Evaluating LEM, FEM, and DFN approaches for rock slope stability across GSI ranges for open pit mine

Rock slope stability is a critical aspect of safe and economical open-pit mining, particularly in structurally complex hard-rock deposits where discontinuities govern failure mechanisms. This study presents a comparative assessment of the Limit Equilibrium Method (LEM), Finite Element Method (FEM), and Discrete Fracture Network (DFN) approaches for evaluating the stability of an open-pit iron ore mine in central India over a quantified Geological Strength Index (GSI) range of 15–75. Detailed geological investigations, including face mapping of 260 discontinuities, laboratory testing of core samples obtained from 204 boreholes, and quantified GSI evaluation, were carried out to derive representative rock mass properties. Comparative stability analyses were subsequently performed using Slide2 (LEM), RS2 (FEM), and RS3 (DFN) under identical slope geometry, excavation stages, and boundary conditions. The results demonstrate that the LEM consistently predicts the highest Factors of Safety (FoS), whereas the DFN approach provides the most conservative estimates by explicitly accounting for structurally controlled failure mechanisms. Relative to the LEM, the DFN approach reduced the predicted FoS by approximately 6–20%, while differences between the DFN and FEM ranged from 5–13%, depending on rock mass quality. A gradual transition from structurally controlled behaviour to pseudo-continuum behaviour was observed over the investigated GSI range, with the onset of convergence between the three modelling approaches occurring at approximately GSI ≈ 35 for the investigated mine. The findings indicate that continuum-based methods provide reliable stability predictions for highly fractured rock masses, whereas explicit DFN modelling becomes increasingly important for competent, jointed rock masses where discrete discontinuities govern failure. The study provides a practical framework for selecting appropriate numerical modelling approaches for open-pit slope design based on site-specific geological conditions.