Rainfall Infiltration-Induced Saturation Evolution and Deformation Response of Centerline Tailings Dams: A Numerical Study

Rainfall infiltration is a critical trigger for tailings dam instability, as the migration of the wetting front and the evolution of internal saturation directly govern the mechanical response and deformation behavior of the dam. This study establishes a representative cross-sectional model of a centerline-constructed tailings reservoir and employs FLAC3D for fluid–solid coupled numerical simulation, systematically investigating the dynamic distribution of saturation and deformation characteristics under continuous rainfall conditions. The simulation implements an extreme rainfall scenario with an intensity of 40 mm/d sustained for 80 days, focusing particularly on the time-history variations of saturation at the surface, 1 m, and 3 m depths in critical zones including the dam crest, dry beach area, upstream slope, and sediment retention structures. Results reveal a phased evolution of saturation under prolonged rainfall: during the initial phase (0–40 days), saturation increases rapidly to peak values (approximately 0.45), with surface zones reaching peak saturation earlier than deeper layers. Notably, the sediment retention dam exhibits excellent drainage performance, maintaining saturation below 0.1 throughout the 3 m depth after a slight initial increase. In the later phase (40–80 days), surface saturation slightly decreases (remaining below 0.45), while deeper layers continue to experience gradual saturation increase due to the dynamic equilibrium between continuous infiltration and internal drainage, leading to a distinctive distribution where internal saturation exceeds surface values. Monitoring data from dam slope positions show that downstream areas experience a 20–30% faster rise in saturation compared to upstream sections, attributed to the superposition of internal seepage flow. Displacement analysis indicates that sustained rainfall mainly induces settlement at the dam crest and upstream slope. When considering the saturation-induced softening effect of tailings, local displacement increments are positively correlated with changes in saturation. Through long-duration rainfall simulation, this research elucidates the coupled mechanism between wetting zone evolution and deformation response in centerline tailings dams under extreme conditions, providing essential data support for long-term stability assessment and early-warning indicator development for tailings dams.