Effects of twin-tunnel excavation on an existing horseshoe-shaped tunnel considering the influence of a settlement joint

In urban centres, the construction of a new tunnel would cause ground disturbance and affect any adjacent existing tunnel. Tunnel–tunnel interaction problems have not yet been comprehensively studied. In particular, the response of an existing horseshoe-shaped tunnel to the construction of two perpendicularly undercrossing tunnels remains unclear. In this study, three-dimensional centrifuge tests and three-dimensional numerical analysis were designed and conducted in dry sand to investigate the effects of twin-tunnel undercrossing on an existing horseshoe-shaped tunnel considering the influence of a settlement joint. It is found that the adverse effects caused by the tunnel excavated later were smaller than those by the earlier one. For the existing tunnel without a settlement joint, the settlement at the invert and crown along the longitudinal direction showed sagging and hogging curves, respectively, owing to significant vertical elongation of the middle section. The presence of a settlement joint resulted in an increase in the settlement of the ground surface and the existing tunnel by over 100%. In both cases, larger bending strains were induced in the transverse direction than in the longitudinal direction, particularly around the corners.