## 4 Discussion
### Displacement of the Slope Model in Freeze-Thaw Cycles and Its Microscopic Interpretation  
From Figure 13, we can see that the displacement variation characteristics of the expansive soil slope during freeze-thaw cycling were as follows: (1) freezing shrinkage and thawing expansion occurred during freezing; (2) after freezing many times, the top of the slope dropped and the bottom of the slope rose, which indicates that there was a slow creep of the slope during the freezing process; that is, the soil particles shrank and slid downward. All displacements in this paper refer to vertical displacements. However, the slope surface and the top of the slope moved towards the free surface (displacement was positive) after several freeze-thaw cycles, which indicates that the pore structure in the slope was larger and the soil skeleton structure was looser. It can be seen from Figure 13 that with an increase in the slope ratio, the vertical displacement variation of the slope after several freeze-thaw cycles also increased, which indicates that the larger the slope ratio is, the greater the displacement of the slope in the freeze-thaw cycle is, and the worse the stability is.  
The first displacement trend of the expansive soil slope was the same as the pore change trend of the slope model in the freeze-thaw cycles described in Section 3.1. That is, in the process of thawing, the porosity of expansive soil gradually decreased, and the slope displacement decreased; in the freezing process, it decreased first and then increased. However, during the remaining freeze-thaw cycle, the change trend of the two was opposite. This is because the moisture content of the two was different. In the nuclear magnetic resonance (NMR) test, the expansive soil samples must be in the saturated state, which leads to the phenomenon that the expansive soil particles shrink and the pores increase during the freezing process, while the expansive soil particles increase in the thawing process, but the consolidation settlement occurs. Differently from the saturated soil sample, for the expansive soil slope with low moisture content, the slope displacement fluctuates greatly during the freeze-thaw cycle, which eventually leads to the overall displacement to the free surface in the thawing stage, and the increase in slope ratio increases the displacement amplitude.  
Therefore, for expansive soil slopes with high moisture content, the freeze-thaw cycling leads to the reorganization of the soil particle skeleton and a reduction in porosity, which is manifested as a decrease in the overall displacement. However, the expansive soil slope with low moisture content tends to free face (displacement is positive) after several freeze-thaw cycles, that is, the slope moves upward and the structure is looser. It can be speculated that between these two moisture contents, for a certain kind of expansive soil, there is a constant moisture content \(\omega_{\text{s}}\) that causes the overall displacement of the slope to remain basically unchanged after several freeze-thaw cycles; that is, it reaches a stable state. It is suitable to maintain this saturation when the method of maintaining the moisture content is used to treat expansive soil in cold regions. For different kinds of expansive soil, displacement tests of freeze-thaw cycles under a moisture content gradient are needed to analyze \(\omega_{\text{s}}\). In practical engineering, controlling \(\omega_{\text{s}}\) should have a good constraint effect on cracks.