Simplified analytical method for evaluating the effects of adjacent excavation on shield tunnel considering the shearing effect

Abstract

Adjacent excavations may have adverse impacts on the existing tunnels. Due to the increasing demand for building excavations in close proximity to existing metro tunnels, ensuring the safety and integrity of tunnel structures is a major challenge for city designers and geotechnical engineers. Urban metro tunnels in soft areas of Chinese cities are often constructed using shield tunnelling technology, and the installed precast reinforcement concrete segmental lining are generally connected together by various steel bolts. Due to the presence of joints, the overall tunnel stiffness, including the bending stiffness and shearing stiffness, are significantly reduced. Due to the reduction in shearing stiffness, the tunnel longitudinal deformation can be decomposed into two distinct modes: a bending mode and a shearing dislocation mode. However, current methods for predicting tunnel longitudinal responses to adjacent excavations generally simplify a tunnel as an Euler-Bernoulli beam, which only considers the bending effect and ignores the shearing deformation of jointed tunnels. Moreover, tunnel-ground interactions are commonly considered through the Winkler foundation model, which is unable to account for the interactions of adjacent springs and leads to overestimations of shear forces and bending moments in shield tunnels. In this paper, a new analytical method is proposed that uses a Timoshenko beam to simulate jointed shield tunnel responses when subjected to an adjacent excavation, which can consider both the bending and shearing effects of a shield tunnel. The tunnel-ground interaction is considered by introducing Pasternak two-parameter foundation, which is able to further take account for the interaction between adjacent springs. The tunnel-excavation interaction is analyzed using a two-stage analysis method. First, the excavation-induced unloading stress on the existing tunnel is computed using Mindlin’s solution. Second, the tunnel longitudinal deformation due to the corresponding stress is calculated using finite difference method. The effectiveness of the proposed approach is validated by two well-document case histories, including finite element analysis and field measurement. The predicted results are also compared with those obtained using the traditional methods. Based on the verified analytical solution, a parametric analysis is also conducted to investigate the effects of key factors on the responses of existing tunnels, including excavation-tunnel relative position, ground Young’s modulus and equivalent shearing stiffness.