Pre-failure dynamic analysis of the Fundão tailings dam using the PM4Sand liquefaction model

This study presents the results of a two-dimensional finite element dynamic analysis performed on the Fundão Tailings Dam, which suffered catastrophic failure in November 2015, approximately 90 min after a sequence of low-magnitude earthquakes. The analysis employed advanced constitutive models, NorSand for static loading and PM4Sand for dynamic loading, to simulate the mechanical response of the tailings, aiming to clarify the role of seismic events in the failure mechanism. The nonlinear numerical analyses revealed that the seismic contribution was limited, with computed excess pore water pressure ratio ( r u ) reaching approximately 0.4 in the shallow sand tailings within the reservoir, but remained below 0.03 in the dam's critical zone, where static mobilisation was most pronounced. Near the drainage blanket and the slope toe, excess pore water pressures of up to 30 kPa ( r u ≈ 0.3) suggested localised stress redistribution and incremental deformation, yet were insufficient to induce seismic liquefaction. Predicted maximum horizontal displacements due to the seismic loading were around 10 mm, approximately 1/60 of those obtained under static conditions, indicating minor direct dynamic effects. The results highlight that the low-magnitude earthquakes did not significantly alter the overall stress state of the tailings storage facility and likely acted as secondary triggers, accelerating an already ongoing process of lateral extrusion driven primarily by static conditions. • Fundão Tailings Dam seismic behavior was analysed using PLAXIS 2D FEM code. • Advanced PM4Sand model was calibrated to simulate seismic behavior of tailings. • Seismic sequence likely accelerated failure via lateral tailings extrusion. • Main seismic contribution linked to tailings displacement, not pore water pressure. • Low-magnitude earthquakes show limited impact on the stability of weakened dams.