## 3 Results
### Soil Pressure in the Slopes during Freeze-Thaw Cycling  
Figure 10 shows the change in soil pressure of the three groups of expansive soil slope models in four freeze-thaw cycles. The soil pressure increased in the freezing process and decreases in the thawing process during a single freeze-thaw cycle, and the soil pressure at the top of the \(S_{1}\), \(S_{2}\) and \(S_{3}\) slope models decreased gradually. This is because during the freezing process, the free moisture in the soil gradually changes into ice crystals, and the volume increases. This causes the frost heaving force in the soil, and the frost heaving stress is transmitted to the soil skeleton, which eventually leads to the rise in soil pressure [40]. In the process of a single freeze-thaw cycle, a peak appears at the beginning of freezing, which is the stage of releasing latent heat from ice-moisture conversion in the soil. With the stability of the freezing temperature, the soil tends to be frozen stable, and the positive temperature stage of the external environment will lead to another thawing stability of the slope. This phenomenon can also be analyzed from the perspective of pore pressure change. [PERSON] et al. [41] monitored the pore pressure change process of silty clay during the freeze-thaw cycle. It was found that the pore pressure decreased continuously during freezing and increased continuously during melting. In Figure 10, the rising soil pressure during freezing corresponds to the dissipation of pore pressure, while the decreasing soil pressure during melting corresponds to the gradual increase in pore pressure. This is because the absorbed water freezes gradually during the low temperature process, resulting in changes in unfrozen water thickness and adsorption potential in soil samples. During the melting process, the radius of curvature of the ice-water interface increases, changing the adsorption potential, resulting in an increase in pore pressure.  
The overall effect of multiple freeze-thaw cycles on the soil pressure can be seen in Figure 10. The results of the soil pressure sensors of all slope models were consistent, and the soil pressure decreased with the increase in the number of freeze-thaw cycles. This is because the freeze-thaw cycles can destroy the skeleton of expansive soil and make the structure looser [42]. The weathering cracks on the slope surface can also reduce the soil pressure at E1-E3, but the decrease in measured soil pressure caused by the increase in slope pore cannot indicate that the slope is safer, because the increase in slope pore leads to the decrease in the area of soil pressure box contacting the soil, thus reducing the measured value of soil pressure. It can be seen from Section 3.2 that the freeze-thaw cycles bring moisture migration to the slope surface in the expansive soil slope model, which will cause an increase in the soil bulk density \(\gamma\). The formula for soil self-weight stress is as follows.  
\[\sigma_{z}=\gamma\;\mathrm{z} \tag{2}\]  
where \(\sigma_{z}\), \(\gamma\) and \(z\) are the self-weight stress of the soil, the bulk density of the soil and the depth, respectively.  
Moisture migration in the closed system will lead to an increase in the soil pressure on the surface of the slope and a decrease in the internal soil pressure. The slope increases the gravity above the sliding surface of the slope and reduces the slope stability. It can be seen from Figure 10a that there is a peak soil pressure in the thawing stage, because the ice in the soil melts into a large amount of free moisture in the high temperature thawing stage, which brings excess pore moisture pressure. According to the principle of effective stress, the dissipation of excess pore moisture pressure causes an increase in soil skeleton stress, and finally leads to the peak value of soil pressure.