Finite-discrete element numerical modeling on shield tunnel face stability undercrossing existing pipeline considering discontinuous contact

Tunnel face stability is critical for excavation safety when undercrossing a pipeline. Previous studies inadequately address soil-pipeline discontinuous contact effects. This study elucidates instability mechanisms using coupled FDM-DEM simulations calibrated by physical tests. Results reveal pipeline-induced sheltering effects on failure modes, with sheltered zones transitioning from inverted triangular to spindle-shaped as h/D decreases from 0.5 to 0.1. Evolution of contact force chains show reduced forces ahead of tunnel face and beneath the pipeline drive instability. Surface settlement develops dual troughs flanking the pipeline as δ/D increases from 0.86 to 6.52%. Limit support pressure demarcates three stages at 0.9σ_T0 and 0.2σ_T0. Crucially, discontinuous contact governs deformation: when ground deformation exceeds pipeline displacement, void-induced separation triggers stress loosening. Parametric studies confirm vertical spacing and flexural stiffness influence pipeline settlements. Controlling face support pressure to prevent excessive discontinuous contact is essential for pipeline safety, providing insights for urban tunnelling adjacent to existing structures.