Assessment of seismic vulnerability in aged tunnels affected by lining deterioration

The structural condition of many existing tunnels, built during the post-war period, is increasingly affected by ageing and deterioration processes, including localised reduction in lining thickness. At the same time, growing awareness of seismic risk, which was often not considered in the original design, has emphasised the need to re-assess the structural reliability of these infrastructures under seismic loading. This study investigates the seismic response of a realistic unreinforced concrete road tunnel affected by crown thickness reduction, with particular emphasis on the role of the initial stress ratio and the effect of pre-existing cracking, frequently detected during systematic inspections. Finite-element analyses were carried out accounting for the construction sequence and adopting a quasi-static approximation of seismic loading, with shear-strain levels derived either from the Eurocode procedure or from site-specific local seismic response analyses. An elasto-plastic no-tension model was considered for the lining, differentiating between intact and cracked states, while the surrounding ground was modelled as elasto-plastic with strength and stiffness properties representative of good quality rock-mass. The results show that crown thickness reduction alone increases distortion and tensile plasticisation, but structural crisis develops only when this defect is combined with pre-existing cracking and a larger seismic demand, which leads to local shear failure at the crown. The study underlines the importance of incorporating degradation mechanisms into seismic vulnerability assessments of existing tunnels, supporting the development of prioritised maintenance strategies within infrastructure asset management frameworks.