Venetian Buildings – How to Survive on a Sinking City?

by Nora Woolley

Images are taken by author unless otherwise stated

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Introduction

The city of Venice was created in the year 452 AD. The Huns were attacking Northern Italy, and many people fleeing sought refuge in a nearby Lagoon (Primal Space, 2024). Despite all odds, this civilization atop a Lagoon flourished. I have been to Venice twice in my life, once when I was a middle schooler and again this year as a college student. Both times I found the buildings atop the lagoon next to flowing canals charming and picturesque. I am not the only one who feels this way since currently it spans 118 islands and is one of the most popular tourist destinations in Italy (WPR, 2024).

So what exactly are the kinds of issues that arise when building in on top of a lagoon? And how can engineers help solve them? This paper will discuss challenges like rising tides and brackish water, as well as creative solutions like choice of material and inward leaning buildings.

Challenges

Foundation

A lagoon is not the first choice of a foundation for obvious reasons, most apparently, the fact that the soft soils of a lagoon cannot support large loads. The first settlers in Venice tackled this issue by making a foundation of timber logs, drilled about 5 meters into the water aiming for a thicker layer of clay called the “caranto.” The logs were drilled deep and close to each other, this compressed the clay and forced the water out. Isolated from air and water, the wood petrified instead of rotting. This process was accelerated by the amount of sediment in the clay and water (Foraboschi, 2019). A wooden plank and stone were used to raise this foundation above the sea level. This foundation can be visualized below in figure 1. Today the majority of the original foundation is still standing (Primal Space, 2024). Unfortunately this solution is far from perfect. The caranto is a layer of over-consolidated clay. It has a high strength and stiffness, however the thickness of this layer, averaging to 2-3 meters vertically, is not thick enough to support a large load. In addition, the caranto is not a continuous layer across the lagoon. It can be found at depths from 2-10 meters and in some places is completely missing (Foraboschi, 2019). With their engineering skills, the Venetians could build structures on top of the lagoon, however the low load bearing abilities and minimal compressibility of the base soil does not allow structures to be very large or heavy. In addition, this kind of foundation is prone to tilting back and forth, so buildings will also have to withstand a large diagonal movement like the one in figure 2.

Brackish Water

Soft soil was not the only challenge posed from the environment, another was the brackish water present in the lagoon. Brackish water refers to a kind of water with a higher saline level than freshwater, common in places like a lagoon. While the wooden lower level of the foundation can withstand this high salinity, the higher level suffers damages (Foraboschi, 2019). As I explored Venice, It was common to see buildings with the plaster peeling off, revealing the bricks underneath. This can be seen in figure 3 below. While bricks being exposed may seem like only an aesthetic issue it can have a large impact on the stability of a building. Bricks are particularly porous meaning they absorb liquid and retain material (Foraboschi, 2019). Another thing I saw frequently in Venice was salt left on bricks. This can be seen in figure 4. This retention of salt water can actually impact the compressive strength of bricks. The results of a 2014 study exposing historical Venetian bricks to water and salt are displayed below in figure 5. The study extracted bricks from Venetian buildings and subjected them to four different mixtures featuring varied salinity and moisture levels. The mixtures were intended to replicate the kinds of solutions Venice buildings are subjected to. Results of the study showed that moisture can drastically affect the compression strength of bricks and that this effect can be worsened by the presence of saline (Foraboschi & Vanin, 2014). The implications of this study are extremely relevant for construction in Venice. It means there is an additional challenge to face for keeping structures stable, and one that will potentially increase in difficulty as sea levels continue to rise.

Tides

Another challenge with being so close to the water is that the environment of the Venetian lagoon has always been temperamental. It is connected to the Adriatic sea by three inlets Chioggia, Malamocco and Lido (Lionello, Nicholls, Umgiesser & Zanchettin, 2021). The location of these inlets are shown in figure 6 below. From these inlets the tide rises and falls twice a day, causing a large range in sea levels throughout a Venetian day (Primal Space, 2024). Venice is also prone to meteorological surges caused by a series of environmental stresses that impact the Adriatic Sea, thus impacting Venice. The Adriatic Sea is prone to free oscillations called seiches, which are triggered by wind. The main seiches can cause flooding even when no other weather conditions are at play, and they can last around 23 hours. Environmental factors in the Adriatic Sea can also lead to meteorological surges for Venice. These are mainly caused by low atmospheric pressure as well as a southeast wind called the sirocco (Lionello et al., 2021). So even without outside influence from climate change, Venice is in a vulnerable position for flooding.

