An experimental and analytical study of rate-dependent shear behaviour of rough joints

Understanding the shear behaviour of rough joints is of great significance for dealing with rock engineering problems. In many cases, rock joints are often subjected to dynamic loadings, which are usually caused by explosions, impacts or earthquakes, etc. Until now, the dynamic shear characteristics of rock joints have not been well understood. In the present study, we investigate joint shear behaviour at different shear rates and develop a rate-dependent constitutive model of rough joints. Testing results shows that the shear/frictional strength of joints with meso-roughness (i.e., planar joints) is independent on the shear rate, while the strength of joints with macro-unevenness (i.e., saw-toothed joints) increases with the shear rate. Under identical boundary conditions, the residual strength of joints with macro-unevenness approximates to the kinetic frictional strength of planar joints. Based on the testing results, the shear stress-displacement curve of rough joints was divided into four phases, i.e., linear elastic phase, shear hardening phase, shear softening phase and residual strength phase. A viscous joint model was thus proposed and validated through comparison with laboratory measurements. This new model correlates the static joint shear stiffness with dynamic one, and has the ability to describe the shear stiffness evolution during the whole shearing process. Based on the proposed model, a hypothesis about shear rate effects of rough joints was introduced and then verified through numerical modelling. The findings in this paper could facilitate better understanding the dynamic behaviour of rough joints and be useful for analyzing rock engineering problems with discontinuous rock masses.