A section of closely spaced twin tunnels at Shangmeilin station on Shenzhen metro line 9 was excavated by the shield tunneling method. Part of this section lies closely beneath the tunnel servicing Metro line 4. Due to restrictions on ground surface to control the extra settlement of the existing tunnel in the area, an in-tunnel grouting protection method was adopted in combination with the shield method. This method greatly reduced the effect on the existing tunnels when building the twin tunnels underneath. The settlement of and stress on the existing tunnels associated with the construction of the new tunnels were systematically monitored. Because of the twice under-crossing process, the excavation of twin tunnels had a superposed influence on the existing tunnel and surrounding soil. The behavior of the overlying tunnel was analyzed based on instrumentation records obtained from the project. The settlement profile of the existing structure displayed a “V” shape after the first under-crossing but a “W” shape after the second. The hoop stress of the existing tunnel induced by shield tunneling below had beneficial effects on the stress state of the tunnel structure. By contrast, the longitudinal stress exerts a bending moment on the existing tunnel, with a large tensile stress that had adverse effects on its structure. Based on the monitored data, three deformation modes are proposed to describe the behavior of the existing tunnel. In addition, the differences between the two under-crossing processes are discussed; both the ground loss ratio and width parameter of the settlement trough in the second profile were larger than those of the first profile due to the decrease in soil stability.
When tunneling in densely populated urban areas, new tunnels are inevitably constructed in close proximity to existing underground structures. In these cases, an earth pressure balanced (EPB) shield machine is widely used. During the mechanized tunneling process, the influence of shield tunneling on the surrounding soil or existing structures is not negligible and is closely related to the soil properties, working parameters of the shield machine, and the depth of the overburden soil. In particular, building twin closely spaced tunnels under an existing tunnel can lead to greater tunnel deformation and ground settlement due to the dual disturbances caused by tunnel excavation. In these cases, it is highly difficult to control the deformation of the existing tunnel, which might not only lead to excessive deformations of the existing nearby tunnel but also pose a serious threat to tunnel operation safety. Protective measures must be considered and analyzed in the design phase to ensure safe construction as well as the operation of the existing tunnel.
The construction of a tunnel, which depends greatly on the ground condition, must be carefully designed in terms of risks and uncertainties to ensure the final objective of quality (e.g., Oggeri and Ova, 2004; Wood, 2002). A substantial number of studies have investigated ground movements as well as the deformation of existing tunnels induced by the construction of a new tunnel (e.g., Attewell and Woodman, 1982; Cooper et al., 2002; Fang et al., 2015; Li and Yuan, 2012; Peck, 1969; Schmidt, 1969; Yamaguchi et al., 1998). The implementation of appropriate supplementary countermeasures to reduce the deformation of ground or subsurface structures nearby is crucial. Grout technology is widely used to condition the soil and protect existing structures in EPB shield tunneling. Garshol (2003) indicated that pre-excavation grouting or pre-grouting is conducive to the improvement of ground stability and ground water control in rock. The operational process, supporting devices and main technical characteristics of grouting were presented. Kovári and Ramoni (2004) summarized the construction experiences of urban tunneling in soft ground and concluded that the design procedure demands high reliability, including statistical calculations for the determination of the necessary support pressure or shape, size and quality of the grouted body. Li et al. (2013) presented a case of an in-tunnel jacking above the tunnel protection methodology for excavating a tunnel under a tunnel in service. Kimpritis (2014) explained how jetgrouting can be used as an integral part of complex tunneling projects and summarized the basic framework for the design and execution of jet-grouting in tunneling. Ye et al. (2015) analyzed the mechanism of an unexpected ground surface settlement of approximately 0.8 m in a Double-O-Tube (DOT) tunnel construction site in Shanghai; the ground settlement was reduced to approximately 0.02 m after implementing improved grouting measures.
According to the literature, there is very limited knowledge on the influence of shield tunneling on overlying existing tunnels and the measures used to reduce the overlying tunnel deformation induced by excavation of new twin tunnels below. Most conventional countermeasures cannot be directly adopted because of the confined space, limited headroom, and the required level of safety and efficiency. This paper describes the implementation of an effective protection method of grouting in existing tunnels for excavating under-passing twin tunnels to control the settlement of an overlying tunnel in China metro construction. This method controls the tunnel deformation and thus decreases the additional stress placed on the existing tunnel. The method aims to compensate for the ground loss induced by tunneling below and reinforce the surrounding soil before construction of new twin tunnels. The displacement of the existing tunnel was controlled within an allowable value using this method. In addition, both the cross-sectional and longitudinal changes in the existing tunnel, including deformation and stress as well as the interaction of the twin tunnels, are investigated in this paper. The deformation characteristics of the existing tunnel in the first and second under-passing are compared based on the monitoring data.
2. Project overview
The plan view and cross-sectional view of the existing and new tunnels at the Shangmeilin station in Shenzhen are shown in Figs. 1 and 2, respectively. The existing tunnels in service run east–west and belong to line 4 of the Shenzhen metro. The tunnels are horizontally parallel and were excavated by the shield method four years ago. The distance between the right tunnel and left tunnel is 7.2 m. The outer and the inner radii of the precast segmental lining are 3.0 m and 2.7 m, respectively. The width of each segment is 1.5 m. The depth of the existing tunnels is approximately 12 m.
The new shield twin tunnels were completed in 2015 as part of the Shenzhen metro line 9. The clearance between the right tunnel and left tunnel is 8.0 m. The clear distances from the new tunnels to the existing tunnels is approximately 2.5 m, and there is a skew of 83 °between the new tunnels and the existing tunnels. The thickness of the segment is 300 mm, and the width is 1.5 m. The segmental lining consists of five segments with a key segment (Fig. 3). The ground surface is at the intersection of two crowded roads, Meilin Road and Zhongkang Road. The first under-crossing of the right tunnel of Line 9 began on the night of 14 November and ended on the afternoon of 18 November. The second under-crossing of the left tunnel began approximately one month later, on the night of 12 December, and ended on the night of 16 December. One Herrenknecht AG shield machine and one Wirth shield machine with excavation diameters of 6280 mm and lengths of approximately 7.9 m and 13.0 m, respectively, were used to build the new tunnels. Before driving, the shield machines were thoroughly inspected and maintained, and all cutting tools were replaced to avoid breakdown of the shield machine below the existing tunnel.
As shown in Fig. 2, the subsurface ground at the site is very complex and consists of back fill, silt clay, sandy soil, gravelly soil and completely weathered granite. The mean values of the physical and mechanical parameters of the soils retrieved from the site investigation report are shown in Table 1.
3. Protection schemes in the design phase
The existing tunnel is just below a busy street, as shown in Fig. 1. It was difficult to adopt reinforcement measures to ensure the safety of the existing tunnel when constructing the twin tunnels. In this case, an in-tunnel grouting protection method was adopted to control the deformation of the existing tunnels. This scheme aimed to compensate for the ground losses caused by the excavation of the new tunnel below. As shown in Fig. 4, grouting pipes with a length of 2.0 m were symmetrically installed in the grout holes in the lining segment of the existing tunnel at an interval of 3.0 m. As the excavation of the new twin tunnels of line 9 would cause twice as many disturbances of the surrounding soil, the corresponding angles of influences, as shown in Fig. 3, may be generally expressed as β = °+ 45 / φ 2, where φ denotes the intrinsic soil friction angle (see Fig. 5).