In the field of geotechnical engineering, the residual shear strength is an essential parameter used to evaluate the long-term stability of new and existing landslides. According to the literature, landslides may occur on many types of bedding planes such as shales, sandstone and mudstone, which consist of cementation materials resulting from diagenesis. In particular, the failure behaviour of slips at the contact surface between the cementation soil layers and the upper soil layers has not yet been sufficiently investigated. In this study, we investigate the shear-strength characteristics of artificially cemented kaolin clay, which models the actual movements of slip surfaces between two soil layers with different cement content. To do this, we use an improved ring shear test apparatus. A series of ring-shear tests was performed on non-cemented kaolin as well as a combination of two parts of a sample of cemented and non-cemented kaolin under various test conditions. The results showed that the residual frictional angle calculated on the contact surface between the two kaolin samples approximately 33.6% lower as compared with the corresponding value for pure kaolin. For combined-cement samples, the residual cohesion intercept, which characterizes the effect of cementation, increased linearly with cement content. We also determined the growth stress ratio at the residual state as the shear-displacement rate increases.
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