## 2 Materials and Methods
### Test Scheme  
(1) Selection of test soil samples  
The selected mine is a leaching rare earth mine in Pingnan County, Fujian Province. It is located in the fully weathered layer. The weathering crust of the mine rock mass is thick, and the shell structure is well developed. The fluid collection project was located in the full regolith, from which the soil was sampled. The properties of the soil sample were as follows: the mineral composition was mainly plagioclase, quartz, potassium feldspar, and carbonated lava-like material. The plagioclase was grayish white, and the content was about 30-40%. The potassium feldspar was a light flesh red, with a content of about 20-30%. The quartz was grayish white, and the content was about 20-25%. The quartz chip had been broken locally. The acid lava-like material was mainly composed of fine-grained feldspar and quartz, the content of which was about 20-25%, and had been partially kaolinised. Pingnan rare earth ore samples were selected for the test, which were taken after stripping the surface clay of the mine. The average void ratio of undisturbed soil samples was 0.90, the density was 1.73 g/cm\({}^{3}\), and the average water content was 18% by mass, as obtained via the cutting ring method. The values of c and \(\varphi\) were measured with a GDS triaxial apparatus. The particle gradation of soil samples was measured with a screening  
Figure 4: Control and acquisition module. Figure 5: Monitoring data of jet pressure. method and a laser particle size analyser. The particle gradation curve is shown in Figure 6, and the particle composition distribution of Pingnan soil is shown in Table 1. According to the American Soil Texture Classification Standard [28], the soil texture of that in Pingnan is loam. The test soil samples were compacted according to their original moisture content and void ratio. The shear strength of the soil sample surface was measured with a miniature vane shear apparatus. The permeability coefficient was measured with variable head column immersion tests. The physical parameters of the test soil samples are shown in Table 2.  
Loose soil samples were mixed at a water content of 18% and were stirred evenly. They were added to the test container for layered compaction. The void ratio of the compacted soil sample was 0.9, and the thickness of each layer was 5 cm. Layers were added until the thickness was 40 cm. Then, the upper part of the containers was covered with film to maintain the humidity of the soil samples, which were left to stand for 2 days before testing. (2) Test conditions  
The test used a moving nozzle to erode the remoulded soil samples, and the shape of the erosion groove was recorded. The area from the nozzle bottom to the groove bottom showed the erosion surface morphology, which was the focus of this study. The soil-breaking pressure commonly used in some projects is 5-20 MPa, so the jet pressure \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample Location**} & \multicolumn{2}{c}{**Distribution of Soil Particle Size/\%**} & \multirow{2}{*}{**Soil Texture**} \\ \cline{2-3}  & **Sand** & **Silt** & **Clay** \\  & **(>0.05-2 mm)** & **(0.002-0.05 mm)** & **(<0.002 mm)** \\ \hline Pingnan & 52.76 & 38.37 & 8.87 & Loam \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical composition of soil. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Testing Means** & **Value** & **Unit** \\ \hline Relative density & Pycnometer method & 2.70 & g/cm\({}^{3}\) \\ density & Cutting ring method & 1.73 & g/cm\({}^{3}\) \\ Water content & Drying method & 18 & \% \\ Shear strength & Miniature vane & 10.6 & kPa \\ Cohesion & GDS triaxial apparatus & 23 & kPa \\ Internal friction angle & GDS triaxial apparatus & 23.6 & \({}^{\circ}\) \\ Permeability coefficient & Column leaching test & 0.16 & m/d \\ Plasticity index & Liquid–plastic limit combined method & 9.6 & / \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Physical and mechanical parameters of test soil samples.