## 4 Experiment Results and Analyses
### Failure Mechanism of Rough Discontinuities Possessing Different Joint Wall Strengths  
Figure 7 presents the failure characteristics of discontinuities possessing different joint wall strengths with JRC of 2.8. It can be seen from Figure 7 that the failure of joint specimens with JRC of 2.8 mainly showed up as friction failure. For sandstone-mortar discontinuities, there were different degrees of wear on the surface of both sandstone and cement mortar, and the damage of cement mortar was more obvious. The damage area and damage depth of the joint surface increased with the increase in normal stress. For granite-mortar discontinuities, the damage was mainly concentrated on the side of the cement mortar. The asperity slightly climbed during the shear process. Due to the slip friction between the upper and lower joint blocks, the asperity on the surface of the mortar was ground and microcracks appeared on the asperity of granite. With the migration of ground mortar particles, the mortar particles were crushed and bonded on the surface of the granite. With the increase in normal stress, the degree of friction damage on the surface of mortar was strengthened and the number of ground mortar particles increased. Figure 7e presents the microscopic failure mechanism of the joint specimen with JRC of 2.8. Based on the strength of rock and mortar, granite and sandstone are named as hard rock, while mortar is named as soft rock. Slight dilation accompanied with the sliding friction between asperities appeared in the joint during the shearing process. Due to the strength difference of hard rock and soft rock, the asperity of soft rock was ground and the asperity of hard rock produced microcracks. Figure 8 shows the failure characteristics of joints with JRC of 10.8. Both types of discontinuities possessing different joint wall strengths exhibited shear failure after dilation. For sandstone-mortar discontinuities, sandstone and mortar only exhibited relative friction and wear between asperities after dilation under low normal stress, and slight scratches appeared on the surface of sandstone and mortar. Meanwhile, the edge of the sandstone surface was fractured due to stress concentration. With the increase in normal stress, the  
Figure 7: Failure characteristics of specimen SCR1 under normal stress of (**a**) 0.5 MPa, (**b**) 3 MPa, and specimen GCR1 under normal stress of (**c**) 0.5 MPa, (**d**) 3 MPa, and (**e**) sketch map for illustrating the failure mechanism of discontinuities possessing different joint wall strengths with JRC of 2.8.  
size of the damaged area increased gradually. Under high normal stress, both the asperities of sandstone and mortar exhibited climbing shear failure, and stress concentration occurred at the edge of mortar. For granite-mortar discontinuities, some asperities on the mortar surface were crushed and cut off under low normal stress, and only a small amount of friction damage appeared on the granite surface. In contrast, under high normal stress, the damage area of granite asperities increased significantly and some asperities were cut off. Meanwhile, the edge of the granite broke due to stress concentration. A great deal of asperities on the surface of cement mortar were cut off. Under the interaction of normal stress and shear stress, the generated moment led to tensile cracks in cement mortar. Figure 8e presents the microscopic failure mechanism of the joint specimen with JRC of 10.8. Discontinuities exhibited obvious dilation during the shearing process and the torque generated during the climbing process led to tensile cracks in both hard rock and soft rock. The shear damage of asperities was deeper due to the lower strength of soft rock. Figure 9 presents the failure characteristics of discontinuities possessing different joint strengths with JRC of 18.7. The damage depth and damage area of discontinuities with JRC of 18.7 were greater.