17 / 2025-04-24 11:27:19

A case study of safety progressive reduction of bridge piers due to local scour

The pier on the hydraulic left of the A. Vespucci bridge on the Arno River in Florence was subject to strong local scour phenomena that compromised its stability and safety, requiring the closure of the bridge in 2019 and the implementation of appropriate consolidation interventions (e.g. the installation of 100 piles in the riverbed). The pier foundation is a caisson on piling; specifically, 106 micropiles, each with diameter 0.4 m and 15 m length.
Following recent (2018) bathymetric surveys along the riverbed, it is observed that: i) 15÷21 micropiles of the pier foundation were deemed inefficient and/or absent; ii) the pier was subjected to a maximum localized scour of 7.85 m.
In this last regard, on the basis of the bathymetric surveys carried out over the years by the Arno River Basin Authority, it was possible also to determine the evolution of the erosion processes (by contraction and localized) in the riverbed, in correspondence with the A. Vespucci Bridge, from the year of construction completion to 2017.
The application of design procedures for bridge pier foundations available in the technical literature has allowed to highlight the progressive reduction of the safety factor of the A. Vespucci bridge pier foundation depending on the coupled effects of degradation (or defects) of the foundation itself (i.e. absence of micropiles) and local scour. Specifically, the study performed and described in the paper highlights (in terms of safety factor) the effects of:
 presence of defects in the micropiles (missing/absent piles);
 distribution of defects in the foundation (symmetric or asymmetric absence of micropiles);
 presence/absence of localized scour;
 combination of the previous conditions.