## 3 Results and Discussion
### Mechanism of Surface Erosion  
The influences of jet pressure and nozzle speed on the geometric shape of the erosion surface were analysed. However, the mechanism of erosion surface formation remains unknown. At present, some scholars [24; 29; 30] have used erosion theory to analyse erosion  
Figure 8: Erosion surface shape curves: (**a**) jet pressure 5 MPa; (**b**) jet pressure 8 MPa; (**c**) jet pressure 10 MPa; (**d**) jet pressure 20, 25 and 30 MPa. Figure 7: Photos of erosion groove shapes. by vertical jets. This paper also refers to erosion theory to analyse the mechanism of erosion surface evolution during soil breaking with a moving water jet. Erosion theory holds that the soil at the water-soil interface is mainly damaged by shear force. In the process of soil stripping, the expansion rate of the erosion surface corresponds to erosion speed, which is perpendicular to the shear force. Erosion can be divided into interfacial erosion and infiltration erosion according to its location. The erosion surface in this experiment moved rapidly with the nozzle, so the erosion was attributed to interfacial erosion without considering the infiltration effect within a short time. The erosion effect caused by shear stress on soil is far greater than that caused by compressive stress and is the main damaging factor. Soil particles at the interface are mainly stripped by the action of shear stress, which is tangential to the erosion surface. The erosion speed is the rate at which the erosion area expands to the non-erosion area in a direction perpendicular to the erosion surface. Therefore, the erosion speed and shear stress direction can be judged according to the shape of the surface. The erosion surface formed by the moving water jet is shown in Figure 9. In the figure, point A is right below the nozzle, the ABC curve delineates the erosion surface shape, and the water jet flows along the erosion surface. Within a certain time \(\Delta t\), the nozzle moves from point A to A\({}^{\prime}\), and the erosion surface also extends from ABC to A\({}^{\prime}\)B\({}^{\prime}\)C\({}^{\prime}\). The displacement of the nozzle is AA\({}^{\prime}\) and AA\({}^{\prime}\) = BB\({}^{\prime}\) = CC\({}^{\prime}\). The tangential slope angle at any point on the erosion surface ABC is \(\theta^{\prime}\), and the erosion surface moves at a speed of \(u_{nz}\) as the nozzle moves forward. The erosion speed \(u_{cr}\) is a component of the normal direction of \(u_{nz}\) on the erosion surface. \[u_{cr}=u_{nz}\cdot\text{sin}\theta^{\prime} \tag{1}\]  
In the formula, \(u_{nz}\) is the nozzle moving speed, and \(\theta^{\prime}\) is the tangential slope angle of the erosion surface. According to the shape of the test erosion surface and Equation (1), erosion speed at jet pressures of 5-10 MPa is as shown in Figure 10. In the process of breaking soil with a water jet, erosion speed does not exceed the nozzle moving speed and decreases with increases in jet distance. At a given nozzle speed, erosion speed attenuation curves at different jet pressures have the same trends. The difference is that, as the jet pressure becomes greater, the slope of the attenuation curve becomes smaller. For example, at the same nozzle moving speed of 20 cm/s, when the jet pressure is 5 MPa, the erosion speed decreases to 0 at a jet distance of 8 cm, and when the jet pressure is 10 MPa, the erosion speed decreases to 0 until the jet pressure is 15 cm. At a given jet pressure, with decreases in nozzle speed, the erosion speed attenuation curve gradually becomes gentle. For example, at a jet pressure of 5 MPa and nozzle speed of 20 cm/s, the erosion speed approaches 0 at a jet distance of 8 cm. Moreover, at a nozzle speed of 1 cm/s, the erosion speed is still close to 1 cm/s at a jet distance of 15 cm. This illustrates the phenomenon of greater nozzle speeds being associated with smaller soil-breaking depths at a given jet pressure.