## 2 Project Overview
### Selection and Design of Support Schemes  
A challenge concerning foundation pit design was that the foundation pit was deep and large. To reduce the impact of foundation pit excavation on the subway, separate excavation was required. The foundation pit was located on both sides of the subway \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Stratum Name**} & **Deformation** & **Tangent** & **Secant** & **Unloading** & **Internal** & \multirow{2}{*}{**Poisson**} & **Unit** \\  & **Modulus** & **Stiffness** & **Stiffness** & **Elastic Modulus** & **Friction** & **Ratio** & **Weight** \\  & **(kN/m\({}^{2}\))** & **(kN/m\({}^{2}\))** & **(kN/m\({}^{2}\))** & **(kN/m\({}^{2}\))** & **Angle** (\({}^{\circ}\)) & **Ratio** & **(g/cm\({}^{3}\))** \\ \hline Artificial fill & 4500 & 13,500 & 13,500 & 40,500 & 15 & 0.36 & 1.95 \\ Gravully Sand & 28,000 & 84,000 & 84,000 & 252,000 & 34 & 0.31 & 1.75 \\ Silt & 2000 & 6000 & 6000 & 18,000 & 4 & 0.33 & 1.65 \\ Silty Clay & 10,000 & 30,000 & 30,000 & 90,000 & 16 & 0.38 & 1.94 \\ Sandy Cohesive & & & & & & & \\ Soil & 16,000 & 48,000 & 48,000 & 144,000 & 22 & 0.33 & 1.82 \\ Completely & & & & & & & \\ Weathered Granite & 60,000 & 180,000 & 180,000 & 540,000 & 33 & 0.26 & 1.84 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Structural properties adopted in the numerical analysis. tunnel and was very sensitive to the surrounding environmental factors. Under sensitive environmental conditions, minimizing the deformation of the ultra-large and ultra-deep foundation pits is required in order to ensure the safety of urban facilities such as surrounding operating subways and important pipelines; this is a challenge that must be overcome in foundation pit design. For deep foundation pit engineering at this depth, commonly used support methods include row pile (wall) and internal support, row pile (wall) and anchor rod support, and double-row pile support. First, because the project involved both sides of the subway tunnel, the construction process (hole forming and grouting) would adversely impact the surrounding area, and the embedded depth of the anchor rod in the row of piles (walls) + anchor rod support would bring uncertainty to the project and affect the stability of the tunnel; thus, anchor rod support should not be used near the tunnel. Secondly, because there were buildings around the foundation pit project, the construction space was narrow, and it was challenging to use double-row piles to construct dense horizontal rows. In addition, due to the long north-south direction of the foundation pit, although the form of a pair of supporting beams can offset the force of the unilateral foundation pit wall, construction is difficult, and the long beams make the stiffness problem difficult to solve. Therefore, pile bracing was used in the foundation pit survey of this project, in addition to the T-shaped layout of slant support. In the new support system, the parameters of the row piles (such as pile spacing, pile diameter, etc.) can be calculated based on commonly used methods. However, for safety reasons, it is necessary to set the row piles of the entire support system as the same, although this design may tend to be conservative. In addition, the beam size of slant support needs to be calculated according to the soil pressure at the pile side, and the transmission of the axial force of the whole structure must be met. The plan of the T-shaped pile support combination support system is shown in Figure 1, and the typical section is shown in Figure 2. The support pile adopted a cast-in-place pile 1.5 m in diameter, with a pile length of approximately 3.0 m and a pile spacing of 1.8 m. The spacing of the slant support was 8 m, and the dimensions were 1 \(\times\) 1 m, all constructed using C30 concrete.