Exploring micromechanical behaviors of soft rock joints through physical and DEM modeling

The joint surface of a rock mass is one of the most important factors that strongly affect its shear strength, which is critical in rock engineering. Specifically, the joint roughness coefficient (JRC) is a parameter that represents the profile characteristic of the joint. In the authors’ earlier investigation, any joint profile with a given JRC might be generated randomly for engineering purposes. In this study, a series of direct shear tests in both laboratory and numerical modeling (through discrete element method (DEM)) was conducted for soft rock joints to elucidate the mechanical properties of a randomly-generated JRC profile. A 2D-DEM model was adopted to simulate the joint specimen under the same direct shear conditions as in the laboratory tests. A reasonable agreement is found between the experimental direct shear tests performed on an artificial gypsum plaster model and the numerical modeling that was carried out, showing that the numerical model can be used in the interpretation of the direct shear tests of joint surfaces. Besides, the peak shear strength of the gypsum model also compares well with that predicted by Barton’s equation. Based on the lab test results and numerical simulation, the failure mechanism of the joint specimen is correlated with the normal stress applied. From a microscopic viewpoint, the distribution of contact forces is most concentrated at the early stage during shearing, especially at the time of the peak shear stress. The distribution of shear stresses along the shear plane is not uniform, depending on the degree of joint undulation. The peak shear strength of the soft rock joints mostly comes from the roughness along the joint surface. However, the residual strength is mobilized from reduced roughness and the shearing-off of the joints.