Coal and lignite (brown coal) are geo-resources that have been governing energy production for decades. Geomechanical challenges, particularly slope stability, related to surface coal and lignite mines are critical during operation and will also determine the post-coal era for these areas. Several failure incidents (landslides) have been reported in mining areas, typically associated with a sub-horizontal failure surface on a weak -clay or marl -layer or an interface of low strength. This weak zone controls the soil profile in terms of stability and is common in several mines globally. In this work, this soil profile is analysed in terms of slope stability through the probability of slope failure; such a systematic analysis is currently missing from the literature. Initially, several numerical issues are addressed in the simulation of the problem. Moreover, the essential parameters' effect is examined: the height and the inclination of the slope, the inclination, thickness, and strength of the weak zone, and the strength of the overburden soil. Their effect on slope stability is evaluated by combining the probabilistic point estimate method with the finite element method and the shear strength reduction technique. This work can support a preliminary slope stability analysis and expands the knowledge and understanding of slope stability of a weak zone soil profile. It is concluded that in the presence of a weak zone, the inclination and strength of the weak zone and the water conditions are the most critical parameters and control the stability. Article highlights• Slope stability is performed on a soil profile with a weak zone, describing a common type of coal mines' failure in several countries. • Numerical issues of practical importance are addressed on the simulation and analysis of the problem. • The inclination and shear strength of the weak zone together with the water conditions control the stability of the mines' slopes.

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