## 6 Numerical Simulation of the Temperature Field of a Block-Stone Subgrade
### Numerical Simulation Study of Block-Stone Subgrade with Different Working Conditions  
#### 6.3.1 Thickness of Block-Stone  
According to the numerical simulation results above, the block-stone layer has the effect of heat preservation and insulation, which can effectively reduce the temperature of permafrost deep in the foundation to reduce the degradation rate and maintain the stability of the subgrade. However, the block-stone layer with a thickness of 1 m has a limited role in protecting the lower permafrost. Therefore, to study the effect of different block-stone replacement thicknesses on the temperature field of the subgrade, the numerical simulation analysis was carried out in three conditions of 2 m, 3 m, and 4 m block-stone thickness, respectively. 1. Analysis of ground temperature variation with depth  
Figure 15 shows the temperature at the medial divider as a function of depth for block-stone layer subgrades with different replacement thicknesses on October 10, the 15 th year. Figure 14: Comparison of permafrost table for ordinary and block-stone subgrades. As shown in Figure 15, the effect of different replacement thicknesses of the block-stone layer on the temperature of the upper part of the soil is large. The effect of the block-stone thickness on the temperature of the soil decreases with an increase in its depth. The temperature of the soil in the lower part of the subgrade is lower when the thickness of the block stone is 4 m, and the temperature of the soil in the lower part of the subgrade is relatively more stable than the other thicknesses of the block stone. Under the same depth, the thickness of the block-stone layer increases, and the temperature of the soil gradually decreases. For example, when the depth of the soil is 5 m, the thickness of the replacement block-stone layer increases from 1 m to 2 m, the ground temperature decreases by 0.1 \({}^{\circ}\)C, the thickness of the replacement layer increases from 2 m to 3 m, the ground temperature decreases by 0.31 \({}^{\circ}\)C, and the thickness of the replacement layer increases from 3 m to 4 m, the ground temperature decreases by 0.26 \({}^{\circ}\)C. Comparing the increase in the thickness of block-stone from 1 m to 2 m with that from 2 m to 3 m, the decrease in the temperature of the latter is more obvious, which indicates that the moderate increase in the thickness of block-stone will have a more obvious effect on the protection of the lower permafrost. Analysis of the process of permafrost table change  
In order to deeply investigate the influence of block-stone replacement thickness on the temperature field of permafrost subgrade, contour plots were drawn according to the numerical simulation results, and the 0 \({}^{\circ}\)C contour was taken as the key eigenvalue, which was indicated by a red line in the contour plots, as shown in Figures 16-18. As the simulation time increased, the permafrost table decreased to varying degrees at all locations in the lower part of the replacement 2 m block-stone subgrade. The permafrost table in the lower part of the medial divider decreased by 3.64 m, with an average rate of decrease of 0.24 m/a, and the rate of decrease per 5 years was 0.30 m/a, 0.24 m/a, and 0.19 m/a. The permafrost table in the lower part of the left toe of the slope decreased by 2.52 m, and the average rate of decrease was 0.17 m/a. The rate of decrease was 0.25 m/a, 0.17 m/a, and 0.09 m/a every 5 years. The permafrost table in the lower part of the right toe of the slope declined by 3.19 m, with a descending average rate of 0.21 m/a and a descending rate of 0.48 m/a, 0.25 m/a, and 0.17 m/a every 5 years. The permafrost table in the lower part of the natural ground surface declined by 1.33 m, with a descending average rate of 0.09 m/a and a descending rate of 0.11 m/a, 0.08 m/a, and 0.07 m/a every 5 years. Figure 15: Variation in temperature with depth for different replacement thicknesses of block-stone subgrade.