This environmental temperament is only exacerbated by the obvious issue, rising relative sea level. Relative sea level is what we will use to measure the rise in water level in Venice since it takes into account both the sinking of the city and the rising of the sea levels. Across recent history Venice has experienced an average increase in RSL of 2.5mm annually. While this might seem negligible, it has had a large impact on the floods Venice had experienced. The number of events with water height maxing at 120 cm was 40 during the last decade. In the first half of the 20th century it was under 2 events per decade. The change in RSL and flood frequency is illustrated in figure 7. This change is only predicted to increase with current greenhouse gas trends. Figure 8 illustrates the exponential curve predicted to occur (Lionello et al., 2021). Venice is already seeing the consequences of the rising sea level, see figures 9-11, but the city will have a much higher rate of sea rising to handle in the future. 

Solutions

When looking at all the many challenges of building on Venetian land, one might come to the conclusion that Venice is a dismal place with few successful architectural achievements. However this is far from the truth. Housing over 250,000 people, and bringing in around 20 million tourists (WPR, 2024) Venice is a jewel of Italy and a beautiful city. So how exactly is this city still standing? As I explored Venice early into my project I wondered the same thing. I had heard about the foundation Venice was built on top of, and yet the city was filled with beautiful feats of architecture, stable despite their foundation. To answer thie question, I will analyze the engineering solutions present in Venice. In order to build atop the lagoon the buildings had to be lightweight, couldn’t take up too much space, and had to be able to resist environmental challenges. Because of this Venice has a very unique method of constructing buildings.

Materials

First of all, the materials for building in Venice were meticulously selected over the years. Originally the Venetians used wood to create their homes. It is flexible, lightweight and was readily available. Unfortunately it is also flammable (Primal Space, 2024). Fires destroyed large parts of Venice in the 12th century, causing wood to be replaced with brick for the outside of buildings. Unfortunately, due to the height of the damage, Venice was unable to manufacture a large amount of bricks. Because of this limitation, many of the bricks during the 12th century were imported and reused from fallen Byzantine or Roman towns (Doglioni, 2012). As a result, buildings in Venice display a range of many different kinds of bricks, as seen in figure 12. As I explored the city I came across many different buildings with a wide array of different colored and sized bricks. Naturally, having a large range of sized bricks was not ideal for building, and this led to walls featuring a large amount of material to fill in the gaps. For this task, the Venetians have used lime mortar since 1260, specifically Istrian stone (Doglioni, 2012). Unlike cement, the mortar was flexible and would allow the walls to shift with the moving foundation (Primal Space, 2024). The practice of using small bricks and lots of mortar was used up until around the 15th century when it switched to larger bricks, less mortar and thus, thinner walls (Doglioni, 2012). This adaptation demonstrates an improvement in material management and structural understanding for Venice.

Fiuba

In addition to their creativity in choosing material, the Venetians used innovative architectural solutions, one of which was the connection between their inner and outer walls. Unlike most buildings where the inner wall, floor and outer walls are completely connected together, the outer wall in a typical Venetian house is resting against the floor and connected with a “fiuba.” The use of the fiuba-meaning “buckle” in ancient Venetian has existed since the 13th century, but was found in certain bell towers, rather than in homes at that time. More common use was dated to the 14th century (Doglioni, 2012). The original fiuba is made up of a stone head about 50-70 cm wide 15-20 cm high and 20-30 cm deep, attached to a long iron hook (Doglioni, 2012). The head lies on the outer wall and extends through to connect to the floor. This original stone headed fiuba can be seen on a building in figure 13. While it looks like a last minute addition, it was actually built along with the outer wall (Doglioni, 2012). The fiuba was essential in establishing stability for Venetian buildings, not only because it brought the elements of the house together, but because it was able to strengthen the outer wall. The hooks were placed alongside things like the ends of fireplaces or at the bottom of columns to help the thin outer walls manage vertical loads (Doglioni, 2012). Things like the visibility of the fiuba from the exterior, and the type of head used adapted and changed over time see figures 13-16 below; however the fiuba remains a common feature of Venetian architecture (Doglioni, 2012).

