Nonlinear Viscoplastic Modeling of the Feijão Dam 1 Failure

Two-dimensional viscoplastic nonlinear analyses of the 2019 Feijão Dam 1 failure are performed using the finite difference program FLAC 8.1 with the user-defined constitutive models PM4SiltR and PM4Sand to assess how a series of commonly used engineering approaches can approximate the observed failure. A brief history of Feijão Dam 1, its failure, and the findings from two previous independent failure investigations are summarized. The present study uses the site characterization from those prior studies to develop the dam cross section, obtain material index properties, and establish groundwater conditions but uses alternative techniques for characterizing undrained shear strengths. The simulations show that the dam was marginally stable against long-term consolidated, undrained conditions and that modest loading changes were sufficient to trigger failure with deformation patterns consistent with the observed failure. The simulations further show that the collapse could have been triggered by a modest wetting event, ongoing drilling activities, or a combination of both mechanisms. Result sensitivity to choices in the calibration process and the numerical solution scheme are evaluated. The implications of these results on the use of commonly used engineering approaches for system-level time-dependent analyses and on long-term slope stability assessment procedures in practice are discussed. The results of this study provide support for the use of these analysis methods and engineering procedures in practice despite their simplifications and associated limitations.