Analysis of the Impact of Shield Tunneling Under Existing Subway Station

To ensure the safety of existing subway station structures and the normal operation of the metro, this study focuses on a specific engineering case in Suzhou’s metro, where shield tunnels were initiated at a close distance to pass beneath an existing subway station. Numerical simulation method was employed to analysis the impact of shield tunneling parameters on the deformation of the existing station structure under conditions with and without pre-reinforcement boundaries. Additionally, the study involved a comparison with monitoring data to validate the reliability of the numerical simulation approach. The results indicate that: 1) Controlling the shield tunneling parameters, such as setting appropriate shield tunneling face support force and tail grouting pressure, is beneficial in managing the deformation of the existing station; 2) The elastic modulus of an equivalent grouting layer has the greatest impact on settlement. Choosing the appropriate grouting material during construction is a key approach to control settlement. 3) Strengthening the soil strength in the reinforcement area and increasing the thickness of the track slab have a minimal impact on the settlement of the track slab at the existing station. However, the former is beneficial for the stability of the shield excavation face. The latter enhances the existing structure’s resistance to deformation.


Introduction
With the rapid expansion of urban underground rail transit systems, the construction of new tunnels passing beneath existing subway stations has become a common yet complex challenge.Ensuring minimal settlement and deformation of operational subway stations during shield tunneling is a critical technical issue.While significant advancements have been made in construction techniques and control measures, research in soft soil areas, particularly when shield tunneling originates from beneath existing stations, is still evolving.Recent studies have shed light on various aspects of this challenge.Hongbo and Gelin (2020) explored key technologies in shield tunnel construction in complex geological areas [1].Tang (2017) emphasized the importance of ensuring the safety of existing lines during close-spaced construction and provided risk analysis for shield machine initiation [2,3].Huo et al. (2020) and Yuan et al. (2022) conducted numerical simulation analyses to compare land subsidence trends with field monitoring and to understand the factors influencing surface settlement during shield construction [1,4].2021) investigated the influence of shield tunneling and train load on existing structures and the impact of excavation parameters on shield tunneling [6,7].Zhang et al. (2023) focused on deformation control of subway tunnels undercrossing airport facilities [8].Liu and Lu (2017) highlighted the fire safety design features of subway tunnels [9].Further contributions include research on the mutual influence between long-span highway tunnels and urban rail transit tunnels [10], the analysis of special-shaped shield construction closely crossing multiple operational metro tunnels [11], and the study of construction technology using the freezing method for connecting passages in subways [12].Zhang et al. (2019) provided insights into the maximum settlement and convergence values during construction, ensuring they meet control requirements [13].These studies collectively enhance our understanding of the complexities involved in shield tunneling under existing subway stations.They offer valuable insights into the deformation mechanisms, risk management, and innovative construction techniques, contributing significantly to the field of urban rail transit engineering.

Project Overview
The existing Suzhou Rail Transit Line 2 station is an underground two-level island-style station, which is already operational.The structural form is a single-column, two-span cross-section, extending in an east-west direction.The newly constructed Rail Transit Line 7 station is an underground three-level station, adopting a double-column, three-span cross-section, extending in a north-south direction.It intersects the existing subway station at an approximate angle of 88 degrees.The new station is located on both the north and south sides of the existing station.The new line uses shield tunneling method to pass beneath the operational station.The layout of the new shield tunnel and the existing operational station is shown in the figure 1. Fourteen automated settlement monitoring points have been installed on the track slab of the existing station to simultaneously monitor the settlement of the track slab in real-time as the shield tunnel passes beneath the station, ensuring the safety of the operating trains.

Figure 1. The layout of the new shield tunnel and the existing operational station
The shield tunnel passes beneath the operational station at an oblique angle of about 88 degrees, with a crossing range length of approximately 20 meters.It uses a single circular shield tunnel, with an inner diameter of the tunnel lining of 5.9 meters and an outer diameter of 6.6 meters.The structure of the shield tunnel is constructed using prefabricated C50 reinforced concrete segments, with a segment thickness of 350mm and a width of 1200mm.The segments are connected using bent bolts and are assembled with staggered joints.The spacing between the lines is 16 meters.The minimum vertical clearance between the shield tunnel and the bottom plate of the existing station is 3.29 meters.The newly constructed shield tunnel and the existing operational station are shown in the figure 2: The proposed construction site of this project belongs to the flat and water-networked plain area of Taihu Lake.The shallow fill soil has a thickness generally ranging from 1.60m to 4.90m.Below this is the layer ③ 1, consisting of grayish-yellow, firm to hard plastic clay, which is relatively good in quality.The lower part is mainly composed of gray soil, primarily consisting of soft plastic to plastic clay, slightly dense to dense silty soil, and silty sand layers.The site features a variety of soil types with significant stratigraphic variations.The shield tunneling section mainly passes through ⑤1 silty clay.The existing station's bottom plate is located in the ④1 layer of silty sand interbedded with silt soil.Three meters below the station's bottom plate, triaxial mixing piles were used for reinforcement during its construction.

