## 4 Results and Discussion  
### Effects of Subgrade Settlement  
Figure 8 shows the longitudinal stress on the slab track and force chains in the surface layer of the subgrade under the settlement condition of 10 mm/15 m (settlement amplitude/settlement wavelength). The calculation results of the unsettled condition are also presented to highlight the effects of differential subgrade settlement. As shown in Figure (a)a, the longitudinal stress on the slab track and the values of the force chains in the subgrade are both small when there is no settlement in the subgrade. In this case, the distribution of force chains is relatively uniform and the limited differences between the distributions of force chains are caused by the bulk properties of the gravel in the subgrade. It can be seen from Figure (b)b that the upper side of the track structure is subjected to compressive stress and the lower side of the track structure is subjected to tensile stress in the settlement section. Near the boundaries of the settlement section, there is a small area in which the upper side of the track structure is in a tension state and the lower side is in a compression state. This is because the slab track exhibits upward arching near both boundaries of the settlement section. The contact forces between the gravel grains in the settlement section are smaller than those when there is no settlement in the subgrade. In  
Figure 8: Longitudinal stress on the slab track and force chain in the subgrade. Figure 7: Stress on the loading plate and force chains in the surface layer of the subgrade. the center of the settlement section, there is no contact between the gravel grains and the bottom surface of the concrete base. In other words, a gap appears between the concrete base and the subgrade, meaning the track structure is in a suspended state near the center of the settlement section. However, the force chains in the surface layer of the subgrade near the boundaries of the settlement section are densely distributed and the values of the contact forces are relatively large. This indicates that the gravel contact forces at the center of the settlement section are reduced, but the contact forces between the gravel grains near the boundaries of the settlement range are increased. The average and maximum contact forces between the gravel grains under different conditions are computed. The average and maximum contact forces between the gravel grains in the unsettled condition are 2.83 \(\times\) 10\({}^{2}\) and 2.62 \(\times\) 10\({}^{3}\) N, respectively. Under the settlement condition of 10 mm/15 m, the average and maximum contact forces are \(2.87\times 10^{2}\) and \(7.58\times 10^{3}\) N, respectively. Compared with the unsettled condition, the average and maximum contact forces between the gravel grains under the settlement condition of 10 mm/15 m increased by 1.41% and 189.31%, respectively. This indicates that the average contact force between the gravel grains in the surface layer of the subgrade increases slightly, but the maximum value of the contact force increases significantly under subgrade settlement condition. The large contact force of the gravel grains near the boundaries of the settlement section easily accelerates the plastic settlement of the subgrade, causing expansion of the settlement section. Figure 9 shows the deformation of the slab track under the settlement condition of 10 mm/15 m.  
As shown in Figure 9, the slab track exhibits deformation under the forces of gravity in the subgrade settlement section. The deformation curve of the slab track is close to a cosine shape, but it does not completely coincide with the subgrade settlement curve. The deformation wavelength of the slab track is greater than the settlement wavelength of the subgrade, but the deformation amplitude of the slab track is less than the settlement amplitude of the subgrade. Furthermore, there are significant upward arches in the slab track structure near the boundaries of the settlement section.