Experimental Study on Failure Mechanisms of Shield Tunnel Segments with Initial Cracks Under Surcharge Loading

Accidental ground surcharge loads can induce adverse effects such as segment cracking in underlying shield tunnel structures, with particularly pronounced impacts on pre-damaged tunnel segments. Cracks represent one of the most common initial damage forms in shield tunnel structures. To investigate through-crack failure mechanisms in shield tunnel segments with initial cracks under surcharge loading, this study conducted 1:8 scaled indoor model tests, considering factors including initial crack length, quantity, morphology, and surcharge position. Research findings demonstrate that increased initial crack length and quantity significantly reduce the critical load required for through-crack formation. Specifically, segments with 9 cm longitudinal initial cracks required 50.9% less load to develop through-cracks compared to intact segments. Similarly, segments containing two 9 cm circumferential initial cracks exhibited a 22.1% reduction in critical load relative to those with single circumferential cracks. Initial cracks in pre-damaged segments substantially influence the propagation path of new cracks during subsequent loading failures. The detrimental effects of staggered longitudinal-circumferential initial cracks exceed those of purely longitudinal cracks, which themselves pose greater risks than circumferential cracks alone. Bilateral surcharge loading significantly increases the critical load threshold for through-crack formation compared to unilateral loading. This highlights the severe structural risks associated with uneven load distribution.