PSI - Issue 84

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

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bridge or its users. Additional evidence emerged only after descending below the piers and through UAV inspections. In particular, debris accumulations were observed near the pier footings (Fig. 2a), suggesting past or ongoing activity. Moreover, in two locations, the absence of vegetation allowed the identification of a shallow landslide detachment niche and a debris-flow channel (Fig. 2b), whose source area could not be directly inspected due to limited accessibility. Among the observed phenomena, the shallow landslide was used for the evaluation of the landslide related Class of Attention at Level 2, as the clarity of the indicators allowed a reliable estimation of volume, direction, and kinematic compatibility with the structure, also highlighting the strong link with meteorological triggering conditions. This case highlights how indirect geomorphological evidence, even in the absence of mapped phenomena or structural distress, may be sufficient to substantially revise the outcome of a landslide risk assessment.

Fig. 2. Main signs observed during the inspection of Viaduct A (a) debris near the pier footing of the bridge; (b) two zones with absence of vegetation due to a shallow landslide and a debris flow; (c) some mitigation structure near the bridge: a rock wall stabilized with shotcrete and a rockfall gallery.

2.3. Viaduct B

Fig. 3. Main cartographies related to landslide risk (IFFI and PAI) regarding the context of Viaduct B.

Unlike the previous case, available cartography indicated the presence of instability affecting the bridge area, although with a limited and approximate representation of the affected surface (Fig. 3). During the inspection, widespread water emergence was observed both upstream and downstream of the structure. These conditions have promoted the development of numerous shallow instabilities, identifiable by darker soil coloration and detachment niches (Fig. 4a), distributed along the slopes and the stream corridor. The spatial recurrence of these phenomena represents a strong indicator of the susceptibility of the local soils to instability, particularly under intense rainfall conditions. Although not precisely mapped, the susceptibility of the site to rapid processes was further confirmed by the presence of several mitigation measures. Cortical nets were installed near the left-bank abutment, while additional