## 3 Finite Element Analysis and Field Monitoring
### Model Parameters  
The soil thickness of the layer has been adjusted according to the needs of the site as well as the actual numerical simulation. The soil properties of 5N-1 and 5N-2 are similar, so to simplify the soil layer and make it easier to set up the model, the two soil layers are merged. The mechanical parameters of the soil layer obtained by the survey unit in the field test and indoor geotechnical test are shown in Table 1.  
In the construction of the pilot tunnel, the lining is reinforced concrete consisting of concrete and steel grids to resist the stresses released by the excavation, maintain the stability of the excavation surface and prevent the tunnel from collapsing. In the finite element model, the equivalent stiffness method [41] was usually used to treat the steel grid as equivalent, and the steel grid was converted to the elastic modulus of the concrete. The elastic of elasticity of the lining after conversion:  
\[\mathrm{E}=\mathrm{E}_{0}+\frac{\mathrm{A}_{\mathrm{g}}\cdot\mathrm{E}_{ \mathrm{g}}}{\mathrm{A}_{\mathrm{c}}} \tag{5}\]  
where, \(\mathrm{E}\): elastic modulus of concrete after conversion; \(\mathrm{E}_{0}\): original elastic modulus of concrete; \(\mathrm{E}_{\mathrm{g}}\): elastic modulus of steel grid; \(\mathrm{A}_{\mathrm{g}}\): cross-sectional area of the steel grid; \(\mathrm{A}_{\mathrm{c}}\): cross-sectional area of concrete.  
The pile and the surrounding soil will have a tangential slip tendency and a normal separation tendency. Therefore, the role of soil-structure interaction in finite element numerical simulation cannot be ignored in this study. In the GTS NX software(2021R1,MIDAS Information Technology Co., Ltd., Beijing, China), the pile element is composed of a beam element and a contact element, and the contact element represents the pile-soil contact  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**StructureName of Soil** & **Miscellaneous** & **Residual** & **Castic** & **Fine** & **Conglomerate** & **C25** & **C35** & **C50 Steel** \\  & **Fill** & **Soil** & **Rock** & **Sandstone** & **Rock** & **Lining** & **Beam** & **Pipe Piles** \\ \hline Thickness H (m) & 3 & 13 & 7 & 9 & 38 & / & / & / \\ Poisson’s ratio & 0.4 & 0.35 & 0.3 & 0.3 & 0.25 & 0.25 & 0.2 & 0.2 \\ Volumetric weight (\(\mathrm{N}\mathrm{/}\mathrm{m}^{3}\)) & 19.5 & 20 & 20.5 & 25 & 25 & 25 & 25 \\ Cohesion (kpa) & 13 & 23 & 26 & 45 & / & / & / & / \\ Internal friction angle (\(\beta\)) & 14 & 25 & 28 & 30 & / & / & / & / \\ \(\mathrm{E}_{\mathrm{sol}}^{\mathrm{ref}}\) (Mpa) & 4.536 & 5.184 & 8.856 & 12.96 & / & / & / & / \\ \(\mathrm{E}_{\mathrm{sol}}^{\mathrm{ref}}\) (Mpa) & 3.78 & 4.32 & 7.38 & 10.8 & / & / & / & / \\ \(\mathrm{E}_{\mathrm{tor}}^{\mathrm{ref}}\) (Mpa) & 26.46 & 30.24 & 51.66 & 75.6 & / & / & / & / \\ E (Mpa) & / & / & / & / & 8000 & 30,000 & 32,000 & 35,000 \\ \hline \hline \end{tabular} where: \(\mathrm{E}_{\mathrm{sol}}^{\mathrm{ref}}\), secant elastic modulus in shear hardening; \(\mathrm{E}_{\mathrm{sol}}^{\mathrm{ref}}\), secant elastic modulus in shear hardening; \(\mathrm{E}_{\mathrm{tor}}^{\mathrm{ref}}\), elastic modulus at unloading; E, elastic modulus.  
\end{table}
Table 1: Physical and mechanical parameters.  
relationship. The contact element consists of a tangential spring and a normal spring. The values of normal modulus Kn and tangential modulus \(\mathrm{Kt}\) can be obtained by two-dimensional plane strain in the GTS NX software. Ks is the spring factor of the foundation, which can be derived from field tests. The schematic diagram of the pile interface contact element is shown in Figure 5.