Impact of small-angle shield undercrossing on existing tunnels

Abstract

A three-dimensional finite element model was developed to investigate the effects of small-angle tunnels on existing tunnels in terms of surface settlement, arch bottom settlement, and ellipticalization of existing tunnel linings. The model simulated two small-angle tunnels crossing two existing tunnels. The maximum surface subsidence was observed at the recently excavated tunnel centers. The presence of existing tunnels reduced surface subsidence caused by ring-by-ring excavation. Surface settlement followed a “W” curve pattern, with two local minima gradually getting close as the tunnel was excavated. The maximum value of the arch bottom settlement of the existing tunnel gradually approached the intersection of the existing tunnel and the newly excavated tunnel. When the absolute value of the rate of change of the arch bottom settlement was high, the tunnel produced a large inter-ring misalignment. The line connecting the centers of the existing tunnel and recently excavated tunnel was parallel to the major axis direction of the ring after the deformation of the lining on the existing tunnel. Therefore, a small-angle tunnel passing through an existing tunnel may result in misalignment and torsion of the existing tunnel, requiring careful monitoring of the inter-ring bolt force to ensure safety.