Effects of above-crossing tunnelling on the existing shield tunnels

Tunnelling in the dense urban areas frequently results in over-crossing or bypassing the existing tunnels. It is obvious that the over-crossing tunnelling will adversely affect and even damage the existing tunnels if the induced deformation exceeds the capacity of tunnel structures. Increasing concerns have been raised about the interactions between the over-crossing tunnelling and underlying tunnels. In order to obtain a better mechanical understanding of the effects of the over-crossing tunnelling on the existing tunnels and provide a quick but low cost assessment alternative method for evaluating the behavior of underlying tunnels prior to construction, a simplified analytical method is proposed in this study. In this simplified method, the tunnel is simply considered as a continuous Euler-Bernoulli beam with a certain equivalent bending stiffness. The unloading stress at the tunnel location caused by the over-crossing tunnelling is computed through Mindlin's solution, ignoring the presence of the existing tunnel. Then, the tunnel-soil interaction due to the relief stress is analyzed based on the commonly-accepted Winkler foundation model. The applicability of the presented method is validated by three well-documented case histories. Results of these case studies show a reasonable agreement between the predictions and observations. Finally, a parametric analysis is also preformed to investigate the influences of the different factors on the behavior of the existing tunnels, including clearance distance, advancing distance and multiple tunnels construction. (C) 2016 Elsevier Ltd. All rights reserved.