# Experimental Study on the Soil-Structure Responses Induced by Tunnelling in Limited Space
## 3 Model and Test Descriptions
### Preparation of Model Tunnel and the Model Underground Structure  
Given the cost and difficulty of model fabrication, aluminum alloy was finally chosen to be the material for the model underpass and model diaphragm wall. In fact, the actual connection between the underpass and the diaphragm wall was rigid. Hence, in this study, the overlapping parts between the model diaphragm wall and the underpass were welded together tightly to ensure integrity. The detailed information of the model underpass is illustrated in Table 2. The thickness of the model diaphragm wall was 0.015 m. The length of the model underground structures (i.e., model underpass and diaphragm) along the advancing direction of the tunnel was 0.6 m. Moreover, the picture of the model underground structures is presented in Figure 5. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & \begin{tabular}{c} \(E\) \\ **(MPa)** \\ \end{tabular} & \begin{tabular}{c} _\(\gamma\)_ \\ **(kN/m3)** \\ \end{tabular} & \begin{tabular}{c} _\(\varphi\)_ \\ **(c)** \\ \end{tabular} & \
\begin{tabular}{c} \(C\) \\ **(kPa)** \\ \end{tabular} &
\begin{tabular}{c} _Water Content_ \\ **(\%)** \\ \end{tabular} \\ \hline Prototype clay & 4.32 & 17.6 & 18.5 & 33 & 58.7 \\ \hline Model clay & 0.143 & 17.3 & 18.3 & 1.105 & 53.6 \\ \hline Similarity ratio & 30.00 & 1.02 & 1.01 & 29.86 & 1.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Typical mechanical parameters for prototype clay and model clay. Figure 4: The view of the model soil: (**a**): model sand; (**b**): model clay. Figure 5: Typical view of the model underground structures. As for the model tunnel, many scholars pointed out that polyvinyl chloride (PVC) tubes can be used to simulate tunnels in model tests. However, many researchers have indicated that the connection effect from the longitudinal and circumferential joints in the segment should be taken into consideration. Hence, the grooving method was widely employed to simulate the reduction of joint stiffness induced by bolts. Since the tunnel-soil-underground structure interaction was the main research topic in this study, the model tunnel was supposed to be assembled to reflect the actual condition to a great extent. According to the findings reported by [PERSON] et al. (2018) [38], 3D printing technology can be an extremely effective method for revealing the detailed features of a scaled model. Based on the aforementioned advantages, 3D printing technology was adopted in this model study. The model segments consisted of 5 standard segments (67.5\({}^{\circ}\)) and 1 top segment (22.5\({}^{\circ}\)), which was in accordance with the distribution of typical tunnel segments in Shanghai. It is well known that the compression stiffness, tension stiffness and bending stiffness of the bolts are the most important characteristics of the joints between the segments. Therefore, according to the similarity theory described in [PERSON] et al. (2014) [39], the final number of longitudinal bolts was selected to be 12 and the number of transverse bolts was chosen to be 6. The material used for the model bolts was iron and the diameter of the model bolts was 1.2 mm. The length of the model tunnel was 0.6 m, which was equal to the length of the model underpass and diaphragm wall. The detailed photographs of the model tunnel are illustrated in Figure 6. The detailed information of the model tunnel is listed in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Type**} & **Thickness of the** & **Thickness of the** & **Thickness of the** & **Thickness of the** & **Material** \\  & **Top Slab (m)** & **Middle Slab (m)** & **Bottom Slab (m)** & **Side Wall (m)** & **References** \\ \hline Prototype & & & & & \\ underpass & 1 & 1.2 & 1.2 & 0.8 & reinforced concrete \\ \hline Model underpass & 0.01 & 0.015 & 0.015 & 0.009 & aluminum alloy \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Typical parameters for prototype underpass and model underpass.