## 4 Results and Discussions
### Influence of Erosion Time on Rock Breaking Effect  
The rate trend of granite erosion by the SPWJ and CWJ is shown in Figure 18, and it is an important index for characterizing the erosion and crushing performance of the water jet. As presented in Table 3, the destruction ability of the SPWJ and CWJ under different erosion times was studied. As shown in Figure 18c, the breaking rates of the two types of jets have similar laws. With the extension of erosion time, the volume of erosion spalling increases first and then decreases. This change is related to the process of granite erosion. At the initial stage of jet erosion of the granite specimens, the jet forms shallow pits on the surface of the rock samples, a small hole is formed by drilling, and the jet further expands the small hole. The target distance of the jet increases owing to the drilling of the small hole, and the space in the hole also prevents deep erosion by the jet. At this time, the jet starts to enter the granite's internal pores, and cracks are generated by the jet, which overcomes the tensile strength in the rock body, expanding and cracking the rock mass, peeling off large granite rock blocks, and further prolonging the target distance of the jet contacting the rock mass surface. At this time, with an increase in the target distance, the ability of the jet is weakened, and the erosion efficiency of the jet decreases gradually. When the erosion time of the jet is 30, 60, or 90 s, the erosion crushing volume and depth of the SPWJ are larger than those of the CWJ, and the area of the crushing pit is not significantly different from that of the CWJ. Then, when the erosion time of the jet is prolonged to 120, 150, and 180 s, the crushing performance of granite by the SPWJ and CWJ is quite different, as shown in Figure 18a-c. With an increase in time, the erosion depth, crushing area, and spalling volume of the SPWJ change smoothly, thus indicating that the SPWJ is less sensitive to erosion time. In a short time, the optimum crushing volume will be reached, and its crushing efficiency will be at its maximum. However, for the CWJ, with the extension of erosion time, when the erosion time exceeds 90 s, the crushing ability is significantly improved, the depth and area of the eroded granite and the volume of the exfoliated granite are significantly greater, and the sensitivity to erosion time is strong. The extension of erosion time can effectively increase the ability of the CWJ to erode and break the granite. This occurs because, for the SPWJ, jet erosion peels off the granite, which can form multi-layered swelling and cracking rings. As shown in Figure 13, shallow pits are formed at the initial stage of erosion damage, and the rock samples are further cracked. However, owing to the intermittent jet emission, it is difficult to store water in rock fissures, and with an increase in time, the crushing effect is limited. For the CWJ, the  
Figure 18: Erosion rate of granite by squeezed water jet and continuous water jet: (**a**) erosion pit area; (**b**) depth of erosion pit; (**c**) erosion pit volume; (**d**) accumulated volume loss of erosion pits; (**e**) unit input pressure failure volume. continuous injection can cause more water bodies to enter the cracks, which are expanded, and the spalling volume of granite increases with the extension of erosion time. As shown in Figure 18d, to further simulate the practical application of the jet in engineering, the crushing rate of the accumulated erosion time of the jet is analyzed, and it is found that the SPWJ has an advantage in the initial stage, but with the accumulation of time, the CWJ will peel off more granite. As shown in Figure 18e, the crushing volume of SPWJ is lower than CWJ at the unit input pressure when the erosion time is 120 s, and it is higher than CWJ at other erosion times. In summary, under the same pressure level, compared to continuous water jets, SPWJ can effectively erode and destroy granite, resulting in larger fracture pits.