## 4 Results and Discussion
### Pore Size Distribution
This result is supported by the study of influences of the drying rates on shrinkage [22]. As we all know, the occurrence of cracks in soil is usually controlled by mineral composition and structural composition. Then the distribution of water content in soil is always nonuniform in the process of rapid dehydration due to the difference of minerals and structure in soil samples, which leads to the emergence of tensile cracks. At the same time, dehydration leads to the gradual connection of some medium pores in the soil, which forms new large pores. Therefore, with the process of dehydration, the large pores in the soil samples show a trend of decreasing first and then increasing, while the medium pores show a trend of decreasing until dehydration is stable. On the other hand, in the evaporation environment, the small pore volume decreases slightly, and the overall change is not big. The main reason is that small pores are controlled by the degree of clay minerals consolidation. The particles of clay minerals are small, and the water molecules attached to the surface mostly exist in the state of bound water. In the dehydration process, the free water molecules in soil are the first to be dehydrated, and the energy required for dehydration of bound water is much higher than that of free water molecules, so it is more difficult to become dehydrated. Therefore, under the environment of 50 \({}^{\circ}\)C-10%, water molecules in small pores are relatively less affected by the environment, so the volume of small pores does not change much in the process of dehydration. As a result of high moisture content under the environment of high temperature and humidity, the dehydration rate of expansive soil is relatively slow. This slow process makes the soil moisture content change relatively evenly in the soil, resulting in the relatively uniform shrinkage. In the process of dehydration, large and medium pores become small or even closure due to the dehydration action. Therefore, the volume of large and medium pores decreased until dehydration is stable. The water molecules inside the pores mostly exist in the state of bound water, and the dehydration environment of 50 \({}^{\circ}\)C-70% humidity has little influence on the bound water, so the fluctuation of the small pore volume is small. Figure 6: The volume of large, medium and small pores varies with the duration of dehydration under different environmental conditions (**a**) 50 \({}^{\circ}\)C-10%, (**b**) 50 \({}^{\circ}\)C-70%. Based on the MIP test, the variation trend of expansive soil porosity with the duration of dehydration in different environmental conditions was also obtained (Figure 7). According to Figure 7, under both high-temperature dry and high-temperature humidity dehydration conditions, the porosity of expansive soil samples decreased firstly, then increased and finally tended to be stable during the dehydration process. This process is opposite to the result of freeze-thaw cycle which kept the same dehydration condition [15]. The porosity of the expansive soil was 39.5% in the initial state, and decreased to 23.4% after dehydrating for a period of time under the environment of 50 \({}^{\circ}\)C-10% humidity. The porosity increased to 26.4% when the dehydrated mass of the sample reached dynamic balance. When the dehydration test was carried out at the condition of 50 \({}^{\circ}\)C-70% humidity, the porosity of the expansive soil sample decreased from 39.5% to 22.8%, and then increased to 28.6% when the dehydration of the soil sample reached dynamic equilibrium. In this paper, we can find that the microscopic pore structure of expansive soil samples undergo a process of shrinkage and deformation due to the dehydration process under the conditions of high temperature drying and high temperature humidity, so the porosity decreases in the initial stage of dehydration. On the other hand, the uneven dehydration process is caused by the difference in mineral structure of the sample itself, which leads to the formation of tensile cracks in the soil.