Probabilistic Finite Element Assessment of Municipal Solid Waste Landfill Stability: Effects of Slope Geometry and Waste Heterogeneity

Municipal solid waste (MSW) exhibits substantial variability in composition, density, degradation state, and mechanical properties, introducing uncertainty into landfill slope stability assessment. This study investigates the influence of waste heterogeneity and slope geometry on the probabilistic stability of MSW landfill slopes. Five slope configurations, from 1V:4H to 1V:1H, were evaluated using four finite element representations of MSW properties: a homogeneous model (C1) and heterogeneous models comprising 4, 13, and 40 waste layers (C2–C4). Analyses used PLAXIS 2D with the shear strength reduction method in an automated Python–PLAXIS framework, where C4 properties were resampled from a compiled literature database over 1000 Monte Carlo realizations per slope configuration. The steepest configuration (1V:1H) increased storage capacity by 54.3% and reduced required footprint by 31.2% relative to 1V:4H, highlighting the capacity-stability trade-off. As slope steepness increased, mean factor of safety decreased from 1.459 to 0.439, while simulated probability of failure increased from 15.4% to 98.1%, with the largest increase between 1V:3H and 1V:2H. The stochastic model produced mean factors of safety up to approximately 72% lower than the homogeneous representation; decomposition using an additional diagnostic configuration (C1b, homogeneous at the C4 database mean) indicates that this reduction is dominated by the spatial representation of heterogeneity (73–79% of the total difference across configurations), with the property database accounting for the remainder (21–27%). Results should be interpreted as comparative responses conditional on the adopted database and spatial representation.