## 4 Experiment Results and Analyses
### Failure Mechanism of Rough Discontinuities Possessing Different Joint Wall Strengths
Plenty of asperities were cut off on the surface of hard rock and soft rock, and massive fragmented rock particles were produced by extrusion and crushing of asperities on the discontinuity. Under high normal stress, multiple tensile cracks appeared in the mortar, and asperities of mortar were completely chewed off due to the interpenetration of tensile cracks. Figure 9e presents the microscopic failure mechanism of the joint specimen with JRC of 18.7. Under the effect of the torque generated during the shear process, asperities of hard rock were partially sheared due to the higher strength of the hard rock. Meanwhile, tensile cracks inside the soft rock were interpenetrated and asperities of soft rock were completely chewed off. Figure 8: Failure characteristics of specimen SCR2 under normal stress of (**a**) 0.5 MPa, (**b**) 3.0 MPa, and specimen GCR2 under normal stress of (**c**) 0.5 MPa, (**d**) 3.0 MPa, and (**e**) sketch map for illustrating the failure mechanism of discontinuities possessing different joint wall compressive strengths with JRC of 10.8. It can be realized from the above analysis that discontinuities possessing different joint wall strengths needed to overcome the dilation and friction resistance of the surface asperity under low normal stress, and the damage was mainly manifested as the wear and shear of asperities on the soft rock. The shear strength of discontinuities was mainly influenced by the strength of the soft rock. The upper and lower blocks of the discontinuities came into close contact with the increase in normal stress. Plenty of asperities on the soft rock were sheared and crushed, while asperities of hard rock were partially embedded in the soft rock and tightly engaged with the soft rock subjected to higher normal stress. Moreover, asperities of the hard rock also successively experienced dilation and shear failure under sustained shear effect, which greatly improved the peak shear strength and shear stiffness of the discontinuities. The shear strength of discontinuities possessing different joint wall strengths was mainly governed by the shear strength of the hard rock. On the basis of the above analysis, the shear process of discontinuities possessing different joint wall strengths can be divided into four stages combined with the shear process of the GCR3 specimen, as illustrated in Figure 10. (a) Stage of tight matching: The upper and lower blocks of the discontinuities possessing different joint wall strengths came into light contact and coupled compaction under the effect of normal stress. (b) Stage of climbing wear of soft rock and crack formation of hard rock: As the shear proceeds, the joint walls of discontinuities dislocate relatively accompanied by dilation. Meanwhile, part of the asperities of the soft rock is worn down and cracks appears in the asperity of the hard rock. (c) Stage of partial shear of soft rock and crack propagation of hard rock: Asperities of the soft rock are partially cut off when the shear stress endured by the soft rock reaches its shear strength. Asperities of the soft rock are completely cut off and cracks in asperities of the hard rock further expands under high normal stress. (d) Stage of entire cutting of asperities: As the shear stress of discontinuities increases continually, asperities of the hard rock are partially cut off. Meanwhile, obvious tensile cracks occurred due to the tensile  
Figure 9: Failure characteristics of specimen SCR3 under normal stress of (**a**) 0.5 MPa, (**b**) 3.0 MPa, and specimen GCR3 under normal stress of (**c**) 0.5 MPa, (**d**) 3.0 MPa, and (**e**) sketch map for illustrating the failure mechanism of discontinuities possessing different joint wall compressive strengths with JRC of 18.7.  
stress in the soft rock exceeding the tensile strength. Interpenetration of tension cracks causes the soft rock to be completely chewed and crushed; hence, the discontinuity was completely broken. The summary of these four stages is shown in Figure 10e.