Abstract: Due to the poor stability and low soil strength of sandy formations,shield tunneling in such environments often leads to support imbalance issues,causing widespread ground settlement and even excavation face collapses.This study established a numerical model for shield tunneling based on the discrete-continuous coupling analysis method (PFC3D-FLAC3D).It investigated the development of excavation face instability under flexible boundary conditions,analyzed the ultimate supporting pressure under different cover-to-diameter ratios (C/D),and explored ground settlement,soil arching effects,and principal stress changes under various degrees of excavation face instability.The results indicate that the unstable deformation zone of the excavation face is significantly influenced by depth.As the C/D ratio increases,the unstable zone evolves from a cup shape to a bulb shape.The ultimate supporting pressure initially increases and then stabilizes with increasing C/D.The width coefficient of the ground settlement trough linearly increases with C/D.During shield tunneling,soil stress above the excavation face deflects horizontally,forming a soil arch that resists deformation,which tends to move upwards as the supporting pressure decreases.The discrete-continuous coupling analysis method proposed in this study effectively overcomes the limitations of single computational methods,saving substantial computational resources.The simulated results closely align with actual engineering conditions,providing valuable insights for future calculations of the ultimate supporting pressure in shield excavation faces.