A comparative assessment of seismic performance in existing tunnels with lining thickness reduction

The seismic vulnerability of ageing underground infrastructures is a growing concern, as many existing tunnels were built without seismic provisions and now show significant degradation. This study investigates the seismic response of a typical two-lane road structure characterised by the absence of an invert arch and lined with unreinforced concrete, affected by localized thickness reductions at the crown, a common damage scenario observed in post-war constructions. The ground was assumed to be a good-quality rock mass (GSI = 65), and an overburden condition equal to four times the tunnel diameter was investigated. A comparative assessment was carried out between equivalent quasi-static analyses and advanced dynamic simulations, both performed under plane strain conditions. In the quasi-static approach, seismic effects were introduced through incremental shear strain fields. Two values were considered: one derived from the simplified Eurocode procedure and another obtained from local seismic response analyses. In contrast, the dynamic analyses accounted for the time-dependent propagation of seismic waves using spectrum-compatible acceleration records. The lining was modelled using an elasto-plastic constitutive law, and the surrounding ground followed a Mohr-Coulomb criterion. The results highlight the limitations of simplified procedures that estimate seismic-induced shear strain solely as a function of peak ground acceleration. In contrast, shear strain values derived from local seismic response analyses provide a more representative estimate of the deformation demand at tunnel depth, in both quasi-static and fully dynamic analyses, indicating proximity to a shearing failure mechanism at the reduced-thickness crown.