How do you build a Metropolis if there’