## 4 Discussion  
Based on the road project of National Highway 332 in the Great Khingan Mountains, this study captured the temporal and spatial dynamic changes of temperature, moisture, and settlement of permafrost subgrade in the forest region, and established a thermo-hydro-mechanical coupling numerical model of permafrost subgrade with vegetation cover. After the completion of subgrade construction, the subgrade will aggravate the thawing settlement deformation every time it goes through a freeze-thaw cycle. After a long period of accumulation, the safety of road operations will be seriously threatened. Therefore, this numerical model can be used to simulate the change process of the displacement field of the permafrost subgrade in the next 10 years and reveal its settlement behavior. In winter, the external temperature gradually decreases, and part of the water in the underground rock and soil mass freezes into ice, resulting in gradual frost heave and uplift of the subgrade. In summer, the external temperature gradually rises, and part of the ice melts into water, leading to the gradual transformation of the subgrade into subsidence. It  
Figure 10: Comparation between numerical results and field observation results: (**a**) at the center of the subgrade; (**b**) at the left slope toe of the subgrade. can be seen that the settlement amplitude is greater than the uplift. Therefore, the subgrade deformation is mainly caused by thawing settlement [30]. At the moment of maximum subgrade settlement, that is, in November of each year, the subgrade settlement cloud maps of 3, 5, and 10 years after construction were drawn, as shown in Figure 11. To study the influence of vegetation cover on subgrade settlement, the settlement data of slope tos on both sides were intercepted to draw the settlement curve, as shown in Figure 12. It can be seen from the figures that the thawing settlement at the slope tos will continue to increase within 3 years after the subgrade construction. The trend of subsequent increase gradually slows down. After the fifth year, the subgrade settlement is stable with little change. In addition, as shown in Figure 12, the settlement of the subgrade with vegetation cover is less than that without vegetation cover, because the temperature of the subgrade  
Figure 11: Cloud map of annual maximum settlement of subgrade: (**a**) 3 years; (**b**) 5 years; (**c**) 10 years. Figure 12: Subsidence of the soil at the base of the embankment slope every year in November. with vegetation cover is lower than that without vegetation cover. Similarly, the volumetric water content of subgrade with vegetation cover is also lower than that without vegetation cover. Therefore, the settlement deformation amplitude of the subgrade with vegetation cover is smaller than that of the subgrade without vegetation cover. It can be seen that vegetation cover is conducive to reducing the settlement of frozen soil subgrade in the warm season. In general, the settlement behavior of permafrost subgrade from construction to operation can be summarized into three periods: the degradation period, recovery period, and stability period. The degradation period occurs because the water and heat balance of permafrost is destroyed by the subgrade construction, which causes the ground temperature to rise, and the upper limit of permafrost to move down, resulting in a relatively rapid increase of subgrade settlement within 3 years after the subgrade construction. As time goes on, the magnitude of the settlement of permafrost will decrease with the weakening of the influence of subgrade construction on permafrost. At this time, the subgrade settlement is in the recovery period. Finally, the hydrothermal transformation of permafrost will reach equilibrium again. Then, the subgrade settlement is in the stability period. In the process of establishing the numerical model of the permafrost subgrade in the forest region, ABAQUS can calculate some simple coupling problems of the temperature field, moisture field, and stress-strain field.