Laboratory investigation on anisotropic shear behavior of natural and splitting joints under different normal stresses

Mechanical splitting test has been widely used to generate splitting joint for investigating shear behaviors in the laboratory. In fact, some differences are within shear behavior between splitting joint and natural joint, especially anisotropic characteristics. To quantitatively compare these differences, the 3D engraving technique was employed in the present study to repeatedly reconstruct three natural joints and three splitting joints. Then direct shear tests were conducted to analyze their shear behaviors under different shear directions and normal stresses. The results indicated that there were substantial dissimilarities between two types of joints in shear behavior. For the splitting joints, the peak shear strength exhibited slight variations in different shear directions under low normal stress. These variations were less pronounced compared to natural joints. As the normal stress increased, the anisotropy of peak shear strength in splitting joints gradually diminished. For the natural joints, the anisotropy intensified in as the normal stress increased. Additionally, the dip angle of micro-asperities of splitting joint surface mainly presented a normal distribution. The shape of directional parameter, theta*max/(C + 1), was circular. The joint surface was uniform damage under different shear directions. During shear test process, most micro-asperities contributed to the shear behavior without displaying obvious anisotropy. There were no key micro-asperities on the splitting joint surface. Conversely, the dip angle of micro-asperities of natural joint surface exhibited a predominantly irregular distribution. The shape of the directional roughness parameter was also irregular. The damage on joint surfaces presented strip-shaped failure, indicating pronounced anisotropy. These damage areas consisted of the micro-asperities with high height or high dip angle, which were key microasperities influencing the shear failure. The nonuniform distribution of these key micro-asperities of natural joints contributed to the prominent anisotropy in shear behavior.