Keywords

1 Introduction

The excavating of tunnel close to existing tunnel unavoidably impacts the nearby soil, causing the deformation and additional force of existing tunnel structures, which can pose hazards to the safety of the tunnel during normal operations. Common occurrences include uneven longitudinal settlement, structural cracks and damage, different degrees of opening in segment joints, as well as seepage and leakage [1,2,3]. Due to the complexity of the tunnel project, it is crucial to precisely ascertain the impact of shield tunnel construction on the existing tunnel structure, and appropriate and effective measures should be adopted to avoid structural damage.

Researchers have conducted studies on the influence of adjacent construction on existing tunnel structures using methods such as field monitoring, physical model testing, finite element method. For field monitoring, Shi et al. [4] based analysis on the monitoring data of the uplift displacement of the subway tunnel below a certain foundation pit excavation in Shanghai. The monitoring data showed that the existing tunnel in the pit bottom covering area showed a significant uplift, while the tunnel part not covered by the foundation pit settled; Liu et al. [5] conducted a comprehensive field monitoring of the deformation response of deep foundation pits and adjacent tunnels, including lateral displacement of underground continuous walls, tunnel crown settlement, elastic horizontal displacement, segment convergence, and opening width of segments. Based on the data, they analyzed the process of tunnel deformation; Li and Yuan [6] used a measurement system to implement displacement monitoring and analysis of the entire process of shield tunneling under existing tunnels; Zhang [7] analyzed the field monitoring data of Shanghai Metro Line 11, crossing Line 4 from above and below. By conducting the numerical simulation on the project, scientifically set shield parameters; Jin et al. [8] reported and analyzed the monitoring data of the construction of a shield tunnel under an existing tunnel in Shenzhen and compared the deformation of the existing tunnel and the ground caused by the construction of the shield tunnel underpass.

In terms of physical model testing, Huang et al. [9] used a centrifuge experiment to obtain the response the foundation pit project and proposed the concept of safe distance between the foundation pit project and the existing tunnel below; Kim et al. [10] conducted a scale model experiment to study the influence of tunnel gap, angle, and lining segment stiffness on the interaction during the process of shield tunneling under existing tunnels in clay; Ng et al. [11] combined the working conditions of shield tunneling under existing tunnels, used centrifuge experiments to analyze the changes in the impact of soil loss on existing tunnels in sandy soil layers.

About finite element method, Chakeri et al. [12] used a finite element software to simulate the construction process of the Torside Tunnel near Line 4 using the New Austrian Tunneling Method. The results showed that the settlement rate of Metro Line 4 was directly related to the tunnel spacing. At the same time, the simulation software calculated the stress and strain curves of Metro Line 4 during the underpass process; Lin et al. [13] took a double-line shield tunnel in Changsha obliquely crossing an existing tunnel as a support and analyzed the deformation response of the double-line tunnel obliquely crossing the existing tunnel through numerical simulation; Lai et al. [14] took a special case of a tunnel close to and approximately parallel to an existing tunnel as a research object and studied the deformation characteristics based on the tunnel based on monitoring data and FDM numerical simulation.

In this paper, field monitoring and analysis are conducted on the convergence deformation of the right tunnel induced by the excavation of the left tunnel, aiming to explore the patterns of influence on the deformation of the existing tunnel caused by the construction of the adjacent tunnel.

2 Engineering Background

2.1 Engineering Project

The newly excavated shield tunnel of metro line 2 in a specific city, was constructed using an earth pressure balance (EPB) shield machine. The tunnel was launched at Jinghu Station and completed at Houshu Road Station, traversing three passages (2 pedestrian lanes and 1 vehicular lane). Total length of tunnel is 1946 m. The MJS (Metro Jet System) method is employed to enhance the stability of the tunnel, reinforcing the shield tunnel structure both above and below the shield tunnel. The spacing distance between the axis of lines measures 14.2 m. The outer and inner diameter of shield is 6.7 m and 5.9 m. The segment has a thickness of 0.4 m, a ring width of 1.2 m, and is constructed using C50 concrete. As depicted in Fig. 1, the tunnel is primarily buried in silty clay from 20 m to 26.7 m.

Fig. 1.
figure 1

Cross section of tunnels

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The parameters of the soil are shown in Table 1.

Table 1. Soil parameters
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2.2 Field Monitoring

To precisely investigate the impact of the left shield tunneling on the structure of the right tunnel, laser range finders are installed in right tunnel to monitor the convergence deformation of tunnel structures. During tunneling of left line, the convergence deformation of the right tunnel is monitored every ten minutes. The data acquisition and transmission system comprise a data acquisition instrument, an acquisition instrument and communication cables. The convergence monitoring layout are shown in Fig. 2.

