PSI - Issue 84

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

1249

The objective of VAL4 is therefore to perform a detailed safety verification of the tunnel lining or of the lining– intervention system, with the aim of: • quantifying the safety level of the structure; • identifying any structural deficiencies; • defining immediate intervention strategies, where necessary; • designing compensatory interventions when verification requirements are not fullfilled. 3. VAL4 procedure for lining inspection segments without interventions 3.1. Load assessment The first step of the VAL4 is the lining load estimation. If the final lining is found to be loaded, a targeted assessment shall be performed, including a detailed reconstruction of the geological setting, advanced structural analysis of the final lining, and numerical simulation of the construction stages. If, conversely, the final lining is found to be unloaded (i.e. the average value of the compression stress is lower than a specific threshold), the following operational scenarios are identified: • case 1: availability of design load information: in this case, the design value of the acting load is known and is directly assumed as reference value for the subsequent verifications; • case 2: absence of design load information but availability of design resistance data for the final lining: in this case, a back-analysis of the tunnel lining is carried out to estimate the acting load compatible with the lining resistances In any case, a load amplification factor shall be applied to account for uncertainties related to historical design assumptions and simplifications adopted in the calculation model. • case 3: absence of information on both design loads and design resistances, but availability of geological information: in this case, it is proposed to adopt the Terzaghi classification, assigning to each homogeneous lining segment the corresponding rock mass class and the related load range suggested in the literature (where posible, the use of more advanced rock mass classifications such as RMR and GSI is recommended). To account for uncertainties, the maximum load value within the range associated with the assigned class should be assumed. This value shall be then amplified by an amplification factor accounting for model uncertainties. • case 4: absence of any information: in this case, it is necessary to design and develop a geognostic investigation campaign to characterize the rock mass. Finally, load combinations shall be set up in compliance with the NTC2018. The definition of the level of knowledge adopted in this paper refers to the one provided by the NTC2018 and the related Circular. In particular, as reported in §8.5.4 of the NTC2018, three levels of knowledge are introduced: LC1, corresponding to a limited level of knowledge; LC2, corresponding to an extended level of knowledge; and LC3, corresponding to a comprehensive level of knowledge. For the execution of VAL4: • the levels of knowledge are defined for each ispection segment, with reference to a set of representative parameters (i.e. as-built, avalaibility of structural tests, georadar, inspection outcomes, ecc.). Some parameters are invariant from one segment to another (such as those related to the original design of the structure); other parameters, instead, depend on the inspected segment and vary from one to another, as they are related to local characteristics of the lining. The proposed assumption is therefore to evaluate the level of knowledge for each inspection segment; • for each segment, two levels of knowledge are defined, related to the definition of structural verifications: one referring to the verification of the most stressed section, and the other related to the verification of sections affected by significant defects; • the levels of knowledge depend on the grade of detail of the information collected with reference to the parameters introduced above and summarised in Table 1; 3.2. Level of knowledge evaluation