Geometric properties
To reduce the influence of boundary effects, the computational range of the model is set at 3 to 5 times the tunnel depth from the left and right boundaries.The three-dimensional numerical model used in this simulation has dimensions of 245.8m × 79.2m × 65m.The construction direction along the newly built Line 7 station is 79.2m, and the computational depth is 65m.The effect of groundwater is not considered.Both horizontal and vertical boundaries are constrained with displacement boundaries.The model grid is divided using a hybrid mesh, as shown below.

Material Parameters and Construction Process
Considering the randomness and complexity of the physical and mechanical properties of geotechnical materials, it is very difficult to fully simulate the mechanical properties of these materials and to conduct numerical simulations strictly according to actual construction steps.In the modeling and calculation process, some conditions are optimized and assumed: (1) The soil layers adopt the modified Mohr-Coulomb constitutive model; the parameters of the soil is listed in table 1.The surrounding soil material is considered a homogeneous, isotropic continuous medium, with initial The shield excavation step length is simulated using a 1.2 m wide segment ring, and the segment lining is modeled using solid elements of linear elastic material, ignoring the lateral and longitudinal connections between the segment linings.(4) The simulation of shield tail grouting, which fills the gap between the segment and the soil, utilizes synchronous and secondary grouting methods.In simulating the grouting process, this paper adopts an equivalent homogeneous lining grouting thickness layer at the same center position as the segment to replace it.Furthermore, this material is considered as an elastic material in the simulation, with a Young's modulus of 100 MPa The analysis process of simulating shield tunneling under an existing operational subway station using finite element software mainly includes: establishing the overall model of the station structure under shield tunneling, loading and solving, and reviewing the results of the numerical simulation analysis of shield tunneling passing through the station.The numerical simulation process is based on the actual situation on site and simplified as follows: during construction, the left line tunnel is excavated first, and after advancing 10 rings, the right line tunnel is excavated from the opposite direction.The newly built left and right line tunnels successively pass under the existing station structure.During the shield tunneling process, the earth pressure is set to 0.14 MPa.Numerical calculations are performed to analyze in detail the deformation during different stages, including when the newly built tunnels of both lines are excavated beneath the existing station structure, leave the existing station structure, and when the double-line tunnels are connected.

Result Analysis
Figure 4 illustrates a comparison between different Young's modulus of the equivalent grouting layer and the actual measured results.The measured results show that after the tunnel excavation is completed, the maximum settlement of the track slab at the existing station is 2.5mm, and the settlement trough is close to a 'V' shape.The results of the numerical simulation indicate that the elastic modulus of the equivalent layer greatly influences the calculated results.When the elastic modulus is set to 100 MPa, the maximum settlement is approximately 2.5mm, close to the measured results.When the elastic modulus is set to 50 MPa, the maximum settlement is about 3.2mm, and when it is set to 20 MPa, the maximum settlement is about 4.8mm.The settlement troughs in the numerical simulation results are all in a 'W' shape.The common thickness of the bottom plate in subway stations is 0.9m and 1m.As can be inferred from Figure 5, increasing the thickness of the base slab has a minimal impact on deformation.When the elastic modulus of the reinforcement body is less than or equal to 200 MPa, the settlement slightly increases with the increase in the elastic modulus.However, when the elastic modulus increases to 2000 MPa, the settlement significantly decreases, and the settlement trough becomes 'U' shaped.It should be noted that the elastic modulus of the reinforcement body formed by mixing piles is usually closer to 200 MPa, and it is difficult to achieve a value of 2000 MPa.Continuously increasing the modulus of the reinforcement body to reduce settlement is not cost-effective.However, this is helpful for the stability of the excavation face.

Conclusion
The study presented in this paper provides significant insights into the impact of various factors on the deformation control of existing stations during shield tunneling operations.The key findings can be summarized as follows: Luo et al. (2021) used ROCSCIENCE software and field measurement to study the deformation law of subway construction under passing existing lines at a short distance [5].Daxin et al. (no date) and Liu et al. (

Figure 2 .
Figure 2. The station and tunnel cross-section diagram

Figure 3 .
Figure 3. Numerical Model of Station and Tunnel Geostresses in each stratum existing only due to self-weight, without considering tectonic stress.(3)The shield excavation step length is simulated using a 1.2 m wide segment ring, and the segment lining is modeled using solid elements of linear elastic material, ignoring the lateral and longitudinal connections between the segment linings.(4) The simulation of shield tail grouting, which fills the gap between the segment and the soil, utilizes synchronous and secondary grouting methods.In simulating the grouting process, this paper adopts an equivalent homogeneous lining grouting thickness layer at the same center position as the segment to replace it.Furthermore, this material is considered as an elastic material in the simulation, with a Young's modulus of 100 MPa

Figure 4 .
Figure 4. Settlement of different different Young's modulus of the equivalent grouting layer vs measured results

Figure 5 .
Figure 5. Settlement of different different modulus of the reinforcement area

Table 1 .
Geotechnical properties of soil layers