## 1 Introduction
Block-stone subgrades mainly use natural convection to cool the permafrost subgrades, i.e., cold air convection is used to cool down the subgrades in the cold season and hot air is blocked in the warm season [10], thus cooling the permafrost foundations and enhancing the stability of the subgrades [11]. [PERSON] et al. [12] analyzed the change in ground temperature under the joint action of block-stone and subgrade fill in permafrost areas with different ground temperatures and obtained that the block-stone subgrade plays a positive role in cooling the lower soil. For the long-term thermal stability of subgrades, a large number of scholars have also conducted relevant studies through numerical simulations, which have promoted the application and development of cooling measures in the permafrost zone. [PERSON] et al. [13] carried out a two-dimensional finite element analysis of subgrades with different depths of insulation in a permafrost area in Tibet, where the lower permafrost was protected by changing the size and location of the insulation. [PERSON] et al. [14], through the establishment of a finite element analysis model on different subgrades and melt depth change rules, obtained the composite subgrade cooling effect is optimal. [PERSON] et al. [15] obtained that the separated ventilation pipe subgrade has a better cooling effect by establishing a numerical model, which can ensure the long-term thermal stability of the subgrade and its lower permafrost. [PERSON] et al. [16] simulated the ground temperature, principal strain, deformation, and factor of safety for four typical seasons during the operation of the embankment,and the results showed that the crushed rock interlayer was effective in cooling the slope embankment and its foundation. As the development and storage conditions of permafrost in Northeast China are more complicated, the above management measures are not fully applicable [17], so it is necessary to propose control measures suitable for the permafrost areas in Northeast China. In the context of global warming and permafrost degradation, research around the cooling effect of block-stone subgrades in actual engineering and the long-term thermal stability of block-stone subgrades using numerical simulation has attracted extensive attention from scholars in recent years [18; 19], but the related research in northeast China is relatively small and still in the development stage [20]. In this paper, the K60+230 section of the Shiwei-Labudalin highway is taken as the research object. By analyzing the monitoring section's temperature field distribution and variation law, a heat transfer model of the permafrost subgrade is established, considering the phase transformation of ice water and the natural convection heat transfer of air in the block-stone layer. The temperature field of the subgrade for 15 years is simulated by COMSOL Multiphysics 6.0. By comparing the simulation results with the monitoring data, the variation law of the temperature field of the permafrost subgrade and the influence law of the block-stone subgrade on the lower soil temperature are analyzed, which provides a scientific basis for the design, maintenance, and reinforcement of highway subgrade in cold regions.