# Study on Cold Resistance Performance of Composite Subgrade Structure in Seasonal Frozen Regions
## 1 Introduction  
Seasonal frozen regions account for 53.3% of the territorial area of China [1]. A large number of engineering practices and subgrade damage investigations [2, 3] showed that frost heaving and boiling are the main responses to frost damage of highway subgrade in seasonal frozen regions of China. Repeated freezing and thawing of subgrade soil results in a reduction in the durability of road structure and severely affects the service performance of roads, which not only causes hidden dangers to normal operation, but also wastes a lot of money for road maintenance. The prevention and control of frost damage is mainly aimed at the key factors leading to frost damage. Currently, prevention and control measures of subgrade frost damage in China include the chemical improvement method, subgrade structure improvement method, replacement method, etc. [4]. The chemical improvement method involves adding chemicals to the soil to change its physical and chemical properties. [PERSON] et al. [5] solidified light clay in Changchun with base-seal curing agent and found that the modified soil had more significant anti-freezing effect. [PERSON] et al. [6] found that adding gypsum additive to solidified material could improve the strength of cement-reinforced soil when strengthening soft soil foundation using a cement-soil mixing method. By analyzing the reinforcement mechanism, the results showed that an appropriate amount of gypsum additive can be added to the solidified material in the foundation reinforcement project in order to improve the microstructure characteristics of the modified soil and optimize the foundation reinforcement effect. Current research on the chemical improvement method is aimed at specific soil types in certain areas, while it is apparently limited by construction technology and other conditions. The subgrade structure improvement method is the improvement of soil for subgrade filling. [PERSON] et al. [7] studied a heavy frost-damaged section filled with fly ash and found that the freezing depth of fly ash subgrade was 30% to 50% lower than that of soil subgrade. [PERSON] et al. [8] studied two kinds of semi-rigid base course materials, namely, lime-stabilized soil and lime-fly ash-stabilized soil, and they obtained the optimal volume of lime which could prevent frost heave progress. [PERSON] et al. [9] analyzed the erosion resistance of soil strengthened with lignin, discussed the problems of subgrade strengthened by lignin, and forecasted the prospect of its application. [PERSON] et al. [10; 11] studied the technology of foundation stability when excavating tunnel in water-bearing soft rock and loose sand. The use of these technologies can improve the stability of the subgrade and ensure the safety of the project. The subgrade structure improvement method can improve the cold resistance performance of subgrade to some extent; however, some of the methods have poor stability under the circumstance of repeated freezing and thawing, which leads to a sharp reduction in the improvement effect. Furthermore, it lacks actual projects to verify its long-term stability. The replacement method refers to replacing all the frost heave-sensitive soil within the freezing depth of subgrade with weak frost heave soil or non-frost heave soil. A test carried out in the Xinjiang Corps reclamation area showed that replacing soil with aeolian sand between 60% and 70% of the maximum freezing depth and a pavement structure layer can effectively prevent and control frost heave damage of tertiary highways and below [12]. Some researchers studied the performance of coarse-grained soil under the circumstance of repeated freezing and thawing and found that replacing coarse-grained soil could reduce the occurrence of frost heave. However, under the condition of repeated freezing and thawing, the water content of coarse-grained soil would increase sharply, which means that frost boiling would still happen [13; 14].