Identifying Indirect Landslide Threats to Infrastructure: Insights from the Experience in the Belluno Province

Inspections play a key role in the risk assessment of bridges and viaducts, particularly when dealing with landslide-related hazards. The most commonly recognized interaction mechanism between landslides and bridges is the direct one, typically associated with slow-moving phenomena affecting a significant portion of the foundation system and leading to detectable structural distress. However, other landslide processes may interact with infrastructure through interaction mechanisms that may remain latent until activation. This is the case for shallow rainfall-triggered landslides, debris flows, or rockfalls, which may impact a structure only following sudden activation under specific triggering conditions. For these processes, the information available during routine inspections is often insufficient to identify potential threats, as source areas are frequently located upslope, hidden by dense vegetation or complex geomorphological settings. Moreover, the structure may exhibit no visible signs of distress, as no load has yet been applied and the hazard remains latent rather than manifest. In such cases, a careful geomorphological analysis, together with a preliminary assessment of potential runout paths and impact mechanisms, becomes essential. With specific reference to Level 2 assessments within the Italian Guidelines framework, this paper presents lessons learned from a series of field inspections carried out in the Belluno province, northeastern Italy. This mountainous region is characterized by widespread slope instabilities, frequent rainfall-triggered processes, and challenging accessibility conditions. The experience gained highlights key aspects for recognizing predisposing conditions, interpreting indirect geomorphological evidence, and identifying early warning signs of potential landslide–bridge interaction, even in the absence of observable structural damage. These insights aim to support inspectors in developing more informed and effective inspection strategies for bridge and viaduct safety in landslide-prone areas.