Failure evolution and mechanical response of permeable ribbed double-arch tunnels with insufficient secondary lining thickness

Under the strategic backdrop of low-carbon environmental protection, the permeable ribbed double-arch tunnel, as a novel structure, effectively harmonizes large-span engineering construction with ecological conservation. However, influenced by complex construction environments and procedures, this structure is prone to defects such as insufficient lining thickness, which severely threaten its long-term operational safety and slope stability. Addressing this issue, this study investigates the catastrophic mechanism and mechanical behavior of the composite lining structure under varying conditions of insufficient thickness, utilizing a combination of 1:40 scaled model tests and numerical simulations. Key findings include: The essence of insufficient lining thickness is local stiffness degradation, which weakens the "composite beam effect" and triggers load redistribution. Damage initiates at the thinnest secondary lining and propagates to the debonding edge of the initial lining, accelerating the failure process of the rib arch. Defects significantly reduce rock pressure and exacerbate pressure imbalance. Outer tunnel defects directly degrade bearing capacity, reducing the failure load and rib ductility by 16.7% and 31%, respectively. In contrast, inner tunnel defects exhibit "hysteresis." Under large-scale defects, rock plastic deformation increases by 23.5%. This causes initial lining peak stress to surge by 169.7% and steel arch stress by 175%. Simultaneously, secondary lining central damage escalates by 476%, increasing reinforcement stress by 85%. Consequently, the yield initiation time of the steel arch advances by 12%, accelerating structural instability. This study elucidates the disaster-causing mechanism of eco-friendly ribbed arch tunnels, providing a theoretical basis for defect assessment and reinforcement.