PSI - Issue 84

Lorenzo Brezzi et al. / Procedia Structural Integrity 84 (2026) 1159–1166

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sector between approximately 740 and 840 m a.s.l. is associated with thinner debris cover and sparse vegetation. This structural and geomorphological configuration promotes planar and wedge failures, as well as shallow landslides capable of evolving into rapid flow-like phenomena. The susceptibility of the area to rapid instability processes is clearly documented by official inventories and hazard zoning. The Italian Landslide Inventory (IFFI) reports multiple rapid flow phenomena and shallow landslides in the immediate vicinity of the viaduct (Fig. 1b), while the Hydrogeological Basin Plan (PAI) classifies the entire slope as subject to very high hydrogeological hazard (P4), indicating a context of severe and recurrent instability (Fig. 1c). Historical evidence confirms that rapid instability phenomena have already affected the infrastructure. In 2003, an event characterised by intense solid transport caused damage to protection works and erosion at the viaduct abutment embankment, triggering the implementation of several stabilisation and protection measures along the slope. Despite the progressive reinforcement of the mitigation system, a residual hazard persists, as demonstrated by a documented rockfall event in 2012, when a block overcame multiple rows of protection nets and reached the roadway. These observations, together with the geomorphological setting and inventory data, support the selection of the site as a representative case study for investigating the indirect interaction between rapid landslides and bridge infrastructure.

Fig. 1. (a) View of the analysed highway viaduct and its proximity to the unstable slope; (b) landslide inventory map from the Italian Landslide Inventory (IFFI); (c) landslide hazard map from the Hydrogeological Basin Plan (PAI). 3. Modelling framework The numerical modelling framework adopted in this study is aimed at investigating the runout of rapid landslide scenarios and their potential indirect interaction with the analysed viaduct under conditions characterised by large deformations and significant parametric uncertainty. Given the need to simulate post-failure dynamics involving mass rearrangement, fragmentation and flow-like behaviour, a particle-based, meshless approach was selected. In particular, the Smoothed Particle Hydrodynamics (SPH) method was adopted (Pastor et al., 2014), as it allows the simulation of large displacements without the numerical limitations associated with fixed mesh connectivity and is well suited to the simulation of post-failure landslide propagation. The analysis focuses on the exploration of plausible interaction scenarios rather than on the back-analysis of a specific historical event. Two potential initiation scenarios were defined on the basis of geomorphological evidence, landslide inventory data and the interpretation of high-resolution orthophotos, with the aim of identifying zones characterised by recurrent instability and potential detachment conditions (Fig. 2). The first scenario represents a single-source debris-flow configuration, consistent with the most frequently observed instability patterns in the area. The second scenario, instead, considers a more severe configuration involving the simultaneous activation of multiple source areas, representative of particularly extreme triggering conditions. This latter scenario is intentionally conceived as a conservative, worst-case configuration to investigate how multiple contemporaneous detachments may modify the extent and characteristics of landslide– infrastructure interaction with respect to a single-source event. Due to the absence of site-specific calibration data, the rheological behaviour of the mobilised material was described using a Voellmy-type model (Voellmy, 1955), with parameter values derived from ranges reported in the literature (Sosio et al., 2008). The uncertainty affecting the governing rheological parameters was explicitly addressed