## 3 Results
### Moisture Variation Regularity of K67+300 Subgrade Section  
After installation and commissioning, the moisture sensors could accurately measure the volumetric water content of unfrozen water in underground rock and soil mass, excluding solid ice. Moisture data were also obtained from October 2019 to December 2020. Figure 8 shows the changing trend of volumetric water content during the observation period, respectively, at different depths within 2 m underground in section W1 (with vegetation cover) and section W2 (no vegetation cover).  
With the increase in depth, no matter whether there was vegetation cover, the volumetric water content of underground rock and soil mass increased. It reached the maximum when the underground depth was 1.5 m, and decreased when the underground depth was 2 m.  
The volumetric water content of underground rock and soil mass changes with the seasonal change in external temperature. From October 2019, the surface temperature gradually decreased and transferred to the underground. The capillary water in the unfrozen soil migrated to the surface at a lower temperature under the action of matrix suction, which led to the continuous decrease of the water content of underground rock and soil mass. By the middle of February 2020, the volumetric water content of rock  
Figure 8: Variation of the monthly average volumetric water content of profile W1 (with vegetation cover) and profile W2 (no vegetation cover) at the same depth within 2 m underground.  
and soil mass was the lowest under the continuous influence of negative temperature. After entering the spring thawing period, the ground temperature gradually rose, and the ice and snow gradually melted. Furthermore, the precipitation in this region was mostly concentrated in summer. Permafrost in the forest region was very sensitive to temperature, so the volumetric water content of rock and soil mass gradually increased under the continuous influence of positive temperature. By the middle of October 2020, the volumetric water content of rock and soil mass was the largest.  
At the same depth, in the cold season, the value of moisture sensors in profile W1 (with vegetation cover) was larger than that in profile W2 (no vegetation cover). In the warm season, the opposite was true. This phenomenon was more obvious within 1 m underground. It is because the vegetation cover has the functions of heat insulation, wind, snow resistance, and transpiration of moisture, which has a great influence on the water and heat cycling process of rock and soil mass. When the cold season comes, the canopy, shrub, and moss cover can prevent the cold air from entering the rock soil mass, and the root water in the upper part of the soil layer freezes to release a lot of heat energy, delaying the occurrence of freezing. When the temperature rises in the warm season, the vegetation insulation layer also prevents the warm air from entering the rock and soil mass. Snow in the forest and ice melting in the upper part of the soil layer consumes a lot of heat energy, which hinders the melting process. The transpiration of vegetation becomes more intense because of the temperature rise, which makes more water in the soil be absorbed and transported away. As a result, the volume water content of rock and soil mass will decrease. It can be seen that when the active layer of permafrost melts in the warm season, the volume moisture content of the underlying rock and soil mass of the subgrade with vegetation cover is lower than that of the subgrade without vegetation cover, which is more beneficial to the stability of the subgrade.