Stress Relaxation Behavior and Mechanism of Rock Joint Planes with Different Roughness

Stress relaxation is an important part of time-dependent behavior of rock joints, and also an important factor affecting the long-term stability of engineering rock masses. To study the stress relaxation characteristics of rock joints with different roughness and further explore the stress relaxation mechanism of rock joint planes, samples with artificial joint surfaces based on Barton’s standard profile lines were prepared by pouring cement mortar into shape. Then, shear stress relaxation tests with a multi-stage loading process were conducted to investigate characteristics of stress relaxation curves and stress relaxation rate curves for joint surfaces with different joint roughness coefficient (JRC) under different initial stress levels, the relationships between initial stress and relaxation stress were obtained, and the mechanism of stress relaxation was analyzed from the test phenomena. The results show that with the increase of the initial stress, the relaxation stress decreases first and then increases; that is, the stress relaxation ability of the rock joint planes decreases first and then increases. This phenomenon is related to the deformation and crack development of joint planes during the shear tests. The stress relaxation is a process in which the test machine constantly adjusts to keep the deformation unchanged and the development of internal cracks leads to the reduction of internal resistance, further resulting in the decrease of shear stress; moreover, the stress relaxation ability of the rock joint planes increases with the increasing of JRC.