PSI - Issue 84

I. Vangelisti et al. / Procedia Structural Integrity 84 (2026) 1239–1246

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Any variation in one or more of these parameters requires the identification of a new segment for which a dedicated test shall be carried out. It is nevertheless recommended to perform at least one stress test every 200 m. Based on the outcomes of the stress measurements, denoting by σ the stress value recorded on the tunnel sidewalls for which the capacity/demand ratio is higher than 1.5 in the most loaded section, defined in compliance with specific technical evaluation, the following actions may be undertaken: • confirmation of the reduction in the attention class when the average stress value obtained from the tests does not exceed 0.5 MPa; • execution of a simplified lining verification when the average stress value obtained from the stress tests lies between 0.5 MPa and σ ; • execution of VAL4 verifications when the average stress value obtained from the stress tests exceeds σ . The definition of these stress intervals is derived from finite element analyses of typical tunnel geometries conducted by the working group, as well as from previous studies and technical documents. The proposed threshold values may nevertheless be modified on the basis of documented analyses that take into account tunnel-specific features. The threshold value σ shall be defined on the basis of assessments of proven validity, aimed at investigating the relationship between the stress state measured by the stress tests and the corresponding stress state induced in the resistant concrete cross-section. The recommended workflow for the evaluation of the stress state of lining inspection segments exhibiting a reduction in the attention class is summarised in Figure 3. 5. VAL4 priorization criteria In this chapter it is introduced a novel prioritization procedure for the execution of VAL4 assessments for lining inspection segments that, according to the criteria discussed in the previous sections, require such further investigation. The prioritization is first carried out at the single lining-segment scale and, subsequently, at the overall tunnel scale. At the individual lining-segment scale, the prioritization criteria are defined as follows: • attention class (High or Medium-High); • rock mass quality (high, medium or low): o high rock mass quality for Terzaghi Classes I, II, and III; o medium rock mass quality for Terzaghi Classes IV and V. Where significant water inflows are present for these classes, the rock mass quality is classified as Poor; o low rock mass quality for all other Terzaghi classes; • concrete quality index, defined as the combination of the characteristic strength of the material and the difference between the minimum lining thickness and the design thickness (Low, Medium, or High). In particular, the concrete quality index is defined in accordance with Table 1. Four priority levels are proposed, namely Priority 4 (Low), Priority 3 (Medium–Low), Priority 2 (Medium–High), and Priority 1 (High).

Table 1 – Evaluation criteria for the concrete quality index. Concrete thickness reduction ≥ 20 cm

0 cm < Concrete thickness reduction ≤ 20 cm

No concrete thickness reduction MEDIUM

f ck ≤ 10 MPa

LOW LOW

LOW

10 MPa < f ck ≤ 20 MPa

MEDIUM

HIGH HIGH

f ck > 20 MPa

MEDIUM

HIGH

As shown in Figure 2, two cases may require the execution of VAL4 assessments: • case A: lining inspection segments with a confirmed High attention class and no available information on the stress state of the lining; • case B: lining inspection segments with a confirmed High or Medium-High attention class that has not been downgraded because the lining is found to be loaded.