Numerical Evaluation of Control Measures for Tunnel Deformation Induced by an Oversized Deep Excavation

Abstract

Deep excavations conducted close to tunnels will cause an adverse impact on tunnels, and countermeasures must be taken during the excavation to ensure the safety of the tunnels. However, the effectiveness of measures to control the vertical displacement of tunnels is still unclear, especially when a cut-and-cover tunnel is tied to a deep excavation. In this study, a three-dimensional (3D) numerical method is used to evaluate the effectiveness of control measures for excavation-induced tunnel deformations. Based on a recent case of a deep excavation in Shanghai, a quasi-three-dimensional numerical model considering the soil–structure interaction and fluid–solid coupling was built by the use of finite difference software, i.e., FLAC3D v5.. Four countermeasures, i.e., zoned excavation with partition walls and cross walls, piles tied to the walls, soil reinforcement, and dewatering in the confined aquifer, were implemented in this model. The numerical model was validated by field data and the effectiveness of these measures was evaluated comprehensively. Results indicate that the zoned excavation can reduce the vertical displacement of the tunnel, while the cross wall will cause an adverse impact on restraining the tunnel heave. Installation of tie piles can decrease the uplift of tunnels, and the maximum tunnel heave decreases with the increase of the length of the tie piles. Soil improvement is the most effective countermeasure that can reduce the tunnel heave significantly. Dewatering in a confined aquifer can be adopted as a countermeasure to restrain the tunnel heave during excavation.