Abstract: During the construction of mountain tunnels, slope-collapse incidents above the tunnel portal and cut-and-cover sections occur with notable frequency. Using the cut-and-cover segment of a highway tunnel in a mountainous region as the engineering context, this study investigates a localized rockfall event that occurred on the southeastern side slope. A 1 m longitudinal equivalent finite-element model is developed to represent three representative conditions: (1)the pre-collapse state, (2)the instantaneous dynamic impact induced by the collapse, and (3)the post-collapse condition following debris removal and re-leveled backfill. The analysis systematically examines the crack-width distribution and the evolution of internal forces within the lining structure under each condition. The findings indicate that, under the original undisturbed condition, the cut-and-cover lining is governed primarily by axial compressive forces, with crack widths remaining small and locally manageable. The collapse-induced impact markedly increases tensile demand at the arch foot and arch crown, causing the maximum crack width to exceed the limits prescribed by relevant design codes and revealing a potential risk of brittle structural failure. After the backfill is re-leveled, a more stable uniformly distributed load is reestablished, resulting in a significant reduction in crack width and restoring compliance with code requirements. The structural stress state consequently returns to a stable condition. These results provide a valuable reference for the assessment and mitigation of similar engineering accidents.

Key words: Cut-and-Cover Structure, Slope Collapse, Finite Element Analysis, Crack Width, Overburden Pressure