Study of Stability of Surrounding Rock due to the Approach of Two Excavation Faces and Face Reinforcement in a Large Cross-Section Tunnel in Weak Stratum

Abstract

When the two excavation faces of a tunnel constructed from both the entrance and exit simultaneously in soft surrounding rock approach, the excavation surface disturbance areas are superimposed, stress and deformation in the surrounding rock become extraordinarily complex, which results in substantial deformation and instability. By combining the study of the surrounding rock stress and displacement caused by both unidirectional and face-to-face excavation in the Shizilong Tunnel, the stability of the tunnel face with two excavation faces approaching one another in a bidirectional construction is analyzed by numerical analysis. It is shown that, with the reduction in the length of the unexcavated soil between the two excavation faces, the plastic zone and deformation significantly increase, and the arching effect gradually weaken, resulting in greatly reduced tunnel stability. When the distance between the two excavation faces is less than the limiting distance, an extended range of the surrounding rock approaches plastic failure, and only effective countermeasures can guarantee the safety of the tunnel construction. Further, bamboo anchor pipe grouting is adopted to reinforce tunnel faces with the initial support strengthened, when those two excavation faces in the construction of the Shizilong Tunnel are approaching. Field implementation indicates that the tunnel is excavated safely.