Abstract: To explore the correlation mechanism between the macroscopic strength and microscopic structure of undisturbed loess containing Na2SO4 salt under freeze-thaw cycles, loess samples were subjected to different freeze-thaw cycle tests, permeability tests with different Na2SO4 salt contents, microscopic tests (scanning electron microscope tests), and macroscopic tests (consolidated-undrained shear tests). The grey correlation analysis method was used to determine the correlation between the shear strength index and microscopic tissue parameters of the samples under different salt concentrations and freeze-thaw cycle times. The strength index changes and microscopic pore damage changes were analyzed from a macro-micro perspective. The research results indicate that: 1) Changes in Na2SO4 salt content and freeze-thaw cycles can cause a redistribution of the proportion and area of pores in the soil. As the salt content increases, large pores transform into small and medium-sized pores. As the number of freeze-thaw cycles increases, small and medium-sized pores transform into large pores. 2) Under freeze-thaw conditions, as the number of freeze-thaw cycles or salt content increases, the strength, cohesion, and internal friction angle of the soil significantly decrease. When the number of freeze-thaw cycles increases to 5 or more, the internal disturbance of the soil tends to stabilize. 3) According to the grey correlation analysis, the change in particle distribution has the greatest impact weight on the change in cohesion, and the change in soil particle area circumference has the greatest impact on the internal friction angle. During the freeze-thaw cycle, the correlation between the microstructure characteristics of particles and the internal friction angle is high, while the correlation between cohesion is low. As the number of freeze-thaw cycles increases, the fractal dimension of the area perimeter method has the greatest impact on cohesion and internal friction angle. The particle morphology plays a dominant role in the microstructural characteristic parameters and has the greatest impact on the shear strength of loess.

Key words: microstructure, soluble salt, freeze-thaw cycle, loess