Investigation of the shear behavior and microscopic damage constitutive modeling of sulfate saline soils during the freeze–thaw process

Influenced by the climatic conditions of seasonally frozen regions, sulfate saline soils exhibit salt expansion, frost heave, thaw settlement, and dissolution-induced collapse, which can cause severe damage to engineering structures. These failures are closely associated with the deterioration of macroscopic mechanical properties and the damage evolution of the soil microstructure. To investigate the mechanical response and damage characteristics of sulfate saline soils during the complete freeze–thaw process, macroscopic direct shear tests and microscopic scanning electron microscopy observations were conducted on sulfate saline soils with a salt content of 2% at several temperature points throughout the process. The results indicate that during the freeze–thaw process, the shear strength of sulfate saline soil first increases and then decreases. Correspondingly, the shear stress–shear displacement curves initially evolve from an ideal plastic or strain-weak hardening state to a strain-softening state and subsequently return to an ideal plastic or strain-weak hardening state. Further analysis reveals that sulfate primarily influences the degree of soil damage through crystallization–dissolution processes and by altering the critical temperature and intensity of the water–ice phase transition. In addition, the fractal-based microscopic statistical damage constitutive model can effectively characterize the shear stress–shear displacement behavior and the damage deterioration mechanism of sulfate saline soils during the freeze–thaw process. The findings provide a theoretical basis and useful reference for agricultural planning and engineering construction in sulfate saline soil regions.