# Study on Cold Resistance Performance of Composite Subgrade Structure in Seasonal Frozen Regions
## 1 Introduction
The replacement method is easy to implement and has a wide range of applications, but the anti-freezing effect is directly related to the replacement depth. According to related research [15], the cost is higher when transporting non-frost heave soil over a long distance or when the depth of soil replacement is more than one-third of the frozen depth. Moreover, sand and gravel are usually adopted as replacement materials of wet subgrade in seasonal frozen regions. To ensure that the subgrade is protected from groundwater, the replacement depth is usually deeper, and a larger amount of sand and gravel are needed. The shortage of sand and gravel is continuing, and it is becoming more and more serious. Relevant policies also restrict the mining of river stones, which results in raw material prices and road construction costs skyrocketing. Thus, there is an urgent need to find an alternative to river stone to relieve this pressure [16]. On the basis of meeting basic requirements with regard to the specification for subgrade materials, the subgrade filler in seasonal frozen regions is supposed to have excellent cold resistance durability. According to previous research results from our research group [17], a new type of subgrade filler, which is silty clay modified with oil shale waste residue and fly ash, was put forward. This new type of filler has excellent freezing and thawing resistance stability and can effectively improve the durability of wet subgrade in seasonal frozen regions. Application of the modified soil to highway subgrade can realize the reuse of waste resources, save sand and gravel, and reduce the engineering cost, which is in line with the concept of sustainable development. The above methods are all based on improving the freeze andthaw resistance durability of subgrade filler with certain limitations. If the negative temperature energy can be prevented from entering the subgrade, the freezing of subgrade filler in the working area can be reduced. Meanwhile, if the material with excellent frost resistance can be adopted as subgrade filler, the freeze and thaw resistance durability of the subgrade will be greatly improved, especially the wet subgrade in seasonal frozen regions. In permafrost areas, both the construction of a thermal insulation layer and the increase of subgrade can increase the thermal resistance in order to protect the permafrost. [PERSON] et al. (2018) took an actual highway project as an example and pointed out the problems of thermal insulation layer subgrade adopted as an engineering measure to control the stability of permafrost subgrade. [PERSON] et al. (2019) put forward a composite subgrade consisted of an L-type heat pipe and gravel insulation board in order to maintain the thermal stability of a wide and high-grade highway in a permafrost area and to protect the underlying permafrost. They studied the thermal stability of composite subgrade using an indoor model test and numerical method. Based on previous research results, this paper puts forward a composite subgrade construction, which consists of extruded polystyrene (XPS) plates and silty clay modified with oil shale waste residue and fly ash. The cold resistance performance of this structure under the natural conditions of winter in Changchun was analyzed, and its cold resistance effect was evaluated, aiming at providing solutions and references for subgrade in seasonal frozen regions, especially for wet subgrade.