Micro-scale investigations on the mechanical properties of expansive soil subjected to freeze-thaw cycles

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.