In this study, carbonate saline soil in the Duerbote of Songnen Plain was improved by adding lime and fly ash. The improved soil was exposed to 0, 30, and 60 freeze-thaw cycles, and ordinary triaxial compression tests (UU) were conducted under confining pressures of 100, 200, and 300 kPa. The effects of freeze-thaw cycles, lime content, fly ash content, and confining pressure on the peak deviatoric stress, cohesion, and internal friction angle on the lime-ash improved carbonate saline soil were analysed. The incorporation of lime and fly ash in carbonate saline soil transformed the stress-strain curves from the strain hardening type to strain softening type and also changed the strain corresponding to the peak deviatoric stress of the soil. The effect of lime on the shear strength of the soil was the most significant, and it significantly increased the peak deviatoric stress, cohesion, and internal friction angle of the soil. Similarly, the effect of fly ash on the peak deviatoric stress and internal friction angle of the soil was significant, but the effect on cohesion was very limited. A small amount of fly ash increased the peak deviatoric stress and internal friction angle of carbonate saline soil, but an excessive amount had the opposite effects. The freeze-thaw cycles cause the shear strength of the carbonate saline soil to decrease, but the incorporation of lime and fly ash alleviated this decrease.

Climate change is accelerating its adverse impact on ecosystems and infrastructure systems in cold regions. For extensive carbonate saline soil areas, their response to the freeze-thaw cycle remains uncertain. By considering the continuous intensification of freeze-thaw cycle frequency, the mechanical characteristics of carbonate saline soils are analyzed for different salt content (0.6% to 2.1%) based on the mechanical test in this paper. The purpose is to reveal the change law of shear strength and its parameters of carbonate saline soils under the scenario of continuous freezing and thawing cycles. The micro-characteristics of the carbonate saline soil before and after freeze-thaw cycling were analyzed by scanning electron microscopy, indicating changes in the structural soil properties caused by the combination of freeze-thawing and salinity. The scanning electron microscope images reveal the cumulative effect of frost heaving and salt expansion, i.e., increasing the number of pores between particles, reducing the effective contact between particles, and weakening the interaction force, resulting in cracks development. A series of mechanical tests demonstrate the stress-strain behavior of carbonate saline soils for different numbers of freeze-thaw cycles under different confining pressures. A transformation from strain-softening to strain-hardening is observed with an increase in the salt content from 0.6% to 2.1%. Furthermore, the shear strength of the carbonate saline soil decreases as the salt content and number of freeze-thaw cycles increase. The shear strength degradation mechanism is attributed to the cohesion and the internal friction angle. These shear strength parameters are critical in geotechnical analyses, such as evaluating of load capacity of foundations and slope stability in similar saline soils.