Shear Behaviour of Rock–Tailings Backfill Interface: Effect of Cementation, Rock Type, and Rock Surface Roughness

Determination of interface shear behaviour is critical for the design of many geotechnical structures, including cemented paste backfill structures. However, the shear characteristics of the CPB–rock interface is not fully understood. No studies have been conducted to assess the impact of the characteristics of rock (e.g., type of rock, rock surface roughness), cementation and the coupled effects of surface roughness and degree of cementation on the shear characteristics of the interface between rock and tailings backfill. This paper presents new findings of research conducted to investigate the effect of different types of rock, rock surface roughness and degree of cementation as well as the coupled effects of these factors on the shear properties and behaviour of the interface between cemented paste backfill (CPB) and rock. The results show that roughness and the interaction between roughness and degree of cement hydration have a significant effect on the shear characteristics of a CPB–rock interface. For a certain degree of cementation, the roughness can increase the shear strength due to the increased size of the interlocking structure. The interface samples with roughness, especially those sheared at high normal stress, tend to experience two peak shear stresses during the shear test. Moreover, the shear strength, shear dilation, and adhesion of the interface also increase with degree of cementation. However, according to the revised Barton’s equation, the effective friction angle of interface with roughness experiences a decreasing tendency with degree of cementation, despite the increasing tendency of the friction angle of the smooth interface. In other words, the interface asperity contributes more to the evolution of adhesion, and the contribution of friction angle to the shear strength is partially offset. It is also found that the rock type (considered in this study) has limited effects on the interface shear properties. The failure of the interface between CPB and rock is because of the existence of the interfacial transition zone, which is characterized by high porosity. These new findings have practical importance to the design of the underground CPB structures.