## 6 Numerical Simulation of the Temperature Field of a Block-Stone Subgrade
### Numerical Simulation Study of Block-Stone Subgrade with Different Working Conditions
#### 6.3.2 Block-Stone Particle Size  
One of the main reasons for the natural convection inside the block-stone layer is that the particle size of the block-stone fill is much larger than that of the subgrade fill, and there is a large porosity inside the block-stone layer, which ensures convection movement of air under the effect of the temperature gradient. In order to analyze the effect of block-stone particle size on the internal temperature of the block-stone subgrade, the block-stone subgrade with particle sizes of 4-6 cm, 6-8 cm, and 10-15 cm was established for numerical simulation and comparative analysis. Based on the block-stone subgrade model with 4 m of replacement in the paper, only the particle size of the material was changed, and the rest of the conditions, such as boundary conditions, were the same as in the previous section.  
Figure 20 shows the curves of temperature at the medial divider as a function of depth for the 10 th of October for different block-stone particle sizes of the replacement block-stone subgrade.  
As can be seen from Figure 19, the main difference in the temperature of the lower soil of different particle sizes of the replacement block-stone subgrade is at the surface, and the temperature difference decreases with the increase in depth. By further comparing the temperatures at the same depth under different particle sizes of the block-stone subgrade, it can be obtained that the temperature of the reclaimed block-stone subgrade with 6-8 cm particle size is the lowest, the temperature of the block-stone subgrade with 10-15 cm particle size is higher, and the temperature of the block-stone subgrade with 4-6 cm particle size is the highest. The ground temperature at a depth of 3 m under the natural ground surface is 0.1 \({}^{\circ}\)C, 0.15 \({}^{\circ}\)C, and 0.19 \({}^{\circ}\)C, respectively, indicating that the 6-8 cm block stone has the highest cooling effect and the stability of the roadbed is better. It can be concluded that the cooling effect does not vary with increasing particle size.  
In order to further understand the effect of different block-stone particle sizes on the stability of the permafrost subgrade and the degradation of permafrost within the subgrade, the change curves of the permafrost table under different working conditions were plotted as shown in Figure 21.  
It can be concluded from Figure 21 that the three different particle sizes of block-stone subgrades can lift the permafrost table in the lower part of the subgrade. The 6-8 cm size block-stone subgrade lifts the permafrost table more than the 4-6 cm and 10-15 cm size block-stone subgrades. The subgrade with 4-6 cm diameter has the smallest lifting height, and the subgrade with 10-15 cm diameter is in the middle of the two. Therefore, according to the above simulation results, the 6-8 cm size of the replacement block-stone subgrade shows a better cooling effect and raises the anthropogenic permafrost table compared to the 4-6 cm and 10-15 cm sizes of the replacement block-stone subgrade. That is, in practical engineering, moderately increasing the particle size of the block-stone can enhance the  
Figure 20: Variation in temperature with depth for different particle sizes of block-stone subgrade.  
natural convection effect within the block-stone layer, thus further enhancing the stability of the lower permafrost. However, excessively increasing the size of the block-stone particle may affect the other physical parameters of the block-stone layer, thus weakening the protective effect of the block-stone subgrade on the lower permafrost. This shows that the cooling effect of natural airflow on the lower soil is greater than that of changing the physical coefficient.