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  A Comparative Study on Shield Tunnels with Different Cross - Section Forms

  Compared with circular tunnels, the space utilization of rectangular tunnels is more efficient, with a bigger usable tunnel face and a smaller minimum buried depth, resulting in lower project demolition cost. In the past 30 years, with advanced technology, and the necessity of higher utilization of precious underground resource, tunnel engineers have paid more attention to rectangle mechanized tunnels. In the foreseeable future, rectangular shield tunnels will become a new focus in underground engineering.A quasi rectangle shield tunnel is a new shield tunnel type and was first applied in Ningbo Metro Line 3, China. Compared with traditional tunnels, this new type is superior in internal space utilization and can be used in tunnels with a smaller buried depth. This will lead to a wide application of the technology. The following four factors are taken into consideration in this paper: A. the superimposed stress in front of the excavation face; B. the friction between the shield skin and soil; C. the simultaneous grouting, and D. the ground loss. Based on the Mindlin solution, this paper presents the computerized outcome of the ground surface deformation caused by these four factors. Furthermore, using theoretical analysis and onsite data, soil disturbance and ground settlement induced by different tunneling options such as quasi rectangular shield, DOT (Double-O-Tube) shield and large diameter circular shield are analyzed and compared. The research shows that: (1) Superposition of these four factors causes large ground surface heave near the tunneling area, with a maximum of 15mm; (2) Grouting force plays an absolutely dominant role in ground surface deformation, which can cause a maximum 19 mm ground heave. This offsets the settlement induced by the horizontal excess pressure and the friction between the shield skin and soil; (3) For quasi rectangle shield tunneling, the impact of the shell friction on ground settlement can be 3.5 times the ground settlement caused by excess pressure, while the corresponding ratio is only 80% in a circular shield; (4) The superimposed stress of the quasi rectangular shield is about 50% of the double circular shield, which means that it is superior to DOT shield in ground disturbance control. These results serve as technical reference for future quasi rectangular shield tunneling; (5) Total ground surface deformation induced by these four factors remains controllable.This paper provides theoretical references for soil disturbance and ground movement control, underground pipeline protection, pile foundation crossing and protection in quasi rectangle shield tunneling.

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