PSI - Issue 84

I. Vangelisti et al. / Procedia Structural Integrity 84 (2026) 1247–1254

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service life of the interventions, it is necessary to evaluate the level of knowledge of the lining–intervention system, through inspections of the interventions and of the lining. The verification of the lining–intervention system is carried out with respect to: • load-bearing capacity and durability; • robustness. An extended nominal service life of the intervention of up to a maximum of ten years is recommended. If the verifications are fullfilled, the new reference service life for the intervention is defined; conversely, if the verifications are not fullfilled, appropriate compensatory measures are identified in order to ensure an adequate level of safety of the lining segment under consideration. In order to perform the verification, loads and level of knowledge have to be evaluate: • the loads may be estimate with reference to the procedures adopted for the assessment of lining inspection segments without interventions; • the introduction of a specific level of knowledge for the lining, that can be assessed with reference to the procedure proposed for lining inspection segments without interventions, for the case of verification for significant defects, is recommended. In addition, a level of knowledge should be defined for each material of the intervention, evaluated on the basis of two main type of information: o degree of knowledge of the intervention geometry; o degree of knowledge of the mechanical properties of the material. With reference to the modelling of the lining-intervention system, two main phases may be identified: • modelling of the geometry of the lining and of the intervention, extracted from as-built documentation and any available surveys; • modelling of any defects identified during inspection, affecting the lining and the intervention. The modelling shall appropriately account for the defect type, the classification parameters defined in the Tunnels Guidelines, and the defect location. Finally, the lining stresses are estimated taking into account the load combinations defined by the NTC2018. 4.1. Load-bearing capacity and durability verification The load-bearing capacity verification aims to quantify the maximum admissible load of the intervention at the Ultimate Limit State at the end of the extended nominal service life, taking into account the expected degradation at the end of this period, and to compare it with the acting load previously assessed. For the sake of computational simplicity, it may be assumed that the load acting on the intervention does not vary over time and that, therefore, the load acting at the end of the extended nominal service life is equal to that acting in the initial condition. As previously stated, it is recommended to perform the verification for a maximum extension of ten years. At the end of each verification scenario, the designer shall evaluate the Capacity-to-Demand ratio, which is used for the assessment of the new nominal service life. 4.2. Robustness verification The verification aims to assess the load-bearing capacity of an individual structural element constituting the intervention, under the assumption of the failure of one or more structural elements of the same typology. The key aspects that the designer shall analyse for the robustness verification include: • identification of the structural element to be verified (e.g., anchor, steel rib, etc.); • definition of one or more appropriate collapse mechanisms; • reassessment of the acting load for each scenario as a function of the assumed failures; • evaluation of the load-bearing capacity of the analysed element, with particular attention to: o the definition of an extended nominal service life of up to ten years; o the consequent assessment of the expected degradation at the end of that period; • evaluation of the Capacity-to-Demand ratio. For each scenario, the designer shall determine the Capacity-to-Demand ratio and compare it with a reference threshold, estimated taking into account the uncertainties of the procedure.