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Abstract

Face stability of slurry shield tunnels in cobble ground is strongly affected by heterogeneous grading, coarse-particle migration and cutterhead-induced disturbance, potentially resulting in support-pressure demand and failure patterns different from those in homogeneous soils. This study combines a 1-g physical model test with DEM simulations to investigate face failure and soil arching under dynamic cutterhead excavation. A three-dimensional DEM model incorporating the graded particle structure and key tunnelling processes, including shield advancement, cutterhead rotation, chamber support and muck removal, was established. The model was calibrated by direct shear tests and validated against physical model observations. The effects of cutterhead opening ratio and cover-depth ratio (C/D) on limit support pressure, failure-zone development, surface settlement, stress redistribution and force-chain evolution were examined. The results show that the limit support pressure increases with both opening ratio and C/D, and the normalized limit support pressure is generally higher than values reported for homogeneous soil or static-support models. A larger opening ratio intensifies particle disturbance, expands the loosened zone and weakens soil arching. Increasing C/D promotes an arch-shaped load-transfer path and limits upward failure propagation, with failure remaining underground when C/D ≥ 1.5 in the investigated cases. However, compared with dry sand and relatively uniform granular soils, the arch in cobble ground initiates at a higher position and has a more limited upward extension, indicating lower load-transfer efficiency and a larger loosened zone beneath the arch. Force-chain analysis indicates that the observed face-instability characteristics reflect the balance between cutterhead-induced disruption of the granular load-bearing network and arch-like redistribution of contact forces around the loosened zone.