## 2 Test Design and Scheme
### Test Equipment and Model Building
#### 2.3.2 Model Construction and Sensor Layout  
Soil samples from the site were collected, transported, and dispersed before being air-dried to prepare the models, following compaction standards that varied according to the positions of the subgrade and foundation. Due to limitations in the available test soil and site conditions, the road was treated symmetrically along the centerline. Additionally, considering the infinite extension of the foundation and a similarity ratio of 20, the foundation had a height of 0.6 m, a width of 0.5 m, and a length of 1.3 m. Considering that the infiltration from the side ditch primarily humidified the lower subgrade, the height of the far subgrade was controlled at 4 m. After this control, the model subgrade had a height of 0.2 m, with a length of 1.2 m and a width of 0.5 m. A 0.1 m slope foot was reserved, and the subgrade side slope was approximately 1:1.5. The model diagram is shown in Figure 6.  
To facilitate a more effective comparison of field construction technologies, a 5 cm cement cushion layer is placed at the interface between the model foundation and subgrade. Each layer is controlled to a height of 3 cm, and a rubber hammer is used as the rolling tool. During the rolling process, water is sprayed onto the surface of each layer. Simultaneously, after each layer is rolled, ring knife samples are collected to measure the moisture content and dry density, ensuring that the physical parameters of the produced model closely match those of the prototype compacted soil. The dry density error must not exceed 0.3 g/cm\({}^{3}\), and the moisture content error must not exceed 1%. The specific rolling indicators are presented in Table 4. Based on the road engineering standards and the similarity ratio discussed in this paper, the self-weight of the upper subgrade of the external load model, set at 92.7 N at the top of the model, is determined. The model construction is illustrated in Figure 7.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Range (\%)** & **Measurement Accuracy (\%)** & **Response Time (s)** & **Operating Environment (\({}^{\circ}\)C)** \\ \hline RS485 & 100 & \(\pm 1\) & \textless{}1 & \textless{}40–80 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 2: Technical indicators of soil moisture sensor.  
Figure 6: Schematic diagram of the model.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Frequency** & **Sensor** & **Number of Channels** & **Acquisition Interval (min)** & **Connection Mode** \\ \hline
\begin{tabular}{c} Pore water \\ pressure gauge \\ \end{tabular} & 20 & 10 & Full bridge \\ \hline \hline \end{tabular}
\end{table}
Table 3: JC-YBY-2001 data acquisition parameters.  
A total of three sensor layers are embedded into the model. A soil hygrometer and settlement measuring rod are, respectively, placed at the foundation heights of 20 cm and 40 cm, as well as at the interface between the subgrade and the foundation. A schematic diagram of sensor embedding is shown in Figure 8. The water channel measures 40 cm in length, 10 cm in width, and 10 cm in height. A water injection pipe is installed in the channel to regularly fill it, simulating the humidity and settlement deformation characteristics of the foundation and subgrade during the infiltration of side ditch water in the cold season. The test dates were 13 December 2023-22 December 2023, the water injection time was 8:00 a.m. every day, and the water injection volume was 2 L. To prevent freezing before infiltration, warm water at 30 \({}^{\circ}\)C is used for injection.