Freeze-thaw (FT) cycle is one of the most important factors contributing to the deterioration of expansive soil properties in seasonal frozen regions. In this study, a multiscale approach was used to investigate the impact of FT cycles on the volume deformation, mechanical properties and microstructure of expansive soil with different initial compaction states. FT cycle, unconfined compression, scanning electron microscopy and mercury intrusion porosimetry tests were carried out. The test results indicated that the volume deformation of the frozen expansive soil showed a completely opposite trend and varied in magnitude with different initial compaction states. The failure strength and elastic modulus of the expansive soil sample decreased significantly with increasing number of FT cycles. Under the impact of FT cycles, the porosity of the expansive soil increased significantly and the proportion of macropores grew. The growth of macropores and the generation of microcracks in the expansive soil were the main causes of FT damage. Besides, the FT damage variable was defined by the failure strength and had a good linear relationship with the soil porosity. In addition, the strength of loose and dense soil samples decreased significantly after FT cycles. It is found that an optimal compaction state may exist at around 95% compaction and the water content can be controlled on the dry side of the optimum water content, where FT cycles have minimal effect on the soil strength and microstructure. The study can provide guidelines for selecting the appropriate initial compaction and water content for the expansive soil projects in seasonal frozen regions.

冻融（FT）循环是导致季节性冰冻地区膨胀土性质恶化的最重要因素之一。本研究采用多尺度方法研究FT循环对不同初始压实状态膨胀土的体积变形、力学性能和微观结构的影响。进行了FT循环、无侧限压缩、扫描电子显微镜和压汞孔隙率测试。试验结果表明，不同初始压实状态下，冻结膨胀土的体积变形呈现完全相反的趋势，且大小不同。随着FT循环次数的增加，膨胀土样品的破坏强度和弹性模量显着下降。在FT循环的影响下，膨胀土的孔隙度显着增加，大孔比例增大。膨胀土中大孔隙的生长和微裂纹的产生是引起FT损伤的主要原因。此外，FT损伤变量由破坏强度定义，与土壤孔隙度具有良好的线性关系。此外，松散和致密土样的强度在 FT 循环后显着下降。研究发现，最佳压实状态可能存在于压实度95%左右，含水量可以控制在最佳含水量的干侧，此时FT循环对土体强度和微观结构的影响最小。该研究可为季节性冻土地区膨胀土工程选择合适的初始压实度和含水量提供指导。