PSI - Issue 84

Alessandro Scala et al. / Procedia Structural Integrity 84 (2026) 489–496

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3. Signs of landslide-structure interaction Although the signs observed during inspection are not assigned a specific cause within the method, it is still useful to contextualize them and understand why they are associated with interaction with landslide phenomena. For this type of evidence, it is first necessary to distinguish between the two main categories of landslides, namely slow and rapid landslides (Hungr, 2007). This distinction is a great simplification but becomes crucial, as the range of signs generated by each category is completely different. Rapid landslides, such as debris flows, rockfalls, or rock collapses, interact with the structure almost instantaneously, striking it suddenly and causing displacements or partial or total collapses, thus posing a serious hazard to users (Brezzi et al., 2021). For this reason, it is rare, if not unlikely, to identify precursor signs of interaction directly on the structure. What can instead be observed on the structure, and especially in its surrounding context, are signs caused by similar events that occurred in the past, which therefore represent important precedents. These observations may indicate the site’s predisposition to generate instability events. Signs of this type can include, for example, evidence of debris impacts on the structure or in its surroundings (Fig. 1c), material accumulation on the roadway, or at the base of the piers (Fig. 1a). Actual debris flows may also be observed (Fig. 1b), which could reactivate if debris accumulates upstream. The presence of precariously positioned rock masses or mitigation measures, ranging from rockfall (Fig. 1d) nets to simple warning signs, also represents an indication of the context’s predisposition to instability. Finally, signs of erosion near the foundations or in the surrounding area can be detected, potentially posing a significant threat to the safety of the structure.

Fig. 1. Examples of signs of rapid landslide-structure interaction (a) accumulation of debris; (b) a visible past debris flow; (c) evidence of impact on the road surface; (d) rockfall nets near the bridge.

The situation is different for slow-moving phenomena, which can range from superficial movements to deep translations or rotations, potentially involving hundreds of thousands of cubic meters of soil and generating forces on the structure or its foundations. Although the volumes involved are generally larger than in rapid phenomena, and thus have the potential to cause greater damage, they often require less urgency. This is mainly due to two factors: the rate at which deformations develop, caused by the horizontal forces of the landslides, is lower, making sudden collapse less likely; additionally, the ability to observe such evidence on the structure or in its surrounding context provides sufficient time to plan interventions to mitigate the risk. These signs vary depending on the type of structure, the type of phenomenon, and the type of interaction. Hyperstatic structures, such as arch bridges, are capable of distributing applied stresses, deforming plastically, and generating hinges. In contrast, simply supported bridges primarily show interaction evidence on the connecting elements, such as expansion joints or bearings, or on the more rigid components, such as abutments, piers, and foundations. The main cause of the signs listed below is soil thrust, but they can also result from settlements or scouring (Di Marco et al., 2022), particularly in the case of differential subsidence. Foundation movements, such as translation or settlements, can be identified by the loss of verticality of the piers or abutments resting on them, which in extreme cases may lead to curb misalignment, loss of longitudinal alignment of the spans, and the appearance of horizontal cracks on the piers themselves. Cracks generally represent a sign that frequently appears in hyperstatic structures and can be vertical, oblique, or branched. However, this type of signal is not always caused by the interaction between the structure and a landslide phenomenon, especially in more complex cases. The correlation is more likely if cracks are also observed on the roadway, along mortar joints, if masonry elements are missing or damaged (Fig. 2a), or, in the most severe cases, if the tympanum detaches. For simply supported structures, in addition to the previously described signs, distress can be identified by examining the connecting elements. Bearings, for example, may show cracks, locking, or deformations. The latter