As slopes in the interbedded sandstone and mudstone formation are geologically characterized by proneness to landslides, past elaborations on the failure mechanism of such slopes were mostly confined to the fact that slope slides often occur after rainfalls when rainwater permeates through sandstone formation. Due to sharp difference in water permeability between sandstone and mudstone, the groundwater will accumulate on the interface of the interbedded formation, resulting in increased pore water pressures and reduced safety coefficients. In the process of slides with large displacement, however, change of mechanical properties on the sliding surface is a major factor that affects slope slides. For this purpose, the study aims to develop a ring shear test apparatus that can be applicable to weak rock materials to obtain their residual strength values under the condition of large displacement. The results show that ring shear tests are more effective in obtaining the actual residual strength of weak rock materials and complete post peak shear behaviors than direct shear tests. The results of this study also demonstrate that abrasion between grains of rock materials still continues till their residual strength values reach a stable condition, even though these rock materials are affected by the action of shear strength, and that the content of fine grained materials increases with the normal force in the shear zone.