Incomplete soil arching effect and responses of existing tunnels induced by undercrossing super-large-diameter shield tunneling in soft strata

Super-large-diameter shield tunneling inevitably induces deformations in the surrounding soil and nearby existing tunnels due to ground-tunnel interactions. This study developed and validated a numerical model to simulate these interactions in typical soft soil strata in Shanghai, with a focus on stress and displacement responses during the undercrossing of an existing tunnel by a new super-large-diameter shield tunnel. The study identified an incomplete soil arching (ISA) effect and proposed methods to delineate the ISA, loosened, and compaction zones, categorizing the influenced areas into reinforced, stable, and safe zones. Parametric analyses examined the influence of tunnel spacing ( S ) and volume loss ratio ( V ) on ground deformation, loosened zone height, and existing tunnel deformation. Results indicate that greater volume loss ratios and smaller tunnel spacings amplify ground settlement, while the loosened zone height is affected by both the volume loss ratio and the stratigraphic boundary. Among the considered scenarios, a volume loss ratio of 0.2% minimizes the loosened zone height across various spacings. Changes in the convergence of the existing tunnel occur in two phases, characterized by rapid changes ( S / D of 0.1–0.3, where D is the diameter of the newly constructed tunnel) and gradual changes ( S / D of 0.3–0.7). To mitigate adverse effects on the ground and the existing tunnel, it is recommended to maintain the volume loss ratio below 0.2% and the tunnel spacing over 0.3 D . Additionally, reinforcing the loosened zone is advised to enhance the stability of the existing tunnel.