Objective To investigate the stage-wise effects of ultra-large-diameter shield undercrossing the existing rectangular mined tunnel structures, it is necessary to study the settlement behavior of existing tunnels.

Method Based on the interval shield project of Shanghai Suburban Railway Airport Link Line passing beneath the existing Metro Line 12, 94 settlement monitoring points are laid out. Using field monitoring, settlement variation characteristics of the existing tunnel at different construction stages are analyzed with field-measured settlement data. Meanwhile, numerical simulation via FLAC3D software is employed to investigate the stage-wise settlement rules and structural lateral displacement laws of the existing tunnel under different slurry and grouting pressures.

Result & Conclusion  The existing double-track tunnel presents distinct deformation responses in three major construction stages, i.e. before, during and after shield undercrossing. Variations both in slurry pressure during cutterhead penetration and in grouting pressure during shield tail passage can induce more prominent deformation responses of the existing tunnel. The differential settlement between the double-track tunnels first increases and then decreases, and the stage-wise turning point occurs around the moment when the shield tail departs from the overlapping area. The uneven lateral displacement of existing tunnel structure induced by shield undercrossing project mainly occurs when the shield machine passes through.The maximum lateral displacement appears near the center point of the undercrossing, accounting for 90% of the final displacement, and in the opposite direction of the shield tunnelling advancement.