PSI - Issue 84

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

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bridge. At the time of inspection, however, no structural damage was observable, as the interaction remains conditional upon the activation of a rapid process. This case demonstrates how an active landslide process can be reliably assessed at Level 2 through indirect geomorphological evidence and mitigation-system performance, even when the structure shows no visible distress. It also exemplifies how a rapid process may remain apparently disconnected from the structure under ordinary conditions, until a triggering event activates a kinematically compatible path toward the bridge, potentially leading to sudden and severe consequences.

Fig. 8. Main signs observed during the inspection of Viaduct D (a) the part of the debris flow channel near the bridge; (b) the 4 m high check dam placed 200 m upstream of the bridge; (c) the probable accumulation zone of the debris. 3. Discussion and conclusion The inspection experiences discussed in this paper confirm that landslide risk assessment at Level 2 is strongly influenced by how field evidence is interpreted, rather than by the mere availability of cartographic information or the presence of observable structural damage. Although the Italian Guidelines provide a structured framework for the classification of landslide-related risk, the analysed cases show that specific conditions may lead to a systematic underestimation of hazard, particularly in contexts dominated by rapid slope processes whose interaction with infrastructure may remain latent until a triggering event occurs. As discussed in the introduction, this behaviour contrasts with that of slow-moving, large-volume instabilities, which tend to produce progressive and persistent structural distress over time. Rapid rainfall-triggered processes, by contrast, may leave little or no structural evidence prior to activation, even though they can directly impact the structure and cause severe damage once triggered. A recurring outcome of the case studies is that cartographic inventories and hazard maps, while essential at the census stage, often provide an incomplete representation of the instability processes affecting bridge surroundings. This limitation is especially relevant for rapid landslides, whose surface evidence may be transient, spatially discontinuous, or concealed by vegetation. In such situations, reliance on mapped information alone may result in the erroneous assumption that landslide risk is negligible. Field inspections play a decisive role in overcoming these limitations, provided that geomorphological indicators and contextual evidence are interpreted in an integrated and critical manner. Indirect indicators such as debris accumulations near foundations, absence of vegetation, diffuse water emergence, and the spatial recurrence of shallow instabilities proved to be key elements for recognizing potentially hazardous processes not explicitly recorded in existing inventories. An important outcome of the presented experiences concerns the spatial extent of inspections. The identification of latent landslide threats frequently required observations beyond the immediate vicinity of the structure, including upstream or upslope areas that are not directly visible from the bridge. Potentially unstable niches, accumulation zones, or source areas were sometimes located at a considerable distance from the structure, yet were kinematically compatible with possible interaction mechanisms with the bridge. This confirms that Level 2 inspections should not be limited to the structure itself or to a narrow surrounding area, but should allow, where feasible, the investigation of a wider portion of the slope, potentially supported by UAV surveys. The role of mitigation works emerges as particularly significant. Rather than acting solely as protective elements, mitigation measures consistently provided valuable insight into past or recurrent instability processes and into potential landslide–bridge interaction mechanisms. Their condition and degree of effectiveness proved to be informative indicators of ongoing susceptibility. Assuming mitigation works as inherently risk-reducing elements,