PSI - Issue 84

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

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In the EC quasi-static analyses with intact lining, nonlinear behaviour remains confined to the shoulder regions on both sides, while no plastic points develop in the central portion of the crown (Fig. 7a). When the larger shear strain obtained from the local seismic response analysis is adopted instead (SR case), plasticisation becomes significantly more widespread, forming an almost continuous band of tensile plastic points between the left and right shoulders (Fig. 7b). Introducing a 15 cm cracked inner layer leads to a further enlargement of the plasticised area and, most notably, to the onset of shear failure at the crown (Fig. 7c). The corresponding relative shear strength contours confirm this interpretation (Fig. 8), showing a broader mobilisation of the available shear resistance with increasing seismic demand and with the crown intrados fully plasticised.

(a) 10 cm intact – EC

(b) 10 cm intact – RS

(c) 10 cm degraded – RS

Fig. 7. Plastic points distributions: black dots represent tensile cut-off points and red dots represent shear failure.

(a) 10 cm intact – EC

(b) 10 cm intact – RS

(c) 10 cm degraded – RS

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Fig. 8. Relative shear strength τ rel (ratio between the actual shear stress and its maximum value) along the lining and crown portion.

5. Conclusions This study investigated the seismic response of an existing unreinforced concrete road tunnel affected by localised crown thickness reduction, with a focus on the role of the initial stress ratio and on the effect of typical pre-existing cracking in the inner portion of the lining. The main findings can be summarised as follows: • localised reduction in crown thickness significantly modifies the structural response of the lining under quasi static seismic loading, promoting upward distortion of the crown, stress redistribution, and a wider tensile plasticisation pattern extending from the shoulders towards the crown region; • the influence of initial horizontal stress coefficient K 0 on the global deformation pattern is moderate, but it is more relevant in terms of crown pre-compression. In particular, a lower value of K 0 leads to smaller static axial forces in the crown and therefore to a less favourable condition when seismic bending develops, making this the selected scenario for the subsequent refined analyses; • for the intact lining, the quasi-static analysis based on the Eurocode shear strain produces a plastic response that remains limited, with no evidence of shear crisis in the crown region. By contrast, when the larger site-specific