## 3 Results  
### Temperature Variation Regularity of K67+300 Subgrade Section  
After installation and debugging, the temperature sensor could accurately measure the temperature of underground rock and soil mass. Temperature data were obtained from October 2019 to December 2020. Figure 6a,b show the variation trend of the underground temperature of the left half of the subgrade (with vegetation cover) and the right half of the subgrade (without vegetation cover) with the depth during the observation period. The following information can be obtained. At the same depth, no matter whether there was vegetation cover, the underground temperature decreased with the decrease of external temperature in the cold season and increased with the increase of external temperature in the warm season. When the underground depth was below 0.9 m, the annual underground temperature became negative, which was consistent with the permafrost table of 0.9 m in the geological survey report. When the underground depth was less than 5 m, the underground temperature changed greatly. The buried depth of this layer was shallow, and there was an active layer of frozen soil in the upper part. As a consequence, it was easily affected by temperature, precipitation, vegetation, and other external conditions. The underground temperature was maintained at \(-0.8\)\({}^{\circ}\)C below the depth of 5 m. It can be seen that in this depth range, the influence of the external environment was small. The underground temperature was constant. In the same month, for example, the underground temperature in December 2020 was slightly higher than that in December 2019. In other words, the underground temperature of the subgrade after construction was slightly higher than that before construction. Especially \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & \(\mathbf{\lambda_{u}}\) **(W/(m\(\cdot\)C))** & \(\mathbf{\lambda_{f}}\)**(W/(m\(\cdot\)C))** & \(\mathbf{C_{u}}\) **(J/(kg\(\cdot\)C))** & \(\mathbf{C_{f}}\) **(J/(kg\(\cdot\)C))** \\ \hline Gravel & 2010 & 17 & \(7.2\times 10^{-4}\) & 2.35 \\ Rubble & 2400 & 10 & \(1.1\times 10^{-2}\) & 2.65 \\ Clay & 1700 & 23 & \(1.5\times 10^{-8}\) & 2.05 \\ Gravel soil & 1950 & 15 & \(6.9\times 10^{-3}\) & 2.20 \\ \hline \hline \end{tabular} Note: \(\lambda_{u}\) = thermal conductivity of unfrozen soil; \(\lambda_{f}\) = thermal conductivity of frozen soil; \(\mathbf{C_{u}}\) = specific heat of unfrozen soil; \(\mathbf{C_{f}}\) = specific heat of frozen soil.  
\end{table} 
Table 2: Thermal parameters in the permafrost subgrade model. Figure 6: Variation of underground temperature with depth. (**a**) The underground temperature in borehole T1 covered with vegetation; (**b**) the underground temperature in borehole T2 covered without vegetation. for the subgrade without vegetation cover, the permafrost table after construction was 1.2 m, which was 0.3 m lower than that before construction of 0.9 m. Because the subgrade construction had disturbed the underlying frozen soil, destroyed its original water and heat balance state, and replaced more heat into the ground, resulting in the reduction of the permafrost table. According to Figure 7, the changing trend of ground temperature at different depths of borehole T1 (with vegetation cover) and borehole T2 (no vegetation cover) in 2020 can be obtained. When the underground depth was 0.5 m, the underground temperature reached the lowest value in February and the highest value in August. However, with the increase of burial depth, the influence of the external environment on underground rock and soil mass gradually weakened. In addition, the temperature transfer in the rock and soil layers had a hysteresis effect. These factors led to the lowest underground temperature in March and the highest underground temperature in September. When the underground depth was 0.5 m, the amplitude of the temperature rise and fall in borehole T2 (no vegetation cover) was significantly greater than that in borehole T1 (with vegetation cover).