Numerical and laboratory experiments on the toppling behavior of a massive single block: a case study of the Furnas Reservoir, Brazil

A massive toppling failure occurred at the edge of the Furnas Reservoir at 12:30 (UTC-03) on 8 January 2022, in Brazil. The toppling belongs to single-block toppling with a volume of about 3.32 × 102 m3 and caused 10 deaths and 32 injuries. Field investigation, numerical analysis, and base friction tests were performed to explore the failure characteristics and mechanism of the toppling. A conceptual model of the toppling mechanism was constructed and the toppling process was divided into four stages: foundation erosion and weakening stage, crack propagation and dislocation stage, opening up and rotation stage, and disintegration and collapse stage. A series of real three-dimensional numerical simulations was performed to clarify the toppling evolution and related triggering mechanism using the finite difference program FLAC3D. Two different alternatives of triggering mechanism for the toppling were comparatively analyzed, the first with a reduction in the shear strength of the weak foundation layer believed to represent the foundation weakening mechanism, and the second with removal of the weak layer believed to represent the foundation erosion mechanism. We found that the foundation weakening of the weak layer resulted in a sliding mechanism of the block, while the foundation erosion resulted in a clear toppling mechanism of the block. The base friction test was conducted to investigate the toppling process and to verify the numerical results over a limited time span. The experimental evidence demonstrated a good agreement with the numerical results as well as those observations in the field. We concluded that the slope was in a critical state due to the foundation erosion of the weak layer, while the heavy rainfall triggered the toppling. It is emphasized that the undermining of the slope foundation and/or existed cavities induced by the foundation erosion played a vital role in the formation of the toppling. Moreover, a produced vertical crack or the propagation of an existed crack in the rear part of the slope may be signs of movements and thought of precursors of the single-block toppling. Dealing with the eroded cavities was suggested to be an effective way to prevent the toppling in the Furnas Reservoir, such as backfilling the eroded cavities with masonry rubble and/or grouting. The understanding of the toppling characteristics and mechanism may offer a reference for single-block toppling issues, such as its movement characteristics and failure mechanism, and may be used for stability analysis and disaster identification of potential failures.