(Tabatha de la Rosa)
Venice is one of the most popular tourist destinations in the world, the thousands of tourists arrive to see its picturesque streets and walk by its canals. The city has a remarkable architecture that goes well beyond its looks, the buildings were carefully designed to withstand the physical conditions of the lagoon. Generations of caring Venetians have tended to the buildings, its streets and canals, however the sea level rise and the migration of the local residents to the mainland put the delicate city in danger.
Construction in Venice
The Venetians had to overcome three major challenges when building the city: soft soils, brackish water, and shortage of land. These challenges defined the engineering and architecture of the city through the building style the locals had to acquire in order to build in these conditions.
The soil under the Venetian lagoon can be broken down into five main layers, all of them have different stratigraphy, depth, strength and stiffness. The first layer of soil has an average thickness of 2 m. This first layer is characterized by a highly heterogeneous stratigraphy consisting of a discontinuous layering of clayey silts, clay loams, silty clays, and varying contents of coarse, and fine-grained particles.
The second layer of soil is made out of irregular stratigraphy of under to slightly overconsolidated silts, silty clay, clay loam, medium to fine sand, and some layers of un-compacted peat. The strength and stiffness of the layer are very low, the layer extends 4 to 8 m under ground level.
The third layer, known as caranto, consists of highly overconsolidated oxidized silty clay. The caranto has very high strength and stiffness but the layer is quite thin, 2 to 3 m, making it not able to bear high loads.
The fourth layer is composed of layers of fine silty sands, sandy silts, with arbitrary distribution and thickness. A few thin deep layers are overconsolidated making the overall layer moderately higher strength and stiffness compared to the second layer.
Starting from 50 to 60 m below ground level the stratigraphy becomes more homogeneous, a system of interbedded sands, silts, and silty clay deposits with high strength and stiffness. (Foraboschi
The system of layers presented above proved to not have the bearing capacity nor the compressibility of the soil necessary to construct buildings higher than two stories tall.
The Venice lagoon is made of brackish water, water with more salinity than fresh water but less than seawater, the salts found in the water are the same that are in the bricks. When bricks are put in contact with the brackish water, a solution of the salts of the brick with the ones of the water is formed. Later, when the water evaporates, the salts precipitate out of the solution leaving white powder like deposits of salt. Subflorescence, when the salt crystallization occurs inside the brick, induces cracks inside the brick that can end up with the crumbling of the brick surface. Also, efflorescence, when the salt crystallizes on the surface of the brick, further contributes to the crumbling of the bricks. The crumbling of the bricks is presented in figure #.
Subflorescence and efflorescence reduce the compressive strength of the bricks, therefore, contact between the lagoon water and the bricks had to be blocked.
Venice historically has attracted masses of people through time. During the 14th century, when the city was one of the wealthiest ones in the world, it was the most populous city in Europe. Since then, the city maintained a population higher than 100,000 up until the late 20th century.
Modification of the landscape such as burying canals, extension of shorelines, and creation of new islands were used to try to satisfy the demand. Although these efforts helped, it was still necessary to build high buildings to ensure housing.
When working with soft soils, it is common to use piles that are drilled to the ground as foundations, the piles are frequently used to transfer the loads of the building to stronger layers. Before the 20th century, the piles that could be installed were between 1.5 and 3 m long, not long enough to reach the caranto, the layer with the most strength and stiffness. Venetians used piles for a different purpose then, instead of transmitting the surface loads, the piles were used for ground densification, increasing the bearing capacity of the soil.
The typical density was 5 to 12 piles/m2, commonly made with larch or oak, with a diameter between 10 and 30 cm. The process of pile installation was highly expensive and labor intensive. Once the piles were installed, they were cut horizontally to provide a leveled platform and a zatterone, a wooden deck, was placed above them. (Bernabei et al. 2019) Figure 4 presents a simple schematic with the pile arrangement.
The piles which are made out of wood commonly rot in water, this is not the case in Venice. Although the piles are made out of water-resistant timbers, the reason behind their durability is the occurrence of two natural phenomena:
- Timber only rots when water and air are present. The piles are in an environment with little to no water.
- The water in the lagoon carries silt and silty clay. The piles adsorbed the sediment, petrifying them at an accelerated rate.
Although the inclusion of the piles improved the soil conditions to build taller buildings, the buildings still needed to be lightweight. To reduce the weight of the buildings, the walls were made as thin as possible. In the building process, facades would sit in stronger foundations compared to the rest of the building, leading to differential settling. The thinness of the walls caused stabilization issues, this combined with the differential settling required inclusion of fiube.
The fiube is a 15 to 20 cm tall, 50 to 70 cm wide and 20 to 30 cm deep istria-stone linked to an iron hook that serves as the connection between the walls and the inner floors. The positioning of the fiube was such that when the floor began to sink, the fiube would pull the facade. This action prevented the thin facades from deforming and allowed the building to sink without causing cracks (Doglioni 2012)
To further protect the facade, in this case of the brackish water, a cladding of istrian stone, which has a very low porosity was used. The low porosity of the stone makes it highly resistant to subflorescence and efflorescence.
