Face stability analysis of horseshoe-shaped shield tunnels considering dynamic effects of composite cutterhead and screw conveyor

Horseshoe-shaped tunnels have been increasingly adopted in recent years due to their spacious design and rational structural behavior. In shield tunnelling, multiple spoke-type cutterheads are applied to form an excavation face that closely matches the horseshoe contour. However, face stability during this specific horseshoe-shaped shield tunnelling under dynamic conditions remains unclear. In this paper, the face stability of a horseshoe-shaped shield tunnel in sand is analyzed with coupled finite difference method and discrete element method (FDM-DEM) in three dimensions. A series of triaxial tests are conducted so as to obtain appropriate parameters for soils as reference. A full-scale, refined Earth Pressure Balance (EPB) horseshoe-shaped shield machine equipped with multiple spoke-type cutterheads is applied. Twelve numerical simulations are carried out under conditions of cutterhead rotation, soil conditioning in the chamber, and soil removal by screw conveyors. Numerical results are validated through comparisons with previous literature. The results suggest that cutterhead rotation at higher speeds strengthens soil mobilization, which in turn induces flatter settlement patterns and larger failure zones. Moreover, ultimate support pressure increases with the rotating cutterhead, which also results in a more uneven pressure distribution in the soil chamber. Furthermore, cutterhead rigidity, design, and soil properties are found to have major contributions to the partial collapse in homogeneous soils. Therefore, applying a lower cutterhead rotational speed and optimizing the cutterhead arrangement can improve tunnel face stability.