# Effects of Freeze-thaw Cycles on the Mechanical Properties and Microstructure of a Dispersed Soil
## 3 Result and Discussion
### Direct Shear Test  
The obtained results reveal that the configurations of the shear stress-shear displacement curves and the patterns of alterations in each soil sample under varying pressures exhibit similarities. This finding is depicted in Figure 10. (Furthermore, the relationship curves between vertical deformation and shear displacement can be found in Figure S1 of the Supplementary Material.) It is evident from the figure that, in all cases, an increase in the number of freeze-thaw cycles leads to a decrease in the shear stress of the soil samples. As compressive stress applied to soil samples decreases, shear strength reduction becomes more evident. Additionally, the shear stress-shear displacement curves of soil samples after freeze-thaw cycles show strain-hardening behavior without notable peaks. Cohesion (c) and angle of internal friction (\(\varphi\)) are two critical parameters that determine the shear strength of soils and can be determined using the Mohr-Coulomb criterion. Figure 11 presents the changes in shear strength parameters for each soil sample under varying quantities of freeze-thaw cycles. The findings suggest that the c-value is more profoundly impacted by the number of freeze-thaw cycles compared to the \(\varphi\)-value. The c-value decreases by 65% throughout the process, while the \(\varphi\)-value only exhibits a change of approximately 30%. In general, the \(\varphi\)-value of the soil samples does not undergo significant changes with an increasing quantity of freeze-thaw cycles. However, there is a trend of an initial increase, followed by a decrease, and the \(\varphi\)-value peaks at seven freeze-thaw cycles. The decrease in the c-value can be attributed to two factors. Firstly, the soil used in this experiment has a high concentration of Na+ ions, which can enhance the thickness of the hydration film surrounding soil particles. This increase in hydration film thickness leads to a more significant separation distance between soil particles, consequently leading to reduced soil cohesion [35]. Secondly, the escalation in the frequency of freeze-thaw cycles can disrupt the original structural properties of the soil. The soil samples, before undergoing the freeze-thaw cycles, exhibit a relatively intact structure. In the freeze-thaw process, the existence of expanded pores and fissures in the soil intensifies, resulting in a weakened interconnection among soil particles. Consequently, this leads to a notable decline in the c-value. The behavior of the \(\varphi\)-value follows a pattern of initial increase  
Figure 9: Variations in SDP of dispersed soil after different numbers of freeze–thaw cycles. and subsequent decrease with an escalation in the frequency of freeze-thaw cycles. This phenomenon can be ascribed to the crystallization of substances within the pore solution. With a higher number of freeze-thaw cycles, soluble salt crystals gradually form and serve as a bridging agent among soil particles [36]. This linking effect partially mitigates structural damage and leads to an increase in the \(\varphi\)-value. However, the highest degree of crystalline substance precipitation is reached after seven freeze-thaw cycles. The widening of cracks beyond this point is no longer conducive to crystallizing soluble salts in the pore solution, resulting in a gradual decrease in the \(\varphi\)-value. The changes in the shear strength parameters, including the decline in the c-value and the initial increase followed by a reduction in the angle of internal friction \(\varphi\)-value), significantly affect the macroscopic mechanical strength of the soil samples, according to the Cullen strength criterion. Despite the initial increase and subsequent decrease in the \(\varphi\)-value, the magnitude of this change is much smaller compared to the reduction in the cohesion c-value.