurs when a saturated soil is subjected, for some reason, to an increase in pore pressure and a consequent reduction in effective stress to the point where the material behaves temporarily like a liquid. Investigations carried out at the request of the Federal Prosecution Service, aiming to identify possible causes of the rupture, took into account the construction history, geometry, flow condition, and historical monitoring data. The groundwater level was considered an aggravating factor, although instrumentation data indicated a lowering after 2.5 years of inactivity. In an effort to understand the event, CIMNE researchers numerically modeled Dam I using the PLAXIS program, employing the CASM constitutive model proposed by Yu (1998). The objective of this study is to analyze the phenomenon of static liquefaction that occurred in the Dam I accident. To achieve this, the geometry was reproduced, and the construction periods of the embankments, dam filling, and the most representative water level at the time of rupture were considered. However, small-scale displacements did not indicate instability in the dam. The dam was then conditioned for a 100-year period, but the displacements remained negligible, and the factor of safety increased slightly. A drilling expedition was underway on the day of the failure. The position of the borehole reached a compressible region near the foundation. In this context, the conditions of the borehole were simulated in order to understand the excess pore pressure generated. The maximum overpressure reached by the software allowed observing that the deformations remained negligible, but the increments of deformations experienced a relative increase unprecedented within a short period of time established in the analysis. Additionally, the stress path revealed a reduction in effective stresses after reaching the peak strength. The identification of the liquefaction process in a small region suggests the possibility of progressive rupture along the entire plasticized surface obtained in the numerical simulations, corresponding to the observed failure surface in reality.