## 4 Discussion  
### Analysis of Cracks during Freeze-Thaw Cycles  
Expansive soil is a kind of plastic material. Fissure is the physical characteristic of expansive soil itself. When tensile stress is produced in the soil, the corresponding strain is produced by internal cohesion. When the tensile strain exceeds the limit, cracks will occur [46]. During the freezing process, the moisture content of expansive soil slope decreases rapidly, the free water in the expansive soil forms ice crystals, and the thickness of the moisture film on the surface of particles decreases, which leads to the shrinkage of expansive soil particles and tensile stress. Figure 12a is a schematic diagram of freeze-thaw cracks. During freezing, the temperature and moisture content of expansive soil at different depths are different, which leads to uneven stress between the upper and lower soils. When the stress difference between the upper and lower strata is greater than the tensile stress of expansive soil, cracks will occur. [PERSON] et al. [24] studied the development process of cracks in expansive soil during the freeze-thaw cycle and proposed that the repeated formation of ice lens led to the generation of cracks. This was not observed in this test, which may be because [PERSON] et al. [24] used expansive soil with a high moisture content (liquid limit), so ice lens was easy to produce during the freeze-thaw cycle. [PERSON] et al. [47] used CT to scan the development of cracks in the soil during the freeze-thaw cycle and found that cracks appeared in the soil when the cold front passed, which was consistent with the description in Figure 12a. When the expansive soil is frozen, the free moisture forms ice and its volume increases. Therefore, without considering moisture evaporation and ice sublimation, the development depth of cracks in expansive soil is related to its own expansibility and moisture content. The larger the free expansion rate of expansive soil is, the greater the moisture loss and shrinkage of soil particles in the freezing process, the greater the tensile stress in the slope and the greater the development speed of cracks. Figure 12b shows that cracks will not obviously develop during freezing and thawing when the moisture content of expansive soil is small. With an increase in moisture content, the moisture loss of soil particles in freezing is greater than the increase in ice volume, and cracks develop to a significant degree. When the moisture content of expansive soil is large enough, the cracks of the slope decrease gradually because the shrinkage of expansive soil particles is less than the volume increment of ice. Compared with the fractures of three different slope ratios after freeze-thaw cycles (in Figure 6), the fracture development depth and width of \(S_{1}\), \(S_{2}\) and \(S_{3}\) increased continuously. After freeze-thaw cycles, the fracture depth of \(S_{2}\) was 1.1 times of \(S_{1}\), and the width was 1.25 times that of \(S_{1}\). The fracture depth of \(S_{3}\) was 1.36 times that of \(S_{1}\), and the width was 1.5 times of \(S_{1}\). This is because: (1) during the freeze-thaw cycle, the angle between the slope and the top of the slope is small for the slope with large slope ratio, which easily forms a cold front invasion in both directions of the slope and the top of the slope. This  
Figure 12: Crack development of the slope during freezing and thawing (**a**) formation process of freeze-thaw cracks; (**b**) relationship between cracks and moisture content in the freeze-thaw process. causes the temperature gradient of the slope to increase, resulting in an increase in moisture migration. At the same time, it also leads to a significant difference between the surface and internal shrinkage of the slope, thus forming large cracks; (2) when cracks are formed, the component force of the soil gravity under the cracks in the slope surface direction is greater for slopes with larger slope ratios.