# Simulation of Track-Soft Soil Interactions Using a Discrete Element Method
## 1 Introduction  
With the development of special unmanned tracked vehicles, more unmanned tracked vehicles are being used in soft soil environments, i.e., river dredging tracked equipment. In order to estimate the trafficability of soft soil, the research focus of the return mechanics of tracked vehicles has gradually shifted to the detailed modeling of soft soil.  
In soft ground conditions, the tractive performance of a tracked vehicle is determined by the interaction between the track and the soil. Previous researchers studied the effect of the geometry of the track segment on tractive performance [1, 2], such as grouser height, pitch, thickness and pitch-to-height ratio. Most of the contemporary research has focused on land soil. For soft soil with a high moisture content, the traditional method is still used to test the mechanical properties of the soil. Due to the high sinkage of soft soil, traditional soil shear tests are unsuitable.  
Most previous studies on the interaction between soil and track have been performed through a soil bin in the laboratory or on an actual field. With the development of computer technology, the application of DEM to interaction problems in terrranechanics has become popular. DEM was first proposed and introduced into the study of vehicle terrranechanics [3]. [PERSON] used a two-dimensional DEM to simulate soil, and verified the effect of track traction in combination with a soil bin test [4]. [PERSON] assessed the mechanical properties of soft ground with low moisture, low viscosity and high friction [5]. [PERSON] established avehicle-terrain simulation model based on the DEM and proved the accuracy of traditional drawbar-pull tests [6]. [PERSON] proposed a systematic and accurate discrete element modeling method for sandy roads [7]. The verification results showed that the modeled interactions between track and sandy ground based on DEM simulations were consistent with the results of the actual soil bin test. For fine soil particles, a major disadvantage of DEM simulation in practical applications is the challenge of modeling very small particles; it is computationally impossible and impractical to calculate every particle of true size.  
In order to reduce the amount of calculation, a feasible method is to apply scaled-up particle technology to DEM simulation. At present, the scaled-up particle method is more commonly used in DEM simulation [8; 9; 10]. If all particle sizes are scaled up by a constant factor, but the material properties remain the same, the dynamics of the granular system will be altered [11]. Thus, DEM parameter calibrations are the key challenge for the scaled-up particle method.  
Many contemporary scholars have focused on the parameter calibration method of DEM simulation. DEM calibration consists of the careful choice of the material input parameters needed to reflect the real behavior of the particles in the numerical simulation. Careful calibration is the basis for achieving predictive results. The methods of parameter calibration include the angle of repose (AOR) test, penetration test, compression test and shear test [12; 13; 14; 15; 16]. Due to the large number of material attributes to be set, most of the calibration is focused on selected parameters. More calibration procedures for the DEM parameters have been developed to reduce computational effort [17; 18; 19].  
In the studies reported above, the DEM was applied to the terramechanics of tracked vehicles; the main research objects are limited to land soil and sand. However, the applicability of DEM simulation to soft soil is uncertain. The objectives of this study were: (1) to calibrate scaled-up particle contact parameters consistent with the mechanical properties of soft soil; (2) to validate the shear properties of the scaled-up particle model and (3) to corroborate the soil traction performance of the tracked vehicle according to the modified soil shear DEM simulation.