An Integrated Risk Assessment Framework for Overburden Dump Slopes in Opencast Mines: Coupled Deterministic–Probabilistic Stability Analysis and Physico-Chemical Characterisation — Field Evidence from the Jharia Coalfield, India

Overburden (OB) dump failures in opencast mines represent a persistent geotechnical and environmental hazard in coal-producing nations worldwide, yet an integrated risk assessment framework coupling geomechanical stability, parametric uncertainty quantification, and post-mining land-quality assessment remains largely absent from the mine waste literature. This paper develops and demonstrates such a framework through a field investigation of OB dump slopes in the Eastern Jharia Coalfield (JCF), India. The approach integrates: (i) comprehensive physico-chemical characterisation of dump fill material, (ii) deterministic stability modelling using five Limit Equilibrium Method (LEM) formulations (Bishop Simplified, Morgenstern–Price, Janbu Simplified, Janbu Corrected, Spencer) in SLIDE 6.0 and Finite Element / Strength Reduction Method (FEM/SRM) analysis in FLAC/Slope 8.0, and (iii) full probabilistic risk quantification via Monte Carlo Simulation (MCS, n = 100,000) and First-Order Second-Moment (FOSM) analysis. The framework yields not only a probability of failure (P ^f ) and reliability index (β), but also a practitioner-oriented design chart linking β to mean FOS across a range of coefficient-of-variation values — a directly transferable tool for mine operators and regulatory bodies. Application to the JCF slope confirms critical instability across all methods: minimum FOS of 0.751 by LEM, 0.810 by FEM, and P ^f of approximately 98% (β = −2.29; FOSM-derived, negative sign confirming mean FOS lies on the failure side of the limit state) from MCS — well above the commonly cited 2.3% threshold corresponding to β_target = 2.0, confirming near-certain failure under current geometric and material conditions. Friction angle governs sensitivity, while cohesion offers the most cost-efficient remediation pathway. The framework is designed to be replicable at any OB dump site globally, requiring only standard site investigation data, and is directly applicable to emerging mine-closure and reclamation planning requirements. The dump material (USCS: SP, slightly acidic, negligible cohesion, trace heavy-metal contamination including anomalous mercury linked to subsurface mine fires) is concurrently assessed for reclamation suitability, illustrating the integrated geotechnical-environmental utility of the approach.