Experimental investigation and limit analysis of shield tunnel face failure mechanism in sand

Shield tunneling in urban underground space necessitates tight control over support pressure at the tunnel face and a thorough insight into ground collapse mechanisms. This study conducts a model test and a theoretical validation to clarify the mechanisms of face failure and subsequent ground collapse in sand during earth pressure balanced shield (EPBS) tunneling operations. The experiment investigates the changes in soil pressure and surface subsidence patterns during shield tunneling and collapse stages, to elucidate the entire process of ground collapse triggered by shield tunneling disturbances. A novel methodology was proposed to ensure effective verification of the rotational failure mechanism, focusing on the collapse pit morphology and the critical collapse pressure. The results indicate that: (1) precise control over the shield tunneling and screw conveyor rotation speeds is essential for tunnel face stability; (2) the sand with low moisture content is prone to stepwise ground collapse under shield tunneling disturbances; (3) soil pressure measurements at the cutterhead are more indicative of face failure and imminent ground collapse than those from the soil chamber; (4) there is a consistent alignment between the rotational failure mechanism and observed collapse pit morphology, albeit with slight variations due to tunneling disturbances; (5) the experimentally determined critical collapse pressure is higher than the theoretical prediction, indicating an underestimation of risks in the current model. The study advances the understanding of the face failure mechanisms in shield tunnels, thereby providing insights into the design and safety of shield tunneling within engineering practices.