## 4 Results and Discussions
### SPWJ Erosion Granite Failure Mode  
To understand how crushed granite specimens are eroded by the SPWJ, the macroscopic crushing morphology of granite after erosion by the CWJ and SPWJ was analyzed. Under a jet outlet pressure of 60 MPa, the damaged volume and area of the granite specimens reached a maximum, as shown in Figure 12. Owing to the erosion effect of the high-pressure water jet, there was an erosion hole in the center of each granite specimen. With the formation of the central hole, some granite specimens began to crack under the action of the water wedge, forming a large-scale erosion pit, indicating that the granite specimens are damaged by water jet erosion in the form of \"drilling and cracking\". This type of damage is characterized by a shallow depth and a large erosion fracture area. Large-scale granite fragments were stripped. It can also be observed from Figure 13 that when the erosion pit produced by the CWJ is damaged by \"drilling\", the damaged area is smaller than that of the SPWJ, whereas when it is damaged by \"drilling-expanding\", the erosion pit produced by the CWJ is larger than that of the SPWJ. This is likely due to the water hammer effect of the SPWJ in the initial drilling stage, which generates strong tensile stress and shock waves on the surface of the granite specimen, effectively drilling it. However, after the drilling stage, the opening of the granite crack requires lateral stagnation pressure. At this time, the CWJ has more advantages than the SPWJ, which leads to larger expansion and crack areas and erosion pits after entering the expansion and crack modes.  
A CT scan of the granite sample after SPWJ erosion was performed and the results are shown in Figure 14. SPWJ formed a large number of cracks in the granite and spalled in different ranges, as shown in Figure 14a. As shown in Figure 14b, the jet eroded the granite to create a main hole channel and produced a large number of main holes around the main hole. In the fissure, the granite is cracked by the pressure of the water wedge. The surface of the granite after erosion has formed several completely closed and incompletely closed fissures, as shown in Figure 14c, including the first gradient of erosion pits, the second gradient of complete cracking pits, and the first three-gradient incomplete cracking. The formation of erosion pits is related to the mechanism by which SPWJ decomposes granite, as shown in Figure 15. There are three models of SPWJ erosion granite. SPWJ uses its erosion effect on granite to form drilling holes. This is model I, the drilling model. After the SPWJ drilled the hole, the reverse jet scoured the hole and formed the scouring pit, which is model II. After SPWJ erodes the granite to form holes, cracks are formed around the holes and further enter the rock mass through the cracks, expanding the propagation of the cracks in the rock body and cracking and destroying the granite. The area of the cracking pit is much larger than the diameter of the jet. This is model III, the initiation model.  
Figure 14: Internal fissures in granite after SPWJ erosion: (**a**) macro destructive features; (**b**) destructive features CT side view; (**c**) destructive feature traces.  
Figure 12: Macroscopic morphology of CWJ and SPWJ eroded granite.  
Figure 13: Fractures of CWJ and SPWJ eroded granite.