Numerical Investigation of Slope Effects on the Stability of Sludge Retention Dikes Constructed with Quarry Waste

Abstract

Phosphogypsum storage poses a major environmental challenge for countries engaged in chemical fertilizer production due to the polluting nature of this industrial byproduct. To promote more sustainable waste management, mining companies are exploring alternative uses, including the construction of sludge retention dikes for liquid effluent containment. In this context, the Gafsa Phosphate Company initiated a project to build sludge retention dikes using quarry waste to store phosphogypsum, with potential applications in civil engineering. This study presents a numerical investigation of the mechanical behavior of sludge retention dikes constructed from quarry waste near Metlaoui City in southwestern Tunisia. Two complementary approaches were employed: the finite element method (FEM), using a plane strain model with the Mohr–Coulomb constitutive law, and the limit equilibrium method (LEM) to cross-validate stability results. Geotechnical parameters for both the foundation soil and dike materials were derived from in-situ and laboratory investigations. The analysis focused on optimizing slope geometry to ensure an adequate factor of safety by assessing the influence of slope angle and sludge height on dike stability before and after basin filling. For a slope inclination of 2V:1H, the factor of safety exceeded 1.4 using FEM and reached approximately 1.5 with LEM following sludge deposition. After deposition, the factor of safety increased on the upstream side to 5.55 and decreased on the downstream side to values ranging from 1.55 to 1.25, depending on the sludge height. Despite this reduction, all configurations maintained a factor of safety above 1.2, indicating a globally stable structure. Furthermore, steeper slope configurations were associated with lower factors of safety, highlighting the critical role of slope design in ensuring the overall stability of sludge retention dikes.