## 4 Discussion  
The objective of this study was to evaluate the draught reduction performance of subsoilers with belt motions. It is clear that the belt motion exhibits a significant draught reduction performance within the operating conditions set in the simulation referring to the analysis in Section 3. For the regular tools, the draught force increases as the rake angle increases, and this trend has been widely observed [7; 41; 44; 45]. In the case of belt tools, the draught force can be reduced by 13.18% to 44.64% with rake angles ranging from 30\({}^{\circ}\) to 75\({}^{\circ}\). The advancement of tools applied forces on soil particles and forced them to move. The actual direction in which the surface elements of the belt tools move points obliquely at the free surface in front of the tool [25]. Such a tool motion is responsible for the reduction in draught force. The vertical components of the resultant force applied to the soil are reasonably larger as the rake angle increases. For this reason, the maximum draught reduction ratio was obtained at the rake angle of 75\({}^{\circ}\). An analogous draught reduction performance could be found in other types of belt machines and power-driven discs, although either the cutting mechanism or the operating mode may be partially different [4; 23; 24; 25; 26; 27; 28]. As described in Section 3.1, these results can be explained by the influence of belt motion on soil disturbance characteristics, soil particle velocity distribution, and soil deformation and movement patterns. First, the belt motion changes the range of soil disturbance. The longitudinal failure boundary is the path of the farthest positions where the tool causes disturbance [46]. The soil failure occurs in a linear manner or logarithmic spiral manner of the minimum resistance path. Under the influence of belt motions, the change in the direction of external friction causes the principal stress axis to rotate [20], resulting in an associated rotation of the failure boundary. The results in Section 3.2 indicate that the forward failure distance l decreases accordingly, the rupture angle \(\beta\) increases, and this inevitably leads to a relative reduction in area of longitudinal disturbance. Since the draught force is a function of the rupture angle \(\beta\)[24], the reduced area of disturbance indicates a decrease in the working volume of the soil, corresponding to a lower draught force. This is consistent with results obtained on power-driven discs performing similar soil operations [24]. Second, the belt motion changes particle velocity distribution. In Section 3.3, the magnitude and direction of particle velocity near the tool surface are close to the velocity of the traveling tool, which is consistent with the velocity field obtained by DEM simulations [29; 47], finite element simulations [48; 49], and glass-sided soil bin experiments [20; 50]. Under certain initial conditions, soil particles continuously accumulate on the tool surface, forming a boundary wedge that moves synchronously with the tool [19; 22]. The boundary wedge changes the macroscopic shape of the tool, causing soil to accumulate in front of the tool, making it difficult to replace soil smoothly and exerting a compressive force on the  
Figure 14: Soil deformation and movement patterns: (**a**–**d**) Regular tools; (**e**–**h**) Belt tools (rake angle \(\alpha=30^{\circ}\), \(45^{\circ}\), \(60^{\circ}\), \(75^{\circ}\)). next soil, which leads to an increase in draught force [24; 29; 40]. The results in Section 3.3 indicate that the vertical component of particle velocity increases as the belt motion worked, and thus changed the direction of the force applied to soil particles. This adds \"lifting\" based on \"pushing\" [35; 40; 45]. On the other hand, the soil particles on the tool surface have a greater tendency to move relative to the tool surface, which relieves the accumulation. The particle velocity changed from a nearly horizontal direction into an obliquely upward direction, resulting in a decrease in draught force.