# Simulation of Track-Soft Soil Interactions Using a Discrete Element Method
## 2 Material and Methods
### Parameters Calibration of Scaled-Up Particles
#### 2.1.3 Parameter Calibration by the Accumulation Test  
For soil with a high moisture content, the repose angle of soil particles is affected by the friction and cohesion of particles. Thus, it is necessary to recalibrate the characteristic parameters of scaled-up particles.  
In this paper, the lifting cylinder test was used to calibrate the soil repose angle. First, the cylinder filling soil particles was placed on the surface of the test platform and raised  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Property** & **Clay** & **Sand** \\ \hline Particle size (mm) & 0.05–0.075 & 0.5–1 \\ Wet density (g cm\({}^{-3}\)) & 1.45 & 1.97 \\ Dry density (g cm\({}^{-3}\)) & 0.67 & 1.63 \\ Moisture content (\%) & 91 & 21 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The characteristics of clay and sand.  
vertically at a speed of 5 mm s\({}^{-1}\) by a screw device driven by a stepper motor (57 BL55S06). The soil particles fell freely through and formed a conical pile on the base plate. Second, the same experimental parameters were set in the DEM simulation. The basic contact parameters of particles are shown in Table 2. Finally, the contact parameters that needed to be calibrated were finally confirmed by comparing the accumulation test and simulation. The dimensions of the cylinder and the cylinder model established in the EDEM software with the same size are shown in Figure 2.  
Trial-and-error methods are feasible to change the parameters and comparison results constantly until the simulation results conform to the actual experimental results. Different accumulations were measured through the above experimental steps. Accumulation tests and DEM simulations were carried out, as shown in Figure 3.  
As shown in Figure 3e, the repose angle of sand particles is mainly determined by friction and can be measured directly. Unlike the formation principle of the repose angle of sand particles, clay is a kind of soil with a small internal friction angle. The repose angle of clay particles is mainly determined by the cohesion, which causes the irregular shape accumulation and is difficult to measure accurately.  
To accurately measure the characteristic parameters of mixed soil and analyze the accumulation of mixed soil with different proportions of sand, a method of image edge extraction and pixel superposition through a saliency detection network and grey-binary processing was proposed with MATLAB software. A schematic diagram of the soil contact parameter calibration method is shown in Figure 4.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Property** & **Value** & **Source** \\ \hline Shear modulus of clay (MPa) & 2.85 \(\times\) 10\({}^{4}\) & [PERSON] [27] \\ Poisson’s ratio of clay & 0.3 & [PERSON] [28] \\ Coefficient of restitution of clay–clay & 0.4 & [PERSON] [29] \\ Coefficient of restitution of clay–PVC & 0.4 & [PERSON] [29] \\ Shear modulus of sand (MPa) & 4.3 \(\times\) 10\({}^{4}\) & [PERSON] [28] \\ Poisson’s ratio of sand & 0.3 & [PERSON] [28] \\ Coefficient of restitution of sand–sand & 0.6 & [PERSON] [30] \\ Restitution coefficient of sand–PVC & 0.6 & [PERSON] [30] \\ Density of PVC (g cm\({}^{-3}\)) & 1.3 & Suggested by EDEM [31] \\ Elastic modulus of PVC (MPa) & 2.41 \(\times\) 10\({}^{3}\) & Suggested by EDEM [31] \\ Poisson’s ratio of PVC & 0.383 & Suggested by EDEM [31] \\ \hline \hline \end{tabular}
\end{table}
Table 2: DEM parameters used in the simulations.  
Figure 2: Bottomless cylinder in accumulation test and simulation.  
Figure 4: Flow chart of parameters calibration in test and simulation.  
Figure 3: Soil accumulation test and DEM simulation. (**a**) Accumulation of clay. (**b**) Accumulation of mixed soil 1\(\#\). (**c**) Accumulation of mixed soil 2\(\#\). (**d**) Accumulation of mixed soil 3\(\#\). (**e**) Accumulation of sand.