Search

The motivation of this paper is to propose a new construction method with better control effects and economic advantages to overcome the disturbance problem in constructing small-spaced twin tunnels under narrow site conditions. First, the reinforcement measures commonly used in some typical projects in recent years and their disadvantages are summarized. Next, combined with the construction case of a double-line shield tunnel with a distance of 0.69 m from the Tel Aviv subway in Israel, a construction scheme for building an underground diaphragm wall between the two tunnels is proposed. Furthermore, the deformation and internal force change characteristics of the leading tunnel segment during the lagging shield tunneling are studied, and the interaction mechanism between small-spaced tunnels is explained. Computational results indicate that when the clearing distance between the tunnels increased from 0.096 to 0.7D, the range of segments affected by the subsequently excavated tunnel was reduced by 52%, and segments bending moment and axial force were decreased by 36% and 71%, respectively. When the tunnel depth is not greater than 2.0D, the minimum tunnel spacing should be not less than 0.28D. The required safe tunnel spacing will increase if the tunnel depth increases. Both recorded and simulated data show that an underground diaphragm wall between the tunnels can protect the leading tunnel effectively. The underground diaphragm wall was recommended for similar projects, enabling construction before shield tunneling with no need to reinforce the tunnel interior. [ABSTRACT FROM AUTHOR]

Shield recovery in water-rich sand strata is a challenging issue in the field of shield tunnel engineering, especially when the end of the shaft cannot be reinforced by jet grouting or freezing or when the shield cannot be supported with a steel sleeve. Therefore, it is important to develop an effective recovery approach and adopt suitable techniques to control the risks. In this study, a new method based on filling the receiving shaft with water is proposed for the underwater recovery of an earth pressure balance (EPB) shield with zero end reinforcement from a metro tunnel in Tel Aviv, Israel. Several additional techniques are used to ensure safe recovery of the shield, including the design of a concrete cradle, drilling of pressure relief holes, control of excavation parameters, screw conveyor sealing, portal sealing, tail sealing, and grouting. Furthermore, according to the actual situation on site, filling the shaft with water to 1 m above the water level in the strata can prevent the fine sand from percolating into the shaft. Before the cutterhead approaches the underground diaphragm wall, the driving attitude should be strictly controlled, and the edge hob should be inspected for wear. The necessary thrust of shield tunneling in the underground diaphragm wall and shaft is calculated theoretically. In order to ensure the deformation control of the underground diaphragm wall and the smooth tunneling of the shield, the thrust of the shield excavating the underground diaphragm wall will not be larger than 12 000 kN, and the penetration degree will be limited to 3 mm/r. Qualitative observations and measurements of surface subsidence in the metro tunnel indicate that these risk mitigation techniques are effective and suitable for the underwater recovery of EPB shields in water-rich sand strata. [ABSTRACT FROM AUTHOR]