Mechanical responses of closely spaced large span triple tunnels

Abstract

This paper presents a case study of closely spaced triple tunnels with large sections in Beijing, China. The mechanical responses of three cross-sections were systematically monitored and analyzed, including the surrounding rock pressure, the internal stress in steel frame and shotcrete, the axial force in pillar-reinforcing bolts, the crown settlement, and the horizontal convergence. It was found that the measured data in the class-V ground were 2 to 3 times greater than those in class-III ground. Pipe roof and curtain grouting were timely implemented to ensure excavation safety in the subsequent excavation. With the decrease of pillar width, the middle tunnel excavation influence on the left tunnel was more significant than the right tunnel excavation influence on the left tunnel. The safety status of triple tunnels was assessed, and the strength-based safety factors were far greater than 1. The middle tunnel safety was the least desirable because of the excessive pre-deformation and potential pre-failure caused by leading tunnel excavations. Therefore, improved supports were in-time applied in the middle tunnel in the following construction. Afterward, the arching mechanism of triple tunnels was further studied by 3D finite-difference simulation. It was obtained that due to the multiple interactions among three individual small pressure arches, a combined large pressure arch might be formed across triple tunnels. Strong pre-supports and reasonable pillar reinforcement could be effective methods to restrict arch formation. This case study might severe as a practical reference for similar tunnelling constructions.