Abstract
As the soil-resistance coefficient in a soft soil area is small, overlarge bending moment may cause exceeding transverse deformation for the shield tunnel and cause structural diseases and waterproof failure at the longitudinal segment joints. Hence, a new idea of cross-section design for a minimum bending moment shield tunnel was proposed. This article has first put forward the concept of a zero bending moment shield tunnel. Then, based on rational and feasible hypotheses, a structural mechanical model and an analytical expression of axis for the cross-section of the zero bending moment shield tunnel was obtained, and computational formulas of internal force and key geometry parameters were given. Based on the case of the metro shield tunnel constructed in the Shanghai soft soil area, the zero bending moment shield tunnel was designed, and its characteristics were analyzed. Considering only one cross-section of shield tunnel can be adopted in one metro line, the design method and procedure of the minimum bending moment shield tunnel were put forward. Finally, taking one of the Shanghai metro lines as an example, a cross-section of a minimum bending moment shield tunnel was designed, and its bending moment was compared with the bending moment of a circular section shield tunnel, which had the same horizontal diameter. The comparison revealed that the cross-section of the minimum bending moment can significantly reduce the bending moment of shield tunnel.
Highlights
The cross-sections for most of the existing shield tunnels all over the world are generally circular [1–4]
According to the design specification of metro [9,10], the largest oval deformation for the new constructed shield tunnel upon the acceptance check was 5D‰, where D represents the outer diameter of the tunnel
The above analysis indicates that the cross-section of the zero bending moment shield tunnel is related to the assumed earth pressure
Summary
The cross-sections for most of the existing shield tunnels all over the world are generally circular [1–4]. The cross-section of a shield tunnel inevitably undertakes bending moment. Shield tunnel section (the bending moment at any section is zero), only the normal condition is considered, which is convenient for analysis and calculation. There is zero bending moment in the designed shield tunnel cross-section. It is required to design and calculate the axial equation for the zero bending moment of the shield tunnel. 3. The Design and Calculation of the Zero Bending Moment Shield Tunnel Cross-Section 3.1. S, the vertical tunnel coordinate for cross-section S can be calculated as Equation (8). The Calculation of the Horizontal Diameter of the Zero Bending Moment Shield Tunnel. The Calculation of the Internal Force of the Zero Bending Moment Shield Tunnel.
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