Seismic performance of existing tunnels affected by lining thickness reduction

The ageing of post-war underground transport infrastructures is raising increasing concerns, as many tunnels were built without seismic design provisions and now exhibit significant signs of structural degradation. This study investigates the seismic vulnerability of road tunnels affected by reductions in lining thickness at the crown – a condition frequently reported during recent inspections. A typical two lane tunnel with a 50 cm thick unreinforced lining, without an invert arch, was analysed as an example to define the potential criticality of this defect. Residual thicknesses of 20 cm and 10 cm were considered, assuming a good-quality rock mass (GSI = 65) and two overburden scenarios equal to two and four times the tunnel diameter. The soil-structure interaction problem was addressed through 2D finite element analyses under plane strain conditions, with an equivalent shear strain increment applied to simulate seismic loading. The lining was modelled using elastic and no-tension laws, while the ground was represented as elastic-perfectly plastic with a Mohr-Coulomb criterion. The results indicate that localised crown thinning significantly increases lining vulnerability under seismic loading, leading to critical stress conditions. These findings confirm the need for targeted evaluation methods and maintenance interventions to ensure the safety and resilience of ageing tunnels.