Back-analysis of the rock landslide at the northeastern part of Kef Essenoun open-pit mine (NE Algeria): Post-disaster observations and numerical assessment

In the early morning hours of September 8, 2007, a massive landslide occurred at the northeastern part of the Kef Essenoun (KEs) open-pit phosphate mine in Djebel Onk, northeast Algeria. Although no human casualties or direct material damage were recorded, the disaster had significant economic consequences, including the prolonged cessation of mining operations in the affected area and the abandonment of substantial phosphate reserves, which considerably reduced overall production. Therefore, this research aims to analyze the failure mechanism and stability of the KEs open-cast mine following a major landslide that brought mining operations to a halt. The stability analysis adopts two approaches, post-landslide observation and numerical analysis, intended to identify the probable cause of the disaster and delineate the unstable zone. Numerical simulation was carried out through the limit equilibrium method (LEM) and finite element method (FEM), where the failure surface and the factor of safety (FoS) were chosen as indicators to evaluate the stability of the KEs open-cast mine. Post-disaster geological and geotechnical investigation indicates that the low-strength phosphate-marl layer governs the instability, where the planar failure mode crosses this zone. The numerical simulation shows that the FoS values are significantly less than unity; both the LEM and FEM analyses yield close results, confirming that the studied quarry was unstable. In addition, the FEM (FoS=0.83) is more conservative than LEM (FoS=0.944) and produces a planar sliding that faithfully reflects the realistic failure mechanism. The study demonstrates the importance of integrated back-analysis in understanding open-pit landslide mechanisms and highlights the need to combine multiple analytical approaches to ensure reliable stability assessments.