Research on deterioration mechanism of sulphate saline soil strength under freeze–thaw cycles in Xining Area

In China’s seasonal permafrost regions, widespread saline soils are prone to frost heave, salt heave, and uneven thaw settlement, mainly driven by freeze–thaw cycles and salt content. This study investigates the mechanical behavior and strength degradation of sulfate saline soils in the Xining area under freeze–thaw cycles through field investigations, laboratory tests, theoretical analysis, and establishes a corresponding strength damage model. The results indicate that sulfate saline soils exhibit a thick diffuse double layer and high particle dispersibility. Repeated salt frost heave–thaw settlement leads to significant structural degradation. With increasing freeze–thaw cycles, the abundance distribution proportion of structural units within the range of 0.8–1.0 has increased. Microstructural units become more equiaxed, pores elongate, particle contact area decreases. Macroscopically, the soil undergoes periodic volume changes and progressive strength deterioration. The decrease in cohesion and internal friction angle is 26%–30% and 32%–44%, respectively. Moreover, increasing salt content enhances the heterogeneity in the morphology and orientation of structural units, aggravating structural damage. At the macroscopic level, the amplitude of alternating “salt frost heave–thaw settlement” deformation increases, and the salt heave rate rises to about 11 times that of frost heave, accounting for more than 90% of the total deformation.