## 2 Materials and Methods  
### Test System  
Previous studies [12; 20; 26] have indicated that the key parameters of a moving jet include the nozzle diameter, jet pressure, nozzle moving speed, and soil shear strength. The nozzle diameter and injection pressure are dynamic parameters, which have a significant impact on the power consumption, weight, and cost of the equipment. The moving speed of the nozzle is determined by the requirements of the grooving operation, and the shear strength of the soil depends on the local geological conditions. These four parameters are all important influences on the erosion notch. Considering the stability and cost of a diversion hole erosion notch, a jet scheme of high pressure and low moving speed is given priority. Therefore, combinations of different jet pressures and moving speeds were tested in this experiment to study their influences on the morphological characteristics of soil breaking. Considering the uneven water content of natural mine soil, soil strength varies greatly with depth. In this experiment, the soil was made into large remoulded soil samples in a square box with the unified compaction method. The moving speed of the nozzle, i.e., the duration of action between the jet and the soil, needed to be accurately controlled for accurate jet module movements. Therefore, the water jet soil-breaking test system mainly comprised a test box, a jet pressure supply module, and a control module. A schematic diagram of the test system is shown in Figure 1.  
(1) Test box  
The side lengths of the test box were 1500 mm, 1000 mm and 500 mm. The box was made of steel-plastic composite plates with a thickness of 80 mm connected by high-strength bolts. Therefore, the whole box had high deformation resistance and could be disassembled. (2) Jet pressure supply module  
At present, there are two common ways for supplying jet pressure. One is performed by applying water head pressure to the pressure water storage tank with an air compressor and other equipment, and then by providing a stable jet pressure through a high-pressure pipe and a regulating valve [24; 27]. The second is performed by pumping a certain pressure  
Figure 1: Schemes follow the same formatting. of water directly through a high-pressure pump [7; 20]. Considering that the second way is often used to provide high-pressure water in practical engineering, the pressure supply device in this test uses a high-pressure pump. The jet pressure supply module mainly included a high-pressure pump, a throttle-pressure-regulating valve, ahigh-pressure pipe, and a nozzle, as shown in Figure 2. The high-pressure pump was a plunger pump with a maximum output pressure of 50 MPa. The pump body was a TW5570 triplex high-pressure plunger pump produced by Italia COMET. The plunger was made of ceramic material. The motor had a rated power of 23 kW and a speed of 1450 r/min. The jet pressure in the test was adjusted by a throttle-pressure-regulating valve. The end of the high-pressure pipe was connected to a high-pressure nozzle and a pressure transmitter. The nozzle diameter was d = 1.37 mm. The structure size of the nozzle is shown in Figure 3.  
The control module mainly controlled the jet pressure and nozzle moving speed during the tests, as shown in Figure 4. Monitoring of the jet pressure was achieved by a Firstrate glass micro-melt core pressure transmitter. The pressure transmitter of the core had high impact resistance and a short pressure response time. The output was a 4-20 mA electrical signal, which monitored jet pressures of 0-50 MPa. Control of the nozzle's moving speed was realised by a servo motor controlling a moving platform on a track. There was a photoelectric door and a relay at the end position, which stopped the high-pressure pump when the end position was reached. To test whether the high-pressure pump could provide a stable jet pressure, the pressure transmitter was set close to the front end of the nozzle where the jet pressure of the nozzle was measured.