## 2 Experimental Methodology
### Slope Model Test
#### 2.1.2 Sensor Layout  
The displacement, soil pressure, temperature and moisture content of the expansive soil slope model during freeze-thaw cycles were recorded. Figure 3 shows the slope model monitoring system used in the test. A HG-C1100 micro laser displacement sensor (160 mm range and 70 \(\upmu\)m precision) was used for slope surface displacement monitoring. The displacement sensor was arranged from bottom to top along the slope. Soil pressure was measured by a DMTY resistance strain soil pressure module (range: 0.05-10 MPa), expressed as E1-E3, with a depth of 10 cm, arranged from bottom to top along the slope (Figure 3). The temperature and moisture content were monitored by an integrated sensor, in which the moisture content measurement range was 0-100%, and the temperature measurement range was \(-\)40 \({}^{\circ}\)C to 80 \({}^{\circ}\)C. Five sensor groups were set up for temperature and humidity monitoring of the slope model: W1, W2 and W5 were arranged from bottom to top along the slope surface, while W3, W4 and W5 were arranged at the top of the slope with different depths. In order to detect the cold front intrusion process of slopes at different positions, the sensor was set at 10 cm away from the slope surface. This position was not too deep and difficult to monitor, nor too shallow, resulting in rapid change. At  
\begin{table}
\begin{tabular}{c c c} \hline
**Properties** & **Values** & **Standard Designation** \\ \hline Specific gravity, \(G_{S}\) & 2.73 & ASTM D854—14 [32] \\ Natural moisture content (\%) & 9.14 & ASTM D2216—19 [33] \\ Free expansion ratio & 1.51 & [PERSON] et al. [5] \\ Liquid limit, \(\omega_{L}\) (\%) & 45.83 & \\ Plastic limit, \(\omega_{P}\) (\%) & 26.92 & ASTM D4318—17e1 [34] \\ Plasticity index, \(I_{p}=\omega_{L}-\omega_{P}\) (\%) & 18.91 & \\ Optimum water content, \(\omega_{opt}\) (\%) & 20.17 & \\ Maximum dry unit weight \(\gamma_{d\max}\) (g/cm\({}^{3}\)) & 1.53 & ASTM D698—12 [28] \\ \hline \end{tabular}
\end{table}
Table 1: Physical properties of the selected expansive soil.  
Figure 2: Slope preparation process. (**(a)** layered compaction; **(b)** surface scraping; **(c)** dismantling mold; **(d)** slope cutting).  
the same time, in order to capture the gradual invasion of the cold front, a group of sensors along the vertical depth were also arranged on the top of the slope with the least impact on the slope stability.  
In the test, displacement monitoring used laser measurement which was not in contact with the slope and would not affect the stability of the slope. Along the lateral wall of the model, the temperature and moisture sensors were connected through signal lines to the external signal processing device (Figure 3c). The upper part of the sensor was covered with compact expansive soil, which had no obvious influence on the stability of the slope during the freeze-thaw cycle. During the test, there was no obvious difference between the soil above the sensor and the surrounding soil, and the cracks on the slope surface did not develop along the sensor area. The validity of the test data can be determined by excavating the slope model after the test, and the sensor layout area was consistent with the soil body at the signal line passing through and with other soil bodies.