Keywords: deep foundation pit; sequential unloading; subway tunnel; operational safety

0 Introduction
With the continuous economic development of Shanghai, there is a huge demand for the development and utilization of areas around operating subway tunnels. Compared with general construction environments, areas around tunnels are close to the subway, so the safety of subway operations must be considered during development, especially for large deep foundation pit construction. How to ensure that tunnel deformation meets operational requirements during soil unloading is the biggest challenge in construction.
1 Project Overview
1.1 Project Introduction
A large deep foundation pit project is located in the center of Shanghai. The pit is about 14 m deep, 190 m long, and 60 m wide. Because an operating subway tunnel is located directly below the project, the pit is divided into two small pits, north and south. The horizontal distance from each pit to the subway is about 8 m, and the pits are connected by an 8 m wide vehicle passage (Figure 1). In addition, another tunnel is adjacent to the south side of the pit, with a minimum spacing of about 7 m.
Due to the severe surrounding environmental conditions, to meet the normal operation requirements of the two tunnels, the overall plan of "zoned sequential construction" is adopted: the north pit is divided into S1-A and S1-B zones implemented successively, and the south pit is divided into S2-A and S2-B zones implemented successively, as shown in Figure 2.
1.2 Calculation and Evaluation Criteria According to the "Interim Provisions of Shanghai Municipality on Technical Management of Building Construction along Subway Lines to Protect Subways", due to the comprehensive impact of various loads and loading construction activities such as deep foundation pits, high-rise building pile foundations, dewatering, and surcharging on subway engineering facilities, the absolute settlement of subway structures and facilities must be ≤20 mm (including the final displacement from various loading and unloading). The curvature radius of the tunnel deformation curve must be R≥15,000 m.
2 Numerical Model of Foundation Pit Excavation
2.1 Establishment of Finite Element Model
The general finite element analysis software MARC was used to simulate the entire excavation process of the foundation pit.
2.1.1 Finite Element Material Model
8-node solid elements were used to simulate the soil, shell elements to simulate the diaphragm wall and existing tunnels, and beam elements to simulate the struts. In the numerical analysis, the Mohr-Coulomb constitutive model was used to simulate the site soil, and the soil parameters were taken according to the engineering geological investigation report.
2.1.2 Finite Element Geometric Model
Based on finite element simulation experience, the influence width of general foundation pit excavation is about 3 to 5 times the excavation depth, and the influence depth is about 2 to 4 times. Therefore, the influence range was determined to extend 100 m outward from the pit edge and 70 m in the soil depth direction.
2.1.3 Finite Element Boundary Conditions
This model adopts standard boundary conditions: the bottom nodes of the calculation model are constrained to zero horizontal and vertical displacement, the surrounding sides are constrained to zero displacement in the horizontal plane (x and z directions), and the top surface is a free surface.
2.2 Calculation Cases
Comparative calculations were performed for the impact of foundation pit construction on the existing tunnel structure under diaphragm wall embedment depths of 51 m and 40 m.
According to the actual excavation steps, the simulation was carried out in four steps: calculation of the initial geostress field, layered excavation of the S1-A and S2-A zones, layered excavation of the S1-B and S2-B zones, and excavation of the connecting passage between S1-B and S2-B. Figure 3 shows the initial geostress field.
3 Analysis of Calculation Results
3.1 Scheme 1
3.1.1 Analysis of Overall Deformation of the Foundation Pit
With Scheme 1, after eliminating the influence of initial displacement (1.8 cm), the maximum settlement around the pit was 1.5 cm, and the maximum heave at the pit bottom was 7.2 cm (Figure 4).
3.1.2 Analysis of Tunnel Deformation Results
Compared with the tunnel on the south side of the pit, the tunnel below the pit is affected by both pits and is also closer to the pit. Due to space limitations, only the deformation of the tunnel below the pit is analyzed here.
3.2 Scheme 2
3.2.1 Analysis of Overall Deformation of the Foundation Pit
With Scheme 2, after eliminating the influence of initial displacement (1.1 cm), the maximum settlement around the pit was 2.3 cm, and the maximum heave at the pit bottom was 8.9 cm (Figure 5).
3.2.2 Analysis of Tunnel Deformation
With Scheme 2, the heave of the tunnel below the pit is larger than that in the original scheme, with a maximum heave of about 2 cm.
4 Conclusion
According to the "Interim Provisions of Shanghai Municipality on Technical Management of Building Construction along Subway Lines to Protect Subways", for the original design with a diaphragm wall embedment depth of 51 m (3 times the pit depth), the impact of foundation pit excavation on the existing subway running tunnel and station is within the control standards. If changed to a diaphragm wall embedment depth of 40 m (2.4 times the pit depth), the tunnel deformation just meets the control standard requirements (Table 1). The numerical simulation results show that if structures such as tunnels are within 10 m of the foundation pit, to meet their disturbance requirements, a diaphragm wall design length of 2.5 times the excavation depth is appropriate.