## 3 Soil Parameter Discrete Element Calibration
### Soil Direct Shear Test Discrete Element Simulation  
EDEM 2020 software was used to simulate the soil direct shear process. The generated soil particles were compacted before the direct shear to reduce their spacing, and bonds were generated afterwards. A permeable stone was placed on the surface of the direct sheared soil and the soil was compacted with a force of 12 N. The model was set as Hertz-Mindlin with JKR as well as bonding, the parameters were set as the actual parameters of calibration, and several tests were conducted to measure the peak shear force under the base 0.1 MPa positive pressure condition. The relative error was compared with the cohesive force C of the direct shear test, and the overall stage of simulated bonding key force is shown in Figure 10. Figure 10a shows the initial state before the start of direct shear, with overall force distribution of bonding inside the soil and no special force points, in line with the bonding characteristics of the soil before direct shear. Figure 10b shows the state of bonding of the soil just before the start of direct shear. It can be seen from the figure \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **3\%** & **6\%** & **9\%** & **12\%** \\  & **Moisture Content** & **Moisture Content** & **Moisture Content** \\ \hline Soil-soil recovery coefficient & 0.45 & 0.45 & 0.45 & 0.45 \\ Soil-soil rolling friction coefficient & 0.188 & 0.15 & 0.193 & 0.388 \\ JKR surface energy coefficient & 0.3558 & 0.3560 & 0.3630 & 0.371 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Contact parameters for soil with different water content. Figure 9: Simulation of the rest angle of soil with (**a**) 3%, (**b**) 6%, (**c**) 9%, and (**d**) 12% water content. that the shear surface force is larger, the maximum force point is located on the front and rear of the direct shear direction, and at this time the soil is still in the bonding state and has not yet ruptured. Figure 10c shows the maximum stress period in the process of direct shear. Although the soil is in the bonding state and has reached the limit of damage, the maximum stress is located on the shear surface and expands outward. Trace displacement appears on the front and rear end of the direct shear direction of the shear surface soil due to the presence of bonding forces, and the rest of the region of soil stress is not changed. Figure 10d shows the state after the end of direct shear; at this time, the shear force no longer exists, most bonds have been broken, only a small amount remains in the limit fracture state, the total stress is reduced, and the soil has been damaged by shear. After the direct shear was completed, the force of the lower shear box was observed to find the peak force point, as shown in Table 12. Comparing the peak force of the soil shear process under the 0.2 MPa positive pressure condition of the physical test, the error rate was found to be distributed in the range of 3.71-6.53%, which was within the tolerable range, and the overall change trend was consistent, indicating that the soil sample parameters of the physical and simulation tests of direct shear were basically consistent, and the measured values are real and reliable. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Water Content (\%)**} & \multirow{2}{*}{**Solidity (MPa)**} & \multirow{2}{*}{**Cross-Sectional Area (mm\({}^{2}\))**} & \multirow{2}{*}{**Direct Shear Displacement (mm)**} & \multicolumn{2}{c}{**Peak Shear Force**} & \multirow{2}{*}{**Error Rate (\%)**} \\  & & & & & **(N)** & **Error Rate (\%)** \\ \hline \multirow{4}{*}{6} & 0.34 & & & 149.4 & 143.9 & 3.68 \\  & 0.945 & & & 183.1 & 176.3 & 3.71 \\  & 1.55 & & & 197.4 & 184.5 & 6.53 \\  & 2.155 & & & 209.2 & 198.9 & 4.92 \\  & 2.76 & & 262.9 & 246.5 & 6.24 \\ \hline \multirow{2}{*}{3} & \multirow{2}{*}{1.55} & & & 157.3 & 162.3 & 3.18 \\  & & & 204.9 & 192.8 & 5.91 \\ \cline{1-1}  & & & 165.3 & 174.7 & 5.69 \\ \hline \hline \end{tabular}
\end{table}