PSI - Issue 84

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

477

interception structures were observed along the stream path, aimed at mitigating debris-flow transport from upstream areas. The inspection also revealed large debris accumulations near the piers and the deck, located at a lower elevation with respect to the surrounding terrain, supporting the hypothesis of solid transport along the channel. A critical aspect of this case is the observed condition of the mitigation systems. Several nets along the stream path were partially or completely filled with debris (Fig. 4b), and some anchoring elements were found partially exposed (Fig. 4c), indicating past events and a progressive reduction of the system effectiveness. This case illustrates how hydrological indicators and the degraded condition of mitigation works may reveal an ongoing susceptibility to rapid processes that would otherwise be underestimated in a Level 2 assessment.

Fig. 4. Main signs observed during the inspection of Viaduct B (a) a widespread emergence of water and numerous shallow instabilities; (b) the stream path that could become a channel for solid transport; (c) critical condition of the nets along the stream path.

2.4. Viaduct C

Fig. 5. Main cartographies related to landslide risk (IFFI and PAI) regarding the context of Viaduct C.

The third case effectively illustrates how mitigation works may represent the primary source of information for landslide risk assessment when direct evidence of instability is limited. Although no rapid phenomena are recorded in the IFFI inventory (Fig. 5), several potentially hazardous processes were inferred during the inspection, all characterized by high expected velocity. These processes are suggested by the steep slopes, lithological conditions, and by the type and distribution of mitigation measures observed on site (Fig. 6a). Vegetation extensively covers the slopes surrounding the bridge; however, exposed rock walls (Fig. 6c) and localized soil-covered sectors (Fig. 6b) remain visible, where cortical nets are installed to protect the abutments and the roadway from potential rock detachments or shallow landslides. Additional rockfall nets are also present to protect both the bridge abutments and the downstream road sections. Notably, no evidence of past impacts was observed during the inspection, despite the extent of the mitigation measures, suggesting that the interventions were likely designed following previous instability episodes. The inspection therefore