## 2 Project Overview  
Zhongyilu Station serves as a transfer hub for Lines 8, 12 and 21. It is strategically located at the intersection of Houhu Avenue and the crossroads of the Zhongyilu-Tazihu East Road in the Jiang'an District. A plan view of the foundation pit is illustrated in Figure 1. This foundation pit is constructed using the cut-and-cover method.  
The station features a three-storey underground frame structure, with an overall length of 519.2 m. The structural width and height of the standard section are 49.5 m and 25.29 m,  
Figure 1: Overall plan of Zhong Yi Station.  
respectively. The roof is covered with approximately 3.2 m of soil. The main structure dimensions of the station are as follows: the clear floor height of the first floor from top to bottom is 7.1 m, that of the second floor is 7.4 m, and that of the third floor is 7.7 m. Among these, the thickness of the top slab of the main structure is 0.9 m, the thickness of the floor slabs of the first and second basement floors is 0.4 m, the thickness of the floor is 1.3 m, and the thickness of the side walls is 0.85 m. The primary construction method employed for the station is the top-down method, which integrates support structures and the main structures effectively. The diaphragm wall has a thickness of 1.5 m, while the upper retaining structure is 0.2 m thick. The lattice columns and bored piles had diameters of 900 mm and 2300 mm, respectively, and both the retaining and main structures were constructed using C35 concrete. The key advantages/disadvantages of excavation methods (top-down, cut-and-cover) are shown in Table 1.  
The strata within the excavation depth of the foundation pit are mainly composed of silty clay, silt interbedded with silt sand, and gravel mixed with cobblestones. In contrast, the overall undulation of the top of the bedrock layer is relatively small. The average groundwater level is 2.5 m, and it fluctuates between 1 m and 4 m. The distribution of the soil layers is depicted in Figure 2. The average groundwater level is 2.5 m, and it fluctuates between 1 m and 4 m.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Method** & **Advantages** & **Disadvantages** \\ \hline \multirow{8}{*}{Top-Down Method} & 1. Construction is simple: fewer processes and mature technology. 2. Short construction period: high degree of mechanization. 3. Low cost: The total cost is approximately 60–70\% of that of the cut-and-cover method. 4. High space utilization: suitable for large-span foundation pits. & 1. Traffic disruption: roads need to be closed, affecting urban traffic. 2. Significant environmental impact: loud noise and dust. 3. Weather constraint: heavy rain may cause the foundation pit to be flooded. \\ \hline \multirow{8}{*}{Cut-and-Cover Method} & 1. Minimize traffic impact: construct the roof first; then restore road traffic. 2. Good structural stability: utilizing permanent structures as supports and reducing the need for reinforcement. 3. Sediment control excellent: minimal disturbance, deformation of surrounding buildings \textless{}10\text{mm}\). 4. Applicable to deep foundation pits: when the depth is greater than 20 m, the safety is high. & 1. Long construction period: complex process (piles \(\rightarrow\) roof slab \(\rightarrow\) reverse construction), 30–40\% slower than open excavation. 2. High cost: the construction cost is approximately 1.3 to 1.5 times that of the open-cut method. 3. The construction is very challenging: the precision of node connections is extremely high (for example, the verticality of the steel pipe columns should be \(\leq\)1/500). \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summarizing the key advantages/disadvantages of excavation methods (top-down, cut-and-cover).