## 4 Analysis of Ground Temperature Monitoring Data
### Overall Change in Ground Temperature  
Based on the monitoring data of the subgrade section at K60+230 of the Shiwei-Labudalin highway from January 2020 to December 2023, the contour plots of soil temperature changes at different locations of the subgrade were plotted using Surfer 15 software, as shown in Figure 4.  
Figure 3: Ground temperature sensor layout. The contours in the contour plots are denser at the surface, indicating that the degree of soil temperature variation with air temperature increases with proximity to the surface. In general, in any season, the contour lines at the surface are denser, and as depth increases, they progressively become sparser. The temperature of the shallow soil layer that makes up the highway subgrade is more sensitive to changes in the atmosphere, and it varies more and exhibits clear seasonal patterns. In the summer, the air temperature rises gradually, and the subgrade's shallow soil temperature likewise exhibits a quick increase in trend. However, as the soil's depth progressively increases, the rate of soil temperature rise also gradually decreases and eventually tends to stabilize. In winter, the air temperature gradually drops, and the subgrade's shallow soil layer also experiences a sharp drop in temperature. As soil depth increases, the rate at which the lower subgrade's soil cools down gradually slows down and eventually tends to stabilize. Because it does not experience  
Figure 4: Isotherms at different locations. freeze-thaw cycles and is kept in a relatively steady negative temperature condition, the soil deeper in the subgrade responds to variations in ambient temperature less strongly. From 2020 to 2023, the active layer thickness under the medial divider increased by 0.5 m from 7.5 m to 8 m; the active layer thickness under the right toe of the slope increased by 0.5 m from 4.5 m to 5 m; the active layer thickness under the right natural hole increased by 2 m from 2 m to 4 m; and the active layer thickness under the left toe of the slope increased by 0.5 m from 2 m to 2.5 m. The thickness of the active layer under the left natural hole did not increase significantly. The melting depth at each location is significantly greater than the freezing depth, indicating that the lower permafrost at each place is unstable and that the lower soil collects more heat than it dissipates. Taking the medial divider as an example, the soil of the subgrade started freezing from the top surface first in November 2020, i.e., from the surface layer of the soil freezing from the top downwards, the external atmospheric temperature decreased, and the temperature of the soil in the lower part of the subgrade also decreased. At the same time, the freezing fronts inside the soil continued to develop downward and reached the maximum freezing depth of about 4 m in April 2021. There was a roughly five-month freeze. In 2021, when the temperature of the top surface of the embankment was persistently higher than 0 \({}^{\circ}\)C in May, the soil layer inside the embankment began to melt. From June to August, in the shallow soil of the embankment, the temperature of the soil at a depth of 0-2 m increased rapidly. Compared with the deep soil temperature significant changes, the temperature of the deeper soil recovered relatively slowly, and the medial divider reached a maximum melt depth of about 8 m in October 2023. In 2020, the soil at 0.5 m depth at the medial divider reached the highest temperature of 23.29 \({}^{\circ}\)C in late July and the lowest temperature of \(-\)17 \({}^{\circ}\)C in February. The date of the lowest and highest temperatures attained by the lower soil exhibited a progressive delay as the depth increased, indicating the existence of a hysteresis phenomenon in the temperature change in the soil at the lower section of the subgrade with the depth.