PSI - Issue 84

Antonio Fiorentino et al. / Procedia Structural Integrity 84 (2026) 749–756

752

At the toe zone, where compression stress is prevalent, the earthflow takes on a lobate shape, creating debris accumulations, which evolve with a retrogressive dynamic and create passive failure surfaces due to the longitudinal compression of the material (Sdao and Simeone, 1997; Doglioni et Al. 2013). This geomorphological configuration, generating itself in the presence of a more rigid and resistant material at the toe and a flow coming from upslope, is meaningful of different activation phases of the earthflow. Given the large extent of the phenomenon, it was particularly difficult to reconstruct and, above all, to obtain effective field confirmation of the evolution of the earthflow toe. From the analysis of geomorphological survey and of the cross-section, clear evidence is provided of a distinct debris accumulation around 900 m large, at an elevation of approximately 225–250 m a.s.l., in an area characterized by a gentle, almost flat slope. The study carried out on this case suggests that this area has not been affected by the most recent climate-induced reactivations of the earthflow, unlike the sectors upslope of the viaduct, that condition the geomorphic evolution of the earthflow. The phenomenon experienced several reactivations and a retrospective reconstruction of the main reactivation phases over the last 10–15 years was carried out in order to clarify the recent evolution of the earthflow body and its progressive interaction with the viaduct, as well as the possible role of rainfall episodes associated with the reactivations (Vassallo et Al., 2016). A multi-temporal analysis of Google Earth satellite imagery was performed to investigate the evolution of the affected area. This diachronic approach allowed the identification of four key observation intervals that are particularly representative of the geomorphological development of the earthflow. In 2006, no clear morphological evidence of earthflow activity was detected. In 2015, the first indications of sediment accumulation within the gully became visible, although no major movement features were apparent. In 2019, the onset of more evident activity was recognized, together with a distinct accumulation of displaced material near the viaduct. Finally, in 2023 as better discussed later, the earthflow exhibited a further increase in mobilized material, with the development of depositional lobes along the landslide channel. The previously well-defined gully was progressively infilled by the flowing material, indicating a significant advance and a more mature stage of earthflow evolution. 5. Earthflow – Viaduct interaction A viaduct of the National Road SS653 was built to bypass the earthflow. However, this viaduct, due to the geomorphological evolution of the earthflow, started to be pushed by the earthflow itself. The debris deposit close to the bearing beams along the earthflow channel (Fig.3 a and b) is a direct indicator of the recent activity of the mass movement and clearly points to a critical condition. In the absence of appropriate and timely structural measures, this situation may compromise the overall safety of the infrastructure. The in-situ survey and photogrammetric analysis documents a rigid downslope displacement of the deck on its bearing plane (Fig 3a), while the abutment remains essentially unchanged. This implies relative sliding at the bearings and a possible exceedance of design tolerances, with a consequent non-symmetrical redistribution of actions on the supports. No significant rotations of the box girder are detected, suggesting a predominantly translational kinematic mechanism along the constraint. Therefore, the viaduct exerts a local buttressing effect (counterthrust induced by the accumulation and by the presence of the structure) that reduces the gradient and the transport capacity. Such “stabilization”, however, is only apparent: under specific rainfall conditions, an increase in pore-water pressure u within the shear band reduces the effective normal stress and therefore the mobilizable residual shear strength. The deck translation induced by the earthflow is of about 37 cm (compared to 10 m of the viaduct section), (Fig. 3a and 3c) and currently the implications for the structural response of the viaduct area quite limited. Anyway, relative sliding at the bearings may lead to exceedance of design tolerances, progressive deterioration of bearings and restraint devices, non-uniform load transfer among supports, and misalignment-related issues, with consequences for serviceability and maintenance. In particular, the combination of longitudinal push and reduced residual seating length may trigger local failure mechanisms of restrainers and, in the worst case, loss of support (unseating) of the deck (AASHTO, 2007; Xiang et al., 2019).