Model test study on instability mechanism during shield under-crossing existing pipeline

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This paper aims at investigating the instability mechanism that occurs during the under-crossing of an existing pipeline using model tests and numerical simulations. The strain in the pipeline, earth pressure, surface settlement, and mechanisms of stratum instability during the process of instability have been obtained. The research findings reveal three distinct growth stages in the longitudinal deformation of the existing pipeline, with the strain growth rate during the rapid growth phase being 2–3 times higher than that observed during the slow growth phase. The pipeline displays a deformation pattern with tensile stress in the upper section and compressive stress in the lower section, due to the uneven force distribution along its length. In the absence of pipelines, the distribution of surface collapse exhibits a distinct “V” shaped. As the clear distance between the existing pipeline and the tunnel (CDPT) increases, there is a transition from a “partial W” to a “positive W” type distribution. The presence of the pipeline exerts the most significant inhibiting effect on the soil directly above it. In cases where there is no pipeline or the CDPT is small, soil leakage primarily occurs in front of the excavation. As the CDPT increases, soil leakage occurs around the pipeline and the instability range of the stratum significantly expands. The stable strata above the pipeline gradually widen, and the area with a significant influence range shifts from the right side to the left side of the existing pipeline.