PSI - Issue 62

Stefano Stacul et al. / Procedia Structural Integrity 62 (2024) 569–575 Stefano Stacul, Nunziante Squeglia, Nicola Perilli / Structural Integrity Procedia 00 (2019) 000 – 000

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The quantitative statistical method allows the weight of the various factors that contribute to instability to be determined through bi-variate or multi-variate analyses. It is based on the principle that areas in which landslides have occurred in the past will very likely be affected by landslides in the future and that areas characterized by predisposing conditions similar to those already affected by landslides have a greater proneness to trigger landslides. The bi-variate statistical method was applied by the former Puglia Regional Basin Authority. The merit of this method is that the assessment of landslide susceptibility can be done in an objective and reproducible manner, moreover, it can be used to classify the entire territory. While the main limitation is that the quality of the results is strongly affected by the accuracy of the input data. Hybrid methods, instead, have been applied by the former Basin Authority of the Arno River, by the Regional Basin Authorities of Liguria, Lazio, Southern Campania and by the Sele Interregional Basin Authority. ISPRA collects all the updated data from the District Basin Authorities, then analyzes the methodology and the landslide hazard classification adopted by each Basin Authority and finally applies a Landslide Hazard Reclassification Table for each PAI to classify the entire Italian territory according to the 5 classes of the national LHM: PF4, PF3, PF2, PF1, AA. The final LHM is clearly affected by significant inhomogeneities (Figure 1), mainly due to the different methodologies used for the assessment of landslide hazard. 3. Comparison between LHM classes and the landslide hazard identified during the inspection In the last 3 years the FABRE Consortium has carried out a huge number of inspections of road/highway bridges and viaducts throughout Italy. This work reports information relating to the landslide risk of 467 bridges, of which approximately 150 located in the provinces of Lucca and Pistoia (a total of 208 bridges in the Tuscany Region), 61 in the Calabria Region, 46 in the Marche Region, 41 in the Puglia Region, 37 in the Umbria Region, 21 in the Emilia Romagna Region and the remaining 53 in the Veneto, Sardinia, Liguria, Basilicata and Molise Regions (Figure 2). Following visual inspections carried out by the consortium staff, of the 467 bridges within the database 106 (23%) were considered to interfere with landslides or potential landslides (Figure 3, left). For 372 bridges (80%) the cartographic information (LHM) was consistent with the inspectors' assessment (regarding the presence or absence of landslide hazard). For 77 cases (16%) the LHM indicated the presence of landslide hazard not found on site by the inspectors (False Positives), while for the remaining 18 cases (4%) the LHM did not highlight the presence of hazard which instead was reported by inspectors (False Negatives) (Figure 3, right). Therefore, False Positives represent those cases in which the LHM provides a more conservative indication than that during the inspection, while False Negatives represent those cases in which only the inspector's expert assessment was able to recognize a condition of obvious or potential risk. The latter represents the most critical situation, as the correct classification according to the guidelines becomes strongly dependent on the inspector's experience and the quality of his activities (both in the desk study and in the field). Although False Negatives represent only 4% (18/467) of all bridges studied, it is important to highlight that they constitute 17% (18/106) of bridges in which a landslide hazard was identified. Still paying attention to the 106 bridges affected by landslides, it is interesting to note that the LHM identified for 61 (58%) the presence of PF3 or PF4 areas, for 16 (15%) PF2 areas, for 9 (8%) PF1 areas and for 2 (2%) AA areas (Figure 4, left). Of the 372 bridges not affected by landslides, 77 (21%) are False Positives, however, in only 25 cases (7%) the LHM highlighted areas with hazard classes greater than or equal to PF2 (Figure 4, right). Looking at the geographical distribution of False Positives it is possible to highlight that these are mainly concentrated in the Puglia Region (26/77) and in the Tuscany Region (38/77), particularly in the province of Lucca. False Negatives, instead, are mainly located in the Marche (5/18) and Calabria (6/18) regions. These results suggest some reflections. From these initial data, False Positives appear to be more frequent in those territories in which the competent Basin Authority has made use of methods such as the geomorphological one (i.e., the Basin Authority of the Serchio River, which is the competent one for the territory of Lucca) or the bi-variate statistical one (i.e., the Puglia Regional Basin Authority). False Negatives, instead, are more frequent where the competent Basin Authority has made use of the qualitative matrix method (i.e., the Marche Regional Basin Authority) or in those Regions (such as Calabria) in which the