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La difesa della città di Venezia e dell’ecosistema lagunare: una sfida continua per l’ingegneria geotecnica

Simonini Paolo
Articolo Immagine
Rivista Italiana di Geotecnica
Rivista Italiana di Geotecnica N. 3/2023

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La città di Venezia è caratterizzata da un equilibrio piuttosto precario con l’ambiente lagunare circostante, con un margine di sicurezza eroso a ritmo crescente. La velocità di degrado ambientale è accelerata dalla crescente frequenza delle inondazioni della città storica e della laguna circostante, ora protette dalle barriere mobili. Ciò e dovuto all’innalzamento eustatico del livello del mare, indotto dai cambiamenti climatici, dalla subsidenza naturale e da una subsidenza antropica, quest’ultima particolarmente importante nell’ultimo secolo. L’articolo, basato sui contenuti della Croce Lecture tenuta a Roma il 16 dicembre 2021, sottolinea il ruolo rilevante dell’ingegneria geotecnica, e, in particolare, delle indagini in sito e in laboratorio, del monitoraggio, dell’analisi e dell’interpretazione dei dati e della modellazione matematica di problematiche geotecniche rilevanti per la conservazione della città e della laguna circostante.

The defense of the city of Venice and of the lagoonal ecosystem: an ongoing challenge for geotechnical engineering

