According to the freeze-thaw performance of high-speed railway subgrade filler in a seasonal freezing area, graded crushed stone mixed with cement and fly ash is used as improvement measures. In order to study the anti-freeze-thaw durability of cement and cement fly-ash improved filler, the water stability, freeze-thaw deformation characteristics, and freeze-thaw durability of the filler are tested. The influence of the modifier’s type, dosage, and curing age on the performance of subgrade filler is analyzed. The results show that the cement-modified filler has higher unconfined compressive strength than the cement fly-ash-modified filler. The unidirectional frost heaving of filler decreases with increasing cement content, and the frost heaving rate of filler increases with the addition of fly ash. After 28 days of curing, the water stability of cement fly ash-modified filler is better than that of cement-modified filler, and the freeze-thaw resistance is lower than that of cement-modified filler.

By simulating the impact of water vapor exchange on highway subgrades in areas with frequent rains, this paper studied the impact of drying-wetting cycles on the improved soil of high-grade highway subgrades in Zhangjiakou area with indoor mechanical tests. It obtained the change law of various mechanical parameters, and put forward the cement CBR influence factor to judge the degradation mechanism of cement-improved soil road perfor-mance. The cement CBR influence factor reflects the load-bearing strength of the sample under the same cement content and the contribution of the cement to the project. The larger the CBR influence factor, the smaller the bearing strength of the sample under the same cement content, and the smaller the contribution value to engineering applications. The results show that the impact factor of cement CBR is greatly affected by the amount of cement. As the number of drying-wetting cycles increases, the impact factor of cement CBR first decreases and then increases. The high-dose cement-modified soil can effectively improve the self-healing ability of the soil sample, and has a positive effect in the early resistance to drying-wetting damage, and the elastic modulus is less affected by the drying-wetting cycle. The change law of the internal pore characteristics of the sample indicates that the early drying-wetting cycle process can enhance the water stability of the modified soil. After multiple drying-wetting cycles, the internal pores of the sample are expanded, destroyed and stabilized gradually, and reached a new structural balance again. According to the comprehensive analysis of CBR technical indicators and unconfined compressive strength indicators, it is recommended to use 6% cement as a reference for subgrade improvement in areas with frequent water vapor exchange.