Inward Lean

The fiuba is not the only peculiarity of Venetian design, one notable feature is the inward leaning upper walls in Venice. When exploring Venice, you might be able to find a few buildings where the upper two floors are tilting inwards, as this was commonplace up until the 16th century. One tilting building is shown in figure 17. We are not fully sure whether this is an intentional choice, or something that occurs due to damage over time, however an analysis by F. Dolgini of the Ca’ Corner de la Frescada indicates that it is the former. The building, constructed in the 15th century, features an inward lean of 34-38 cm from the outer wall and an 18 cm lean from the side walls. This is a 2-3% incline from the outer wall and a 1% incline from the inner. A few things lead us to conclude that this leaning is intentional. For one thing the window jambs, stone pillars added to the sills to provide relief, align with the inward facing tilt. Another fact that supports this hypothesis is that the floor remains flat which would most likely not occur if there was structural damage occurring. Finally, the kinds of cracks that would appear in the building following a large deformation are not present. With all this information we can conclude that the inward leaning upper floors present in the Ca’ Corner de la Frescada are intentional (Doglioni, 2012). While this does not necessarily prove that all the leaning in Venetian upper floors is intentional, we can infer that it was a design choice made more than once across the city. This design could have been for a few reasons. Maybe the sloping made the upper floors less heavy, allowing them to build three stories high without putting too much compressive stress on the foundation. Maybe the design contributed to the self locking feature of Venetian architecture which we will analyze next. In my opinion, the kinds of creative engineering I have seen in Venetian buildings leads me to believe this was an intentional choice.

Different Foundations

Another way that engineers in Venice created creative solutions was the self locking mechanism created by the different foundations underneath the buildings. The outer wall often had a deep, yet thin foundation. On the other hand, the inner walls were supported by a more shallow, wider foundation. As expected, this difference leads to the inner walls sinking deeper than the outer walls. While this sounds like a concern structure wise, it is actually beneficial to the building being able to withstand further sinking. This difference in sinking between the walls, combined with strategic placement of fiuba where they are far from the connection between outer and inner wall actually create a kind of self locking mechanism. The way the building hooks the walls as they settle prevents structural strain and deformation of the walls (Doglioni, 2012). Figure 18 shows a visual of how this process works. This kind of engineering showcases the way Venice engineers, even in the past, were well versed in handling the kinds of challenges present when building atop a Lagoon.

Venetian engineers were forced to be extremely creative and adaptable in constructing buildings. The kinds of challenges they faced were almost entirely unique to Venice and thus required unique and creative solutions.

An Alternative Approach

As I explored Venice I frequently saw places where an older building had begun to sink under the rising water or ground level. In some cases there was a replacement door added somewhere on the building, as seen in figure 19, and in some cases the room seemed to be no longer acessable. While these kinds of approaches are common to see across the city, one architect in particular took a very different approach.

Carlo Scarpa was a famous Italian architect that, in the 60s, took a creative approach to the rising Venetian tide levels. He did several projects throughout Venice, however, for this purpose I will be discussing his work on the Querini Stampalia. The Querini Stampalia, previously a palace, is currently a library and museum of large cultural importance to Venice, featuring works by Bellini, Tintoretto and others. However, a few decades ago, it was under a threat, the high water left could permeate the lowest floor of the building at high tide. In 1966 Carlo Scarpa was commissioned to solve this problem and make the bottom floor usable (Montuori, 2020). Rather than try and expel the water from the space, he replaced the doors with iron gates and let the water flow into the building. Scarpa included a raised marble walkway so that visitors could walk along the lower floor. He added plaster panels to the inner walls to avoid water damage, and created a new bridged entrance out of the window (Montuori, 2020). The entrance can be seen in figure 20. This method of channeling and controlling the water rather than trying to get rid of it was an innovative and artful approach. This kind of creativity and adaptability is characteristic of Venetian architecture, and this particular approach to high tides may become a norm for a future Venice.

Conclusion

The greatest challenge and the greatest attraction of Venice is its improbability. A city floating on a swamp sounded impossible in 452 AD and yet it was done. The challenges facing Venice are high in number and worsening over time. If the city is going to survive in the future it will need a multitude of creative solutions, and fast. On the other hand, Venetian architects and engineers have proven throughout history to be adaptable and resourceful beyond their time. Perhaps in the future when rising sea levels threaten other major cities to the same degree, we will be able to look back at Venice and take inspiration from the solutions they discovered.

Sources

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Foraboschi, P., & Vanin, A. (2014). Experimental investigation on bricks from historical Venetian buildings subjected to moisture and salt crystallization. Engineering Failure Analysis, 45, 185–203. https://doi.org/10.1016/j.engfailanal.2014.06.019

Lionello, P., Nicholls, R. J., Umgiesser, G., & Zanchettin, D. (2021, September 1). Venice flooding and sea level: Past evolution, present issues, and future projections (introduction to the special issue). Natural Hazards and Earth System Sciences. https://nhess.copernicus.org/articles/21/2633/2021

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Paolo Foraboschi. (2019). Venetian buildings are “of” and not “on” the lagoon. Journal of Civil Engineering and Architecture, 13(2). https://doi.org/10.17265/1934-7359/2019.02.007

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