Numerical Study of Excavation Face Active Instability in Upward Shield Tunneling

To study excavation face instability in upward shield tunneling, a 3D numerical model was established using ABAQUS software v2023 under different depth ratios (C/D = 1, 1.5, 2, 3, 4), with reference to the upward shield tunneling project of the Midosuji Utility Tunnel Construction Project in Japan. The model simulated soil as an elastoplastic material governed by the Mohr–Coulomb criterion, with dimensions of 60 m × 60 m × 60 m and boundary constraints applied to soil surfaces. This study explores variations in the limit support pressure and soil failure zone during the instability process. Simulation results were validated through scaled model tests (1:50). The study findings reveal that (1) for varying depth ratios, the support pressure on the excavation face decreases as the depth ratio increases, with diminishing reductions at higher ratios. (2) At shallower depths (C/D < 3), the soil failure zone above the excavation face is nearly conic. At deeper depths (C/D ≥ 3), the failure zone resembles a “bullet head” shape. (3) At shallower depth ratios (C/D < 3), surface displacement shows slow-to-rapid transitions. At deeper depth ratios (C/D ≥ 3), surface displacement remains nearly constant. (4) Post-instability, stress concentration at the horizontal tunnel’s top opening causes segments to deform into an inverted V-shape. (5) Soil stress changes are categorized into three zones—stress release, soil wedging, and stress transfer—with each zone expanding as depth increases.