Sea Level Rise (SLR)
Venice is no stranger to the change of the sea level, first reported in 589 AD, acqua alta has been part of the life of Venetians for generations. Acqua alta occurs during the winter when the astronomical tides combined with strong south wind (Scirocco) cause large amounts of water to inflow into the Venice lagoon (Gugliuzzo 2017). One of most memorable acqua altas is the one of November 12, 2019. The event took place due to the overlap of four phenomena: the peak of the astronomical syzygy tide, high sea level in the Adriatic ocean, strong Scirocco and a small cyclone of small dimensions. Figure 7, presents in blue the contribution of the weather, in a dotted line the contribution of the astronomical tide and in red the water level recorded.
Although the acqua alta is a recurring seasonal increase of the water level, there is an overall trend to an increased SLR. The historical sea level rise in Venice can be studied quantitatively by using the paintings of Veronese, Belloto, and most famously Canaletto. The paintings were made using the camera obscura technique, which consists of tracing contour lines thanks to a light beam and applying different perspectives to the elements of the painting. The technique is known for producing real and accurate images (Camuffo et al. 2014). Figure 8, presents a Canaletto painting from 1740 compared to figure 9, a picture taken in 2019. Although the view points are different, it can be qualitatively noticed that the water level has risen by the reduced amount of steps that lead to the water level.
The use of paintings provides us with visual proxies to the 1700’s, however, the comune di Venezia has publicly available data that shows the steady increase in Venice’s mean sea level. Figure 10, shows in red the change in the average sea level in Venice and in blue the notable acqua altas.
It is important to note the inclusion of MOSE, a system that extends for 1.6 km made out of steel flap-gates. When a high water level is predicted, the gates that are usually lying on the floor are raised by introducing compressed air. Multiple studies have been conducted on the performance of the MOSE system. According to Umgiesser, for a SLR of 50 cm, the gates would need to stay closed between 300 to 430 times per year. Such extenuating use of the gates will have negative effects on the ships that arrive at the industrial and touristic ports. Lastly, MOSE is a preventative measure with a lifetime of 100 years, leaving the city to find other alternatives to preserve itself.
Looking into the future
As shown in figure 3, the population that inhabits the historic center of Venice started to decline in the second half of the 20th century. During summer 2022, I had the opportunity to take a guided tour with Engineering Rome across the city. The tour guide who had been living in the city for years, talked about memories of playing children, street markets, and a community that has been fading away. As we walked through the city it was noticeable the disproportionate ratio of tourists to locals and there is a kind of emptiness that lingers among the streets that are not reached by the crowds of tourists. Nevertheless, there is a love and resilience that the locals show towards the city that with the support of robust governmental policy can help them keep the streets alive. When it comes to the rising sea level, MOSE is not going to be able to protect the city from it, new long term solutions will need to be explored. Venice has proven to be a resilient city and the same attention and creativity that built it may be able to save it.
Bernabei, Mauro, et al. “The Wooden Foundations of Rialto Bridge (Ponte Di Rialto) in Venice: Technological Characterisation and Dating.” Journal of Cultural Heritage, vol. 36, Mar. 2019, pp. 85–93, https://doi.org/10.1016/j.culher.2018.07.015.
Camuffo, Dario, et al. Science, Technology and Cultural Heritage : Proceedings of the Second International Congress on Science and Technology for the Conservation of Cultural Heritage, Sevilla, Spain, 24-27 June 2014. Edited by Rogerio-CandeleraMiguel Ángel and George Clapp, Crc Press/Balkema, 2014, pp. 1–18, doi-org.offcampus.lib.washington.edu/10.1201/b17802.
Comune di Venezia. “Le Acque Alte Eccezionali.” Comune Di Venezia., 3 Apr. 2017, www.comune.venezia.it/it/content/le-acque-alte-eccezionali.
—. “Variazioni Del Livello Medio Del Mare.” Comune Di Venezia., 10 May 2017, www.comune.venezia.it/it/content/variazioni-livello-medio-mare#ingl.
Doglioni, F. “Relations between Constructive Peculiarities and Structural Behavior in Venice Buildings.” Informes de La Construcción, vol. 64, no. Extra, Dec. 2012, pp. 57–68, https://doi.org/10.3989/ic.11.070.
Foraboschi, Paolo. “Specific Structural Mechanics That Underpinned the Construction of Venice and Dictated Venetian Architecture.” Engineering Failure Analysis, vol. 78, Aug. 2017, pp. 169–95, https://doi.org/10.1016/j.engfailanal.2017.03.004.
Gugliuzzo, Elina. “The ‘Serenissima’ at Hazard: The Historical Phenomenon of Acqua Alta in Venice.” Humanities Research, vol. VI, no. 12, Jan. 2017, www.academia.edu/82930132/The_Serenissima_at_hazard_the_Historical_Phenomenon_of_Acqua_Alta_in_Venice.
Panwar, Ravi. “Venice: Foundation Details of the Biggest Floating City in the World.” The Constructor, 5 Nov. 2020, theconstructor.org/case-study/venice-foundation-details/224185/.
Statista. “Venice: Population 1050-1800.” Statista, 31 Dec. 2006, www.statista.com/statistics/1281705/venice-population-historical/.
Umgiesser, Georg. “The Impact of Operating the Mobile Barriers in Venice (MOSE) under Climate Change.” Journal for Nature Conservation, vol. 54, Apr. 2020, p. 125783, https://doi.org/10.1016/j.jnc.2019.125783.