## 6 Stability Analysis  
### Finite Element Model Development  
Finite element modeling of the excavation was performed using PLAXIS 2D. Based on axisymmetric simplification principles, a two-dimensional finite element model was established along the centerline of the excavation pit with half of the actual structure. To mitigate boundary effects, the model dimensions were set at 250 m \(\times\) 150 m [43]. Key geometric parameters included the following: excavation depth \(H\) = 21.3 m, retaining wall embedment depth \(D\) = 33 m, and excavation radius \(B\) = 45 m. Boundary conditions were assigned as follows: horizontal displacements were restrained at both lateral boundaries, full fixation was applied at the bottom boundary, and the top boundary remained free. Due to symmetry, the left boundary was designated as impermeable while the right boundary was permeable; the bottom boundary was also impermeable. Soil strata were modeled using solid elements, while the retaining wall was simulated with plate elements.  
To simulate the interaction between the diaphragm wall and the soil, the parameter \(R_{\text{inter}}\) is used [44]. The parameter \(R_{\text{inter}}\) reflects the degree of interaction between the two. The default value of \(R_{\text{inter}}\) in the PLAXIS 2D, 0.67, was adopted [45].  
The soil body adopts the Mohr-Coulomb constitutive model. The Mohr-Coulomb constitutive model parameters for each soil layer were obtained based on the site geological investigation report and indoor geotechnical tests, including triaxial consolidated drained loading/unloading tests, triaxial consolidated drained shear tests, and standard consolidation tests. To simplify the model, selected geologically similar soil strata were merged into identical layers, as presented in Table 4.  
The reinforcement zone adopts the Mohr-Coulomb constitutive model. The equivalent elastic modulus of the reinforced composite, as well as the equivalent angle of internal friction and equivalent cohesion used as the Mohr-Coulomb constitutive model parameters for the reinforced composite, were calculated based on jet grouting column parameters and undisturbed soil parameters [46].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Soil Layer**} & **Thickness** & **Unit Weight** & **Cohesion** & **Friction** & **Elastic Modulus** \\  & & **(m)** & **(kN/m\({}^{3}\))** & **(kPa)** & **Angle (\({}^{\circ}\))** & **(MPa)** \\ \hline
1 & Silty Clay & 2.19 & 18.40 & 12.8 & 7.9 & 12.72 \\
2 & Silty Clay & 8.60 & 18.10 & 11.5 & 7.6 & 11.91 \\
3 & Silty Clay & 20.90 & 18.40 & 14.5 & 9.2 & 13.59 \\
4 & Silt & 3.90 & 19.70 & 3.3 & 29 & 37.07 \\  & Medium & 2.10 & 19.80 & 4.0 & 28.3 & 42.05 \\
5 & Sadium & 12.61 & 20.30 & 3.4 & 28.8 & 36.17 \\
6 & Sad & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Physical and mechanical parameters of soil layers in the FEM model.  
The model was discretized using triangular elements. The mesh parameter settings employed the 'Medium' mesh generation accuracy in the PLAXIS 2D (V2024), and local refinement was applied to the diaphragm wall and the reinforcement zone.  
To accurately simulate the mechanical behavior of the anchorage foundation in actual construction sequences, this analysis accounts for the construction of the diaphragm wall, cap beam, and deep ground improvement, along with staged excavation and dewatering of the foundation pit. Corresponding calculation phases were established as follows: (1) initial geostatic stress equilibrium; (2) construction of diaphragm wall, cap beam, and deep ground improvement; (3) excavation with concurrent construction of 1 st inner lining layer and dewatering to pit bottom; (4) excavation and construction of 2 nd inner lining layer; (5) excavation and construction of \(n\)-th inner lining layer (\(n\leq 8\)).