## 3 Experimental Facilities and Procedures
### Experimental Procedures  
In every test of granite erosion by SPWJ, the SPWJ generated from the squeezed PWJ generator periodically impacted and damaged the rock samples on the clamping device, as shown in Figure 9. The SPWJ-generating device was fixed, a nozzle was arranged at the front end of the pressurization chamber, and then a granite specimen was clamped. The target distance parameter was set by adjusting the position of the clamping frame, and then the clamping device was locked.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Density** & **Elastic Modulus /GPa** & **Poisson’s Ratio** & **Compressive Strength /MPa** & **Tensile Strength /MPa** \\ \hline Granite & 2683 & 49 & 0.27 & 218.2 & 14.10 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Physical and mechanical properties of granite samples.  
Figure 8: Schematic diagram and photograph of the nozzle structure: (**a**) nozzle structure diagram; (**b**) physical drawing of nozzle; (**c**) granite specimen.  
Figure 9: Erosion loading mode of SPWJ.  
Then, the water pump was started, and water was injected into the pressurization chamber. The aluminium alloy baffle in front of the granite specimen was then positioned, followed by starting the hydraulic pump and injecting hydraulic oil into the generator. The device was activated to squeeze the water body and generate a PWJ. The pressurization chamber pressure was monitored, and the inlet oil pressure was adjusted synchronously until it reached the predetermined pressure level, after which the aluminium alloy baffle was removed. Erosion interference of the jet on granite during pressure adjustment was avoided, as shown in Figure 10.  
The SPWJ began to erode granite, and the erosion duration was 60 s in the rock erosion strength test. In the rock erosion failure rate test, the erosion time increased from 30 to 180 s and increased once every 30 s. In the optimum target distance test of rock erosion, the dimensionless standoff distance of the rock was changed from 100 to 300, and \(\Delta\)s = 50, as shown in Figure 10. After each erosion test, the large pieces of rock scattered on the platform were collected, and the eroded granite samples were taken out for measurement. Moreover, six outlet pressures were used in the test: 30, 40, 50, 60, 70, and 80 MPa. In addition, under the same conditions, each rock erosion test was repeated three times. The parameters describing the rock erosion characteristics of the SPWJ were measured and averaged to reduce systematic and accidental errors in the test process and make the results more reliable. The experimental conditions of the rock erosion test with the CWJ were exactly the same as those with the squeezed PWJ, including the jet outlet pressure, target distance, and erosion time. The specific experimental process parameters are shown in Table 3.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**SPWJ/CWJ** & \begin{tabular}{c} **Outlet Pressure of** \\ **Nozzle /MPa** \\ \end{tabular} & \
\begin{tabular}{c} **Dimensionless Standoff** \\ **Distance** \\ \end{tabular} &
\begin{tabular}{c} **Erosion Time/s** \\ \end{tabular} \\ \hline
1 & 30, 40, 50, 60, 70, 80 & 150 & 60 \\
2 & 60 & 50, 100, 150, 200, 250, 300 & 60 \\
3 & 60 & 150 & 30, 60, 90, 120, 150, 180 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Experimental design of SPWJ and CWJ erosion granite.  
Figure 10: Change in dimensionless standoff distance of SPWJ.