## 5 Application of Metro Station Excavation Model Test
### Model Test Design  
According to the size of the foundation pit excavation and the principles of high strength, stability, and easy operation, the test bench was designed; its dimensions were 3 m \(\times\) 1 m \(\times\) 2 m. The test bench adopted a frame-type steel frame, which was composed of box-type outer peripheries and a bottom plate for carrying the model soil. High-strength tempered glass was used to simulate the retaining wall, as shown in Figure 8. Stacked hydraulic lacks were used to apply the load uniformly to the model soil through a force transfer device. The excavation project of the metro station foundation pit was simulated by excavating model soil. The model test system consisted of a steel structure bench, data acquisition equipment, and vertical loading hydraulic jacks. The model soil was made using a layered filling method during which similar materials were buried in the test bench to the design height and evenly tamped layer by layer.  
A uniform distributed load of 14 kN was applied to the adjacent excavation area in order to simulate the initial ground stress field of the soil. The acrylic plate was embedded to simulate the supporting structure, and the earth pressure cells were deployed on the acrylic plate to measure the earth pressure at different layers of the excavation. Figure 9 shows the diagram of the model test. The layout of the monitoring elements and the model soil are shown in Figure 10.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material Type** & **\(P\) (g/cm\({}^{3}\))** & **\(E\) (MPa)** & **\(C\) (kPa)** & **\(\varphi\) (\({}^{\circ}\))** & **\(\sigma_{c}\) (MPa)** & **Poisson Ratio** \\ \hline Foundation pit soils & 2.42 & 2350 & 141 & 24.8 & 25.5 & 0.27 \\ Similar materials & 2.42 & 47 & 2.82 & 24.8 & 0.51 & 0.27 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Mechanical parameters of the raw soils and the similar material.  
Figure 8: Model test bench and model soil backfill.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**\(B\) + \(I\)/\%** & **\(B\)/\(I\)** & \(\beta\)/\% & **PA/\%** & **\(G\)/\%** & **Mesh no.** & **\(S\)/\%** \\ \hline
65 & 7:3 & 40 & 6 & 1.5 & 40 & 27.5 \\ \hline \hline \end{tabular} Note: \(B\) is bartie powder; \(I\) is iron powder; \(S\) is quartz sand; \(G\) is gypsum powder; \(PA\) is cementing agent; \(\beta\) is the proportion of rosin in cement.  
\end{table}
Table 6: Proportion of the similar material in the model test.  
occurred when \(n=0.50\). Before the excavation depth reached 25 cm, the earth pressure had a larger disturbance. Therefore, the corresponding support system should be strengthened during construction in order to prevent excessive earth pressure causing ground failure.  
The model test indicates that the similar material ensures accurate and stable mechanical properties during the model excavation, when the mechanical parameters can be adjusted accordingly in large ranges, providing a reference for the construction simulation of similar underground projects. The monitoring and analysis system adopted in this paper can accurately represent the changes of soil mechanics parameters. The development of a new similar material provides assurance for the success of the model test in terms of materials.