## 3 Mechanical Properties Analysis
### Macroscopic Failure Characteristics and Fracture Surface Morphology  
#### 3.3.1 Low Confining Pressure Conditions (0 and 5 MPa)  
The failure diagram with confining pressure at 0 MPa, is shown in Figure 9. There was no phenomenon in the sample for a long time. With continuous application of the load, microcracks were gradually generated and then the clear and brittle sound of the rock sample cracking could be heard; however, the surface of the rock sample was not destroyed, but particle ejection occurred at the corner of the rock sample, producing flake spalling debris. With continuous loading, the microcracks continued to extend from both ends to the middle. Finally, the microcracks became interconnected; the rock sample lost its bearing capacity instantly and collapsed loudly.  
The relevant failure diagrams for confining pressure at 5 MPa are shown in Figure 10. Owing to the confining pressure, the original fractures compacted and the mechanical properties of the rock samples improved. It can be observed from the fracture surface that the integrity of the sample was good after failure and the sample damage was localized along a fracture surface inclined at an angle between 69\({}^{\circ}\) and 74\({}^{\circ}\). There is also a fracture surface of approximately 90\({}^{\circ}\), resulting in vertical fracture fragments.  
Figure 8: The fitting curve using Hoek–Brown strength criterion.  
Figure 9: _Cont._  
3.3.2. Medium and High Confining Pressure Conditions (10, 15, and 20 MPa)  
The failure diagrams for scenarios where the confining pressure was 10 MPa is shown in Figure 11. A further increase in the confining pressure made the rock sample denser, and the sample remained complete after failure. The inclined fracture surface was generated along the inclined plane at a certain angle, and the inclination angle of the fracture surface was mostly within the range of 61-71\({}^{\circ}\).  
Figure 12 shows the failure diagrams when the confining pressure was 15 MPa. A further increase in the confining pressure further limited the lateral deformation of the specimen, and the specimen after failure remained complete. The fracture surface of the specimen was inclined, and the inclination angle of the fracture surface was 56-68\({}^{\circ}\). It was found that conjugate inclined fracture surfaces formed in some samples with the inclination angles of the two fracture surfaces being approximately the same.  
Figure 10. Test results and failure modes of granite with confining pressure at 5 MPa.  
Figure 9. Test results and failure modes of granite with confining pressure at 0 MPa.  
Figure 13 shows the failure diagrams for confining pressures at 20 MPa. The specimen was initially in a quiet period for longer than the other cases. The specimen remained  
Figure 11: Test results and failure modes of granite with confining pressure at 10 MPa.  
Figure 12: Test results and failure modes of granite with confining pressure at 15 MPa.  
intact after failure and the dip angle of the fracture surface was mostly 61-65\({}^{\circ}\). Some rock samples also produced a conjugate inclined fracture surface.  
Through a study on the failure characteristics and fracture surface morphology of granite under triaxial compression with different confining pressures, it was found that the vertical fracture surface was dominant under low confining pressures, the inclined fracture surface was dominant under medium and high confining pressures, and the dip angle of the fracture surface decreased with the increase in confining pressure. The average angle of the fracture surface decreased from 66\({}^{\circ}\) (10 MPa) to 65\({}^{\circ}\) (15 MPa) to 63\({}^{\circ}\) (20 MPa).