Purpose. The aim of our work is to study the influence of the failure surface choice on the factor of safety for open-pit Phosphate mine case. Methodology. To estimate the influence of failure surface, our study is focalized on the case of the real slope with complex geometry (Kef-Essnoun Mine), where an important sliding has happened. Firstly, the safety factor (FS) was calculated with Limit Equilibrium Method (LEM) and three non-circular potential surfaces were chosen. Secondly, calculation of the safety factor was performed through the Finite Difference Method (FDM). Finally, the different values of FS obtained for the failure surfaces were compared in order to find the closest approach to what happened in our study case. Findings. The FDM is a useful tool for verification of LEM design due to precise calculation of the safety factor and a unique failure surface. Originality. The originality of this work is to use two different approaches, LEM and numerical method (FDM), for analysing the slope stability design and the accuracy of this method in mining engineering field. Practical value. This study illustrates that the results obtained by LEM in the cases (2) and (3) (FS - 0,920 the minimum value) of failure surface are almost identical to those obtained using the FDM (FS - 0,87), which reflects the reality of our case of study. On the other hand, in the case (1) LEM gives a contradictory result regarding that by FDM (FS - 4,345), the sliding does not happen (total stability). The close agreement between the two analysis methods indicates that the FDM can be used as a practical and meaningful verification of conventional LEM of complex slopes.

Purpose. To assess the geotechnical risk of rock slope stability by empirical, numerical and Limit Equilibrium methods with the generalized Hoek - Brown criterion as a strength failure criterion to encompass all structural and geomechanical parameters influencing the stability of the open slope. Methodology. The study conducted risk assessment in three steps: risk identification, risk analysis, and risk evaluation. First, site observation and movement monitoring confirmed the real existence of instability risk. Then, from the in situ and laboratory investigations, an empirical classification called Slope Mass Rating, SMR and a geotechnical model were obtained. Finally, a quantification of this risk was evaluated using the limit equilibrium method and the finite difference method while considering the nonlinear criterion of Hoek - Brown as a criterion of failure. Findings. The non-linear generalized Hoek - Brown criterion can be used with some reliability in these stability studies since it takes into account the conditions of the discontinuities and the rate of fracturing within the rock mass. Originality. The method for geotechnical risk management was used based on the application of different approaches to quantify the geotechnical risk induced by the exploitation of this open pit. To attain the objective, the following techniques were involved: rock mass classification, geological strength index, limit equilibrium analysis. The philosophy of the research implies combining geometrical, structural and mechanical parameters to assess the rock slope stability. Practical value. This work has allowed us to conclude that SMR classification can be used as a prior check taking into account the structural and geometric context of the rock mass (orientation and integrity damage conditions, fracturing rates). The values of the safety factors of the extended Janbu's method and that of Morgenstern & Price are quite close to the method of finite differences (use of shear strength reduction technique).

Purpose. To analyze the sliding from the geotechnical point of view and to identify the plausible causes that influence it. Methodology. To analyze the sliding from the geotechnical point of view and to identify the plausible causes that influence it, our study follows the following chronological order. Firstly, a feedback reconstructs the slip from the geotechnical point of view; a back-analysis is required to confirm the surface failure. Next, a check will be made of the surface failure through the finite difference method and using the shear strength reduction by finite difference method (SSR-FD). A parametric study on the influence of the geometric parameters is performed to see the influence of the latter on the stability. The plausible cause that has directly influenced this sliding is shown. Findings. This paper considers the application of three methods: limit equilibrium method (LEM), finite element method (FEM) and finite difference method (FDM) to perform back analysis and find out the layer of slope failure in an open pit mine. Originality. Three different approaches LEM, FEM and FDM were used to perform the back-analysis the rock mass properties and to investigate the slope failure mechanism and the accuracy of this method in mining engineering field. Practical value. This study has illustrated that non-compliance with the art, open pit mining standards and early planning of the mining method can most often lead to critical situations and catastrophic results. The back analysis of the slip site allowed us to reconstruct the break already observed and to draw maximum lessons on the mode, location and mechanical parameters that triggered this break.