In the city of Venice and surrounding lagoon, which stand in such close contact with the sea, geotechnical issues have assumed a significant role. In fact, its historical buildings, extraordinary characteristics, as well as its future existence, strictly depend on the lagoonal environment and its evolution. Firstly, the natural tidal variation of the sea level within the lagoon affects the city of Venice each day, with the margin of security of this rather precarious equilibrium eroding at an ever-increasing rate. The rate of environmental deterioration is being accelerated by the increasingly frequent flooding of the historical city, caused by rising sea levels due to climate change, by natural subsidence and by a local maninduced subsidence, brought about particularly between 1946 and 1970. The ever-larger gap between the level of the sea and the surface of the islands has caused a loss in ground elevation of 26 cm from the beginning of the last century. This phenomenon became particularly clear when, on the 4th November 1966, an exceptional flood took place, during which the entire city and surrounding islands were completely submerged by a tide that lasted for many hours. The impact on the city of the 1966 flood was so momentous that it induced the Government of the Italian Republic to enact a Special Law for Venice (Law no. 171 of 16 April 1973) aimed at the safeguarding of Venice and its Lagoon (Interventi per la salvaguardia di Venezia / Interventions for the Safeguarding of Venice). This represents the first organic legislation enacted after the dramatic flood of 1966. At around the same time, the Italian Government, along with the National Research Council, commissioned the Italian Oil Company Agip Mineraria to dig a 950 m deep borehole in the Venice area, which was intended to be a preparatory study on the relevance of the subsidence phenomenon. The sampled sediments, which cover a time interval from over 2 million years ago to today, provided material for studies that were at the basis of the subsequent, increasingly detailed research on the evolution of the Venetian territory The 950 m deep borehole was integrated by two other boreholes, the 120 m deep VE1-bis, and then the 400 m deep VE-2, to study the aquifer and aquitard system and to measure geotechnical properties useful for an evaluation of the subsidence problem. During the central decades of the last century, the industrial area of Mestre developed on the closest mainland and, consequently, a large-scale water extraction was performed from the 350 m deep aquifers underlying Venice, reducing the piezometric levels in the deep aquifers and generating ground-surface settlements measuring up to 150 mm throughout Venice over the same period. Aquifer depletion was halted in 1973, and so stopped the associated ground subsidence as well. By then, however, enormous damage had been done. Alongside anthropogenic subsidence, slight natural subsidence also takes place. Natural subsidence is caused by a combination of secondary compression, prevalently occurring in the upper soil deposits, as well as by processes of oxidation of surface peaty soils, which affect especially the marshes and peatlands subjected to tidal cycles. To keep the evolution of subsidence under control, continuous surveying has continued up to nowadays. Several interventions were planned by the Special Law in Venice, such as restoration of quay walls, raising of the islands’ pavements, protection and reconstruction of salt marshes, and the most important intervention, the enormous project MoSE (an acronym for Modulo Sperimentale Elettromeccanico, Experimental Electromechanical Module) involving the design and construction of movable gates located at the three lagoon inlets. These gates, controlling the tidal flow, temporarily separate the lagoon from the sea at the occurrence of particularly high tides, thereby protecting the historic city. New geotechnical investigations were therefore carried out to characterize Venetian soils at the inlets to achieve a suitable design of the movable gate foundations. The Geotechnical Group of the University of Padova, and especially myself, have been involved in planning a small but meaningful part of these investigations, which were based on the recent advancements in geotechnical laboratory and site testing. From the geological and geotechnical investigations carried out, it has been found that the lagoon soils are characterized by a predominant silt fraction, combined with clay and/or sand. These form a chaotic interbedding of different sediments, whose basic mineralogical characteristics vary slightly, as a result of similar geological origins and common depositional environment. This latter feature, together with the relevant heterogeneity of soil layering, suggested that research concentrate on some selected test sites, considered as representative of typical soil profiles, where relevant in-situ and laboratory investigations could be carried out for a careful characterization of the Venetian lagoon soils. The first test site, namely the Malamocco Test Site, was determined in the early 1990s at the Malamocco inlet. Within a limited area, a series of advanced site investigations that included boreholes with undisturbed sampling and laboratory testing, seismic piezocone, dilatometer, self-boring pressuremeter and cross hole tests were performed on contiguous verticals. In addition, a continuous borehole was carried out for very careful soil mineralogical classification. The comprehensive laboratory test program completed at the Malamocco Test Site emphasized the very heterogeneous, high silty content and low-structured nature of the Venetian soils, emphasizing the difficulty to characterize even the simplest mechanical properties with a certain degree of accuracy only from geotechnical laboratory tests. In the early 2000s, a second test site was therefore selected, namely the Treporti Test Site, which was located at the inner border of the lagoon, very close to the Lido inlet. The goal of this new site was to measure directly in-situ the stress-strain-time properties of the heterogeneous Venetian soils. At the Treposti Test Site a vertically walled circular embankment was constructed, thus measuring, along with and after the construction, the relevant ground displacements together with the pore pressure evolution. To this end, the ground beneath the embankment was heavily instrumented using plate extensometer, differential micrometers, GPS, inclinometers, piezometers and load cells. Boreholes with undisturbed sampling, traditional and special dilatometer and piezocone tests were employed to characterize soil profile and estimate the soil properties for comparison with those directly measured in situ. The results of this second special investigation were particularly relevant to accurately calibrate site testing techniques, to measure relevant soil properties and to formulate and calibrate and propose new constitutive models. Only a few of the findings of these studies are presented in the paper. Some major aspect concern, of course, the preservation of the cultural heritage of the historic city. Two cases are discussed in the paper. The first one concerns with the typical footings of historic Venetian buildings are possibly subjected to degradation. In fact, the buildings’ foundations rest on wooden planks or short, smallin- diameter wooden piles, embedded at a very small inclination. Modest in size and closely spaced wooden piles help to improve the mechanical behavior of the soft clayey silt, which characterizes the shallowest layer of the Venice lagoon, thus reducing the expected settlements. Wooden piles were long believed to last indefinitely, as they are permanently waterlogged. Disproving this assumption, recent evidence shows that anoxic bacteria can seriously deteriorate wood even in anoxic conditions. The effects of wood deterioration on the mechanical behavior of the foundation over time were investigated and modelled by numerical analysis, showing that wood degradation leads to stress transferring from pile to soil causing an increase in settlements. The second one describes the condition of San Marco square, which is located on the lowest-elevated island forming the city, and therefore is often flooded during very high tide events. This is the most recent geotechnical study carried out in the Venetian area. In order to design cost-effective and non-intrusive protection interventions for San Marco square, a deep understanding of flooding mechanisms and the relationship between groundwater pressure and tidal oscillations was necessary. The paper presents and discusses the results of geotechnical surveys and measurements of pore pressure oscillations in the soil of Piazza San Marco. The results provided important information that guided the design of the project and could be of interest to similar coastal areas. Results showed that significant pressure oscillations occur in the subsoil, and should not be neglected when stabilizing horizontal architectural structures, such as historical mosaics and paving. All these diverse experiences emerge to provide an overall comprehension of the geotechnical issues related to this extraordinary city, inserted in its equally extraordinary lagoon environment. The information presented here are of course not be complete nor exhaustive.