Antecedent rainfall effects on the stability of soil‒rock mixture slopes: influence of matrix type and volumetric block proportion

The antecedent-rainfall effects on soil–rock mixture (SRM) slope stability remain poorly quantified. This PLAXIS 3D study tracks displacement, pore water pressure (PWP), and factor of safety (FOS) through a six-stage antecedent rainfall sequence (−72 to +72 h; peak 0.134 m/day) across 12 configurations: three USDA matrices (Loamy sand, Sandy loam, and Silty loam) at volumetric block proportions (VBP) of 0%, 15%, 25%, and 35%. The model was benchmarked against centrifuge, physical-model, and field-monitoring studies of unsaturated and block-in-matrix slopes, and its sensitivity to rainfall intensity and duration, water-table depth and matrix strength was assessed. FOS was maximized within a narrow band, at 15% VBP in every matrix. Displacement and stability dissociated: Sandy loam at 35% VBP gave among the smallest peak-rainfall displacements (0.002 m) yet a lower FOS than at 15%, so displacement alone can understate risk. Loamy sand failed (FOS < 1.0) at 0% and 35% VBP. Localized positive-PWP cells around blocks, termed water patch zones, formed preferential failure paths. After peak rainfall, FOS fell for 72 h in Sandy and Silty loam, while Loamy sand displacement and PWP kept rising. From one geometry and one event, the VBP thresholds are model-based indicators for monitoring and field verification, not design values.