A coupled macro–micro prediction model for desiccation shrinkage deformation of freeze–thawed expansive soils incorporating the pore size-volume dual microstructure parameters

The macroscopic desiccation shrinkage of freeze-thawed expansive soils is intrinsically linked to their micro-mesostructural characteristics, necessitating a multi-scale coupling approach for rigorous deformation characterization. In this study, we investigated the effect of freeze–thaw cycle on the microscopic pore structure and macroscopic shrinkage-cracking behavior of unsaturated expansive soil. Specimens with initial water contents of 11–29% were subjected to up to 11 freeze–thaw cycles. Freeze-thaw cycle predominantly affects the 5–75 μm pore range in unsaturated expansive soils. Both the transition from unsaturated to saturated states and cumulative freeze–thaw cycles trigger a bimodal-to-trimodal transformation in pore size distribution curves, and a transition from a wide to a narrow range of the interpeak distance between secondary and tertiary peaks. Comprehensive analysis reveals that fracture-type failure occurred under desiccation versus fragmentation-type deterioration induced by freeze–thaw cycle. Freeze-thaw actions reduced the soil contraction capacity and disrupted the self-healing process of the desiccation shrinkage cracks. Besides, a coupled macro–micro prediction model was developed for desiccation shrinkage deformation of freeze-thawed expansive soils, integrating the dynamic evolution in the pore size-volume dual microstructure parameters. These findings provide critical insights for the assessment of deformation stability and the mitigation of geohazards involving expansive soils in cold-region engineering.