## 4 Results and Discussion
### Pore Size Distribution  
Based on the MIP test, the curves of cumulative pore volume distribution and pore size distribution of expansive soil under different dehydration conditions and duration were obtained, as shown in Figures 4 and 5. Figure 4 shows that the pore size distribution range of expansive soil measured by MIP test was 5-10\({}^{6}\) nm. Seen from Figure 5, the initial state of pore size distribution curve presented a three-peak distribution, and the corresponding distribution ranges of peak values were 5-100 nm, 500-4000 nm, and 4000-10,000 nm, respectively. The corresponding pore size range of peak values kept changing with the duration of dehydration. Referring to the classification method of pores by [PERSON] et al., and [PERSON] et al. [20,21], and based on the tri-modal of pore distribution curve of expansive soil, the pores of expansive soil were divided into small pores (5 nm \(\leq\) d \(<\) 300 nm), medium pores (300 nm \(\leq\) d \(<\) 4000 nm) and large pores (d \(\geq\) 4000 nm). The pore size of the samples used in this study are characterized by the medium pore range, which is different from the soils in western and southern China [20]. Figure 6 shows that under different dehydration environments, the pore volumes of large, medium and small pores varied with the duration of dehydration. In the dehydration environment of high temperature and dry (50 \({}^{\circ}\)C-10% humidity) environment, the large pore volume of expansive soil samples decreased first and then increased with the dehydration process. In the initial state, the large pore volume was 0.093 mL/g, and decreased to 0.021 mL/g on the 7 th day of dehydration. When the soil mass reached dynamic balance after 15 days of dehydration, the porous volume of soil was 0.44 mL/g. In the initial state, the volume of medium pore in the sample was 0.107 mL/g. On the 5 th day of dehydration, the volume of medium pore decreased to 0.042 mL/g. Thereafter, the change of the volume of medium pore in the soil sample fluctuated little. The volume of small pore changed little,  
Figure 3: Moisture content of expansive soil varies with dehydration time under different conditions. whose initial pore volume was 0.066 mL/g and the final pore volume was 0.057 mL/g when dehydration reached a stable state. Under high temperature and humidity (50 \({}^{\circ}\)C-70%) environment, the volume of large and medium pores of expansive soil samples both decreased with dehydration until stable,  
Figure 4: The relationship between the cumulative pore volume distribution and the dehydration time of expansive soil under different environmental conditions (**a**) 50 \({}^{\circ}\)C-10%, (**b**) 50 \({}^{\circ}\)C-70%. Figure 5: The relationship between the pore size distribution of expansive soil and the duration of dehydration under different environmental conditions (**a**) 50 \({}^{\circ}\)C-10%, (**b**) 50 \({}^{\circ}\)C-70%. while the volume of small pores changed little. In the initial state, the volume of large pores was 0.093 mL/g, and it decreased to 0.025 mL/g on the 5 th day of dehydration. Thereafter, the volume of large porous soil fluctuated little until the mass of soil sample reached dynamic balance on the 26 th day of dehydration. The initial pore volume was 0.107 mL/g, and decreased to 0.039 mL/g on the 5 th day of dehydration. Thereafter, the pore volume of soil samples fluctuated little. However, under the high temperature and humidity environment, the pore volume of soil sample changes a little, whose initial pore volume was 0.066 mL/g, and the final pore volume was 0.059 mL/g when dehydration reached a stable state. According to the results of pore size distribution, we can find that the dehydration rate of expansive soil samples is fast in the initial stage due to the environmental influence of high temperature and dry. The soil shrinks because of the rapid water loss, resulting in the rapid shrinkage of the large and medium pores in the soil, and then the volume content of pores shows a decreasing trend.