Risk of falling stone material on the Ramps of Piazzale Michelangelo in Florence

The Ramps of Piazzale Michelangelo (Michelangelo Square) in Florence are a pedestrian connection between the Arno riverside and the higher Piazzale Michelangelo, leaning against the side of the hill called "Monte alle Croci," a hill delimiting Florence to the south. Over the years, this area has experienced a long series of instabilities that have affected some of the most significant testimonies of the city's architecture. The Ramps of Piazzale Michelangelo are popular both with tourists visiting the city and with the residents of Florence, who use them as a picturesque place to stroll.

The Ramps and contiguous Viale dei Colli (Hills Avenue) are one of the most interesting projects of the architect Giuseppe Poggi during the period when Florence was the capital of Italy (1865-1871). The staircases of the Ramps feature stone balustrades, topped with Pietraforte sandstone caps. Geologically, Pietraforte is a turbiditic sandstone characterized by numerous sedimentary layers typical of the Bouma sequence and by the presence of secondary calcite veins. These features are the main weak points where detachment phenomena can occur.

The action of rainwater leads to the dissolution of calcium carbonate, present both in the calcite veins and in the carbonatic cement within the rock. In the first case, this mechanism can result in the decohesion of the rock with the complete opening of veins and possible detachment and fall of blocks (even blocks with volume up to about 50 dm³). In the second case, detachments occur as a superficial exfoliation rather than detachments of entire portions of material.

Recent restoration works, completed in May 2019, focused on the conservation and recovery of architectural elements. However, in July 2020, a wedge collapsed, hitting a vehicle below. Subsequently, as a temporary countermeasure, the parapets were covered with nets to prevent new possible accidents.

For a long-term countermeasure, this architectural problem has been investigated assimilating it to a rockfall scenario. First of all it was necessary to manually detect and evaluate every block's discontinuity to assess susceptibility. An equation for calculating the risk of each identified block was then implemented. Differentiated interventions were proposed for each block based on its possible kinematics.

Using an approach based on statistical analysis of the rockfall’s susceptibility, this study aims to: 1) Quantify the spatial distribution of rockfalls; 2) Build an equation to identify the more dangerous blocks; 3) Propose safety interventions with minimal impact, diversifying them based on the kinematics of each individual block.

 

Keywords: Cultural Heritage, Stone element risk, rockfall, road safety, susceptibility.