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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. [ABSTRACT FROM AUTHOR]

• A model of a large-diameter slurry shield crossing an existing tunnel is developed. • Shield crossing obliquely causes an offset of existing tunnel settlement trough. • The solution of semi-infinite saturated soil is applied to deformation prediction. • Safety indicators are provided as a preliminary evaluation tool for tunnel design. With the expansion of underground space networks, new tunnels especially large-diameter shields under-crossing existing tunnels have been commonly practiced. To obtain a better understanding of the effect of large-diameter slurry shield under-crossing on the existing tunnel and provide a quick and effective tool for evaluating the deformation behaviors of the existing tunnel before construction, in this study a simplified analytical method is proposed. Based on the two-stage method, and considering the characteristics of construction factors such as additional thrust on the shield excavation face of the slurry shield machine, friction force of the shield shell, and grouting pressure of the shield tail, deformation of the existing tunnel caused by the crossing of the large-diameter slurry shield in saturated soil is calculated. The proposed method is verified with field measurement data from a case study. A parametric analysis is conducted to study the influence of typical factors on tunnel responses, including longitudinal bending stiffness, elastic modulus, clearance between two tunnels, and shield crossing angle. The parametric study indicates a significant influence of the crossing angle on maximum settlement and the influence range of settlement of the existing tunnel. Recommendations for safe values of shield crossing clearance are given. The proposed analytical solution can be used as a reference to predict the potential risk of new large-diameter slurry shield crossing on the existing tunnel in engineering practices. [ABSTRACT FROM AUTHOR]