Prediction model for dilation behaviors of soft rock joints considering degradation of actual contact 3D roughness

To predict the shear dilation characters of natural rock joints, a shear dilation prediction model suitable for soft rock joints is proposed to analyze the degradation laws of the actual three-dimensional contact roughness during the shear process under constant normal loads. The change of normal displacement of joints under shear loads is the superposition of joint climbing behaviors at the shear side of triangular asperities and joint closing behaviors after climbing some asperities. The displacement caused by the climbing and closing behaviors is proportional to the maximum possible dilation angle. The essence of the degradation of the maximum possible dilation angle is the degradation of the average equivalent dip angle of the actual contact joint asperities. By analyzing the joint morphology characteristics at the initial shear and residual stress stages, a model for calculating the initial maximum possible dilation angle and the dilation angle at the residual stress stage is proposed. Furthermore, based on the assumption that the soft rock joints do not experience sudden brittle failure, the variation of the maximum possible dilation angle is quantified by studying the degradation laws of the joint asperities during the shear process. The relationship among the joint climbing behaviors, the closing behaviors and the maximum possible dilation angle is quantified, and a joint shear dilation prediction model is proposed. The validity of the new model is verified by the test results. The new model can accurately describe the shear dilation behaviors and the initial shear compaction behaviors of joints.