PSI - Issue 84

Daniela Boldini et al. / Procedia Structural Integrity 84 (2026) 175–182

176

1. Introduction Tunnels built in Italy after the Second World War have become a critical focus for infrastructure management, as many are now over 70 years old and approaching the limits of their design life. The country’s complex mountainous terrain led to an extensive network of more than 9,000 tunnels, stretching roughly 2,600 km, which presents ongoing challenges for inspection, maintenance, and safety assurance. Recent regulations aim to standardise tunnel management practices. The 2022 “Italian Guideline for Risk Classification, Safety Evaluation, and Monitoring of Existing Tunnels”, issued by the Italian Superior Council of Public Works under the Ministry of Infrastructure and Transport, provides a national framework for evaluating structural conditions, assessing risks, and planning maintenance interventions, ensuring compliance across all operators. Implementation of these guidelines has involved comprehensive surveys of tunnel conditions, integrating visual inspections, radar scanning, concrete coring, and in-situ stress measurements (Carigi et al., 2024, 2025). Observed defects commonly include cracks, gravel inclusions, and voids at the lining extrados, particularly at the crown (Alessio et al., 2024). Two factors have emerged as critical for tunnel safety. First, seismic loads were largely neglected in the original 1960s–1970s designs. Second, reduced lining thickness at the crown is widespread, often resulting from construction practices of that era. Concrete placement methods, ranging from conveyor belts to pneumatic pumps, combined with poor compaction and inconsistent material quality, produced incomplete linings with irregular thickness and discontinuous contact between the lining and the surrounding rock. Nevertheless, the majority of these tunnels have retained structural performance over decades, as lining stresses remain low in static conditions, dominated mainly by self-weight. Fig. 1a shows voids and reduced-thickness sections identified during refurbishment of the Monte Bianco Tunnel, opened in 1962. Despite the recent efforts of tunnel stakeholders in the activation of multiple monitoring programs and operational intervention plans (already active on the Italian national scenario), numerical models focused on the behavior of tunnels affected by construction defects and ageing-related degradation remain limited. This paper intends to share preliminary results of a numerical model focused on a realistic tunnel section subjected to severe seismic action.

Fig. 1. (a) reduced-thickness lining defects observed in the Monte Bianco Tunnel after 25–30 cm milling; (b) typical road tunnel cross-section considered in the analysis (dimensions in m). 2. Case study The analyses were carried out on a typical two-lane road tunnel from the 1960s–70s, with a circular cross-section, a 50-cm-thick unreinforced concrete lining and without an invert arch (Fig.1b). To evaluate the impact of structural degradation and construction-related defects on seismic performance, tunnel configurations with residual crown thickness of 10 cm were simulated, representing significant potential voids behind the lining. A single overburden