PSI - Issue 84

Lorenzo Brezzi et al. / Procedia Structural Integrity 84 (2026) 473–480

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allowed the recognition of unstable processes not recorded in existing inventories and highlighted that the potential interaction mechanisms mainly involve the bridge approach zones, which are spatially limited but critical for infrastructure safety. This example shows how mitigation works alone may provide essential insight into potential landslide–bridge interaction mechanisms, even in the absence of direct evidence of past impacts.

Fig. 6. Main signs observed during the inspection of Viaduct C (a) the presence of rockfall nets i and ii near the bridge abutments; (b) detail of the slope in which the rockfall net i is placed; (c) detail of the slope in which the rockfall net ii is placed.

2.5. Viaduct D

Fig. 7. Main cartographies related to landslide risk (IFFI and PAI) regarding the context of Viaduct D.

The final viaduct is affected by a landslide phenomenon that is both mapped and clearly observable during on-site inspection. Although classified in existing inventories as collapses or topples (Fig. 7), field evidence indicates that the actual process is more consistent with a debris flow. The phenomenon was identified directly from the bridge due to the large volume of debris in the riverbed and the presence of a selective check dam more than four meters high, located approximately 200 m upstream (Fig. 8a and 8b). UAV surveys allowed the identification of a probable accumulation and source area approximately 2 km upstream (Fig. 8c), characterized by large boulders. At the time of inspection, nearly two meters of debris, including coarse material and woody elements, were observed upstream of the check dam, significantly reducing its interception and hydraulic capacity. These observations indicate recent activation of the mitigation measure and confirm ongoing activity of the landslide process. Based on these elements, the potential evolution of the phenomenon was assessed. Under intense rainfall, localized collapses and rockfalls at the source area may initiate a debris-flow process through entrainment and acceleration along steep slopes. While part of the material would be retained by the check dam, residual debris could still reach the