In open-pit limestone mines in Japan, limestone layers, referred to as the cover rock and often tens of meters thick, are retained to protect the bedrock. Recently, several cases of slip deformation along the discontinuity between the bedrock and the cover rock have been observed. In controlling the slip deformation, two countermeasures have been empirically used: increasing the thickness of the working bench and constructing buttresses. However, the specific mechanical effects of these countermeasures are unknown, and their optimal designs have not been developed. In this study, two-dimensional numerical models were established to investigate the mechanical effects of the above countermeasures. Effects of working bench thickness were estimated by reducing the lateral extent of excavation. The impacts of the buttress were also discussed by considering the construction width, height, and stage. Additionally, as the strength of the buttress usually determines slope stability, a parameter study was conducted to evaluate its effectiveness under varying contact strengths. The results indicate that both countermeasures can effectively control slip deformation. Increasing the thickness of the working bench significantly enhances slope safety. Buttresses are effective when constructed at the earlier stage of the slip deformation with higher heights. The study concludes by recommending specific countermeasure designs for open-pit mines with similar geological structures experiencing slip deformation.