Background  During the mining of an open-pit mine, denudated layers contain loose Neogene and Quaternary aeolian, alluvial, and diluvial deposits, facilitating water storage and conduction. This is prone to cause slope degradation and instability, severely threatening mining safety.

Methods  Based on results from field surveys and laboratory experiments, this study systematically analyzed the impacts of groundwater on the weak layer along a slope of an open-pit mine in the Huolinhe Coalfield, Inner Mongolia, focusing on its softening effect on mudstones. By establishing a mechanical model for unconsolidated strata, this study derived an equation for calculating the seepage force in the accumulation layer and quantified the impacts of groundwater level on the factor of safety (FoS) of the slope. The findings were validated through numerical simulation, thereby revealing the mechanisms behind the slope degradation and instability caused by groundwater.

Results and Conclusions The results indicate the basal mudstone layer of the slope is severely weathered, with joints well developed. This layer, together with the Quaternary sandy soil layer, forms the weak layer of the slope. The mudstone layer is low-lying due to the structural influence and is impermeable, rendering its overburden rich in water in the long term. Influenced by groundwater, the weakly cemented accumulation layer is prone to deformations and failure under the action of the seepage force, while the mechanical properties will be reduced by groundwater. Therefore, the slope’s topography facilitates water convergence, and the presence of the weak layer represents a major cause of the slope instability. Furthermore, the slope instability is exacerbated by the groundwater-induced seepage force, buoyancy, and rock-soil mass degradation. Accordingly, it is advisable to intensify systematic hydrogeology research and take the prevention and control measures including the interception of external water and drainage of internal water.