Research on the Longitudinal Deformation of Segments Induced by Pipe-Jacking Tunneling over Existing Shield Tunnels

The prevalence of pipe jacking projects that traverse beneath subway tunnels in urban environments is on the rise, and the construction of pipe jacking can have a considerable effect on the deformation and stress experienced by existing shield tunnel segments. This study aims to assess the impact of pipe jacking construction on the longitudinal deformation of these segments, specifically centering on the pipe jacking project that intersects with Metro Line 6 in Chengdu. Initially, seven monitoring sections were established on-site to evaluate the vertical and horizontal deformation characteristics of each section. Then, a numerical model was developed using FLAC3D software to simulate the pipe jacking construction in relation to the existing shield tunnel. This model is designed to further explore the effects of the geological strata and the relative spatial arrangement of the two structures on the deformation of the shield tunnel. The findings reveal that under same geological conditions, the effects arising from the vertical clearance between the pipe jacking and the shield tunnel are significant. Specifically, when the vertical clearance is reduced from 2.4 m (the diameter of the pipe) to 1.4 m, the maximum vertical deformation at the crown experiences a 14.40% increase, while the maximum differential settlement between the pipe rings escalates by 46.66%. It is advisable that the separation between the two during construction should not fall below the diameter of the pipe. This research could serve as a valuable reference for the safeguarding of existing shield tunnels in analogous projects.