Damage evolution mechanism and multiscale characterization of expansive soil under unidirectional freeze-thaw cycles

In seasonal frozen regions, expansive soil freezes in winter and thaws in spring from the ground surface downward. The temperature fluctuation triggers severe engineering failures, such as continuous landslides and collapses on road embankments. Conventional laboratory studies typically expose samples directly to cold environments, resulting in freezing from all boundaries, which fails to replicate the vertical temperature gradient in nature. This study investigated the evolution of internal damage in expansive soil under unidirectional freeze-thaw (F-T) cycles from macroscopic, mesoscopic, and microscopic perspectives using deformation monitoring, mercury injection tests (MIP), and computed tomography (CT) tests. The results indicated that the peak vertical deformations for 19.6%, 21.6%, and 23.6% water contents samples were 1.46 mm, 1.70 mm, and 2.25 mm, which eventually stabilized at 0.70 mm, 1.40 mm, and 1.80 mm respectively. Unidirectional F-T cycles caused the most pronounced pore expansion at the 13.5 cm height of the samples, where section porosity increased by 3.57%, 5.34%, and 6.15% after 50 cycles, accompanied by a maximum 10.3% increase in large pores (> 4 μm). The frost heave damage mechanism was divided into three steps. The first step was primarily characterized by original frost heave due to water migration and ice crystallization. The change in the second step can be attributed to particle agglomeration. Finally, the shift in the third step resulted from the dispersion of agglomerates and the development of microfractures. These findings provide vital engineering significance for reducing disasters and ensuring the safety of transportation systems in seasonal frozen regions.