Fig. 2.
figure 2

Convergence monitoring section layout

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Convergence monitoring sections are established at rings 372, 396, 438, and 446. Laser range finders are installed along the tunnel’s waist at each monitoring section, dedicated to monitoring vertical and horizontal convergence. The installation position of monitoring instruments is shown in Fig. 3.

Fig. 3.
figure 3

Monitoring section layout design

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3 Measurement Results

The monitoring sections for lateral convergence of tunnel segments during the right tunnel shield machine’s passage through the area with pile foundation effects are rings 372, 396, 438, and 446. Convergence monitoring for ring 372 began on January 1, 2022, while monitoring for rings 396, 438 and 446 started on January 19, 2022.

From Fig. 4, for ring 372 and ring 438, there is horizontal expansion and vertical convergence, representing a “horizontal duck egg” convergence deformation pattern. Both horizontal and vertical convergence occur in ring 396 and ring 446, indicating a non-uniform convergence deformation pattern. Horizontal and vertical convergence at monitoring sections are shown in Table 2.

Fig. 4.
figure 4

Horizontal convergence patterns at monitoring sections: (a) R372, (b) R396, (c) R438, (d) R446

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Table 2. Final convergence at monitoring sections
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4 Result Analysis

In this project, the axial spacing between the left and right tunnel axes is 14.2 m, with the minimum distance between the two tunnels (distance from the boundary of the left tunnel to the boundary of the right tunnel) being 7.5 m. The right tunnel is excavated before the left tunnel. To investigate the impact of the new tunnel excavating on the existing tunnel structure, this section further analyzes the horizontal convergence when the left tunnel shield machine approaches the monitored tunnel segments.

The horizontal and vertical convergence for each monitoring section is shown in Fig. 5, where positive and negative values represent the expansion and convergence of segment, respectively. Figure 5 reveals that the convergence (expansion) of each monitoring section gradually develops and stabilizes. Before and after shield machine passing the position of monitoring section, due to the change of surrounding soil pressure, the deformation of tunnel gradually developed; when shield machine passing, the deformation suddenly changed.

Fig. 5.
figure 5

Horizontal convergence at monitoring sections

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During the excavation of the left tunnel while the right tunnel is already in place, it can be observed that as the left tunnel shield machine approaches the monitoring sections stage, there is minimal change in horizontal convergence of the right tunnel segments. However, after the tunnel shield machine passes through each monitoring section, the horizontal convergence of the right tunnel segments gradually increases, while the vertical convergence decreases. After the convergence changes stabilize, the vertical convergence for ring 372 is −3.8 mm, and the horizontal convergence is 3.4 mm. For ring 396, the vertical convergence is −5.3 mm, and the horizontal convergence is −4.1 mm. For ring 446, the vertical convergence is −2.1 mm, and the horizontal convergence is −7.6 mm.

Due to lateral compression, right tunnel segments experience horizontal convergence and vertical expansion. This is the result of the horizontal compression induced by the shield tail synchronous grouting pressure as it passes through each monitoring section of the left tunnel. This pressure causes the surrounding soil to expand outward, transmitting deformation to the right tunnel. Simultaneously, the lateral deformation of the tunnel segments in the left tunnel also applies a compressive force on the surrounding soil, finally causing the convergence of the right tunnel segments.

5 Conclusions

This paper analyses the convergence deformation of the right tunnel induced by the left shield tunneling. Based on the analysis of field monitoring result, the mainly conclusions drawn are as follows:

  1. (1)

    The convergence (expansion) of each monitoring section gradually develops and stabilizes over time. For the upper part with the influence of pile foundations, the convergence of tunnel segments exhibits “horizontal duck egg” pattern and non-uniform convergence pattern. When reaching a stable state, the maximum horizontal convergences for rings 372, 396, 438, and 446 are 6.6 mm, −4.0 mm, 1.9 mm, and −1.5 mm; The maximum vertical convergences are −7.4 mm, −6.7 mm, −9.8 mm, and −1.3 mm, respectively.

  2. (2)

    When the shield machine reaches and passes, there is no apparent change of horizontal convergence in the existing tunnel. However, after the tunnel shield machine passes, due to the horizontal compression caused by synchronous grouting pressure and lateral deformation of the left shield tunnel segment, surrounding soil experiences lateral compression deformation. This causes the horizontal convergence of the right tunnel to have an increase in horizontal convergence and a decrease in vertical convergence.