Excess stress models for capturing nonlinear shear and compressive behaviour of rough joints

Abstract

Understanding the dynamic deformation behaviour of rough joints is of great significance for dealing with the rock engineering problems caused by explosions, impacts or earthquakes, etc. Until now, the dynamic shear and compressive characteristics of rock joints have not been well understood. The nonlinear deformation mechanisms and strain rate effects of joints are two main issues in our knowledge of joint dynamic mechanics. In this study, I have investigated the rate-dependent shear and compressive behaviour of joints and proposed two nonlinear excess stress models of joints, which gave me new insight into the physical mechanisms of joint strain rate effects. Load transfer between asperities during shearing has been observed and detailed in the present study. Together with the damage of asperities, these two dominate the nonlinear shear behaviour of joints. The proposed shear model of joints has formularized this nonlinearity and revealed that static and dynamic shear behaviour of rough joints can be correlated by a rate-dependent component that reflects the acceleration of irreversible/plastic deformation. As for the joint compressive behaviour, testing results show that joint strength and stiffness increase with the rise of loading rate, while energy dissipated from joint deformation decreases with increasing loading rate. That means joints become more brittle at higher strain rates, bringing about low energy dissipation. The proposed joint compressive model has incorporated this rate-dependent feature of joints and also related this hardening behaviour to joint irreversible/plastic deformation. Based on the proposed joint models, a hypothesis is put forward to explain the mechanisms underlying the joint strain rate effects. This hypothesis has been tested by a series of numerical and experimental direct shear tests of rough joints at different shear rates. New testing techniques and modelling methods were employed to provide reliable information about crack propagation and contact-stiffness variation during shearing. It is evidenced that the shear rate effects are attributed to more unstable propagation of microcracks with increasing loading rate. The outcomes of this study would help to understand the static and dynamic shear/compression mechanisms of joints and potentially provide a mechanically sound frame for the studies on the rate-dependent behaviour of joints, such as wave propagation across joints, fluid flow through joints, and dynamic response of rock masses.