Stability Analysis of Upstream Tailings Dam Using Numerical Limit Analyses

Abstract

The catastrophic failure of the Córrego do Feijão Dam 1 in January 2019 has raised many questions regarding the safety of tailings storage facilities constructed by upstream methods. The stability of these structures is strongly affected by the internal stratigraphy and properties of the hydraulically deposited, segregated tailings materials (from slimes to coarse sands) and by control of seepage conditions. The authors conducted an independent review of site investigation data reported by an Expert Panel (December 2019) and evaluated the stability for a critical section of the Feijão Dam 1 using 2D numerical limit analyses under various representations of fill stratigraphy and shear strength properties. The results show that the structure has a low factor of safety, FS=1.11–1.16 on average, based on conventional assumptions of drainage conditions for fill strata. The computed failure mechanism is strongly affected by the presence of a layer of normally consolidated, low permeability slimes near the base of the fill, and by the lack of drainage provisions within the starter dam. The analyses show a clearly defined critical mechanism of failure over the full height of the fill and suggest that the dam has smaller margins of safety than previously published limit equilibrium results. The analyses do not resolve or identify causes of the actual dam failure, which occurred more than three years after the end of construction but do suggest that more reliable methods of slope stability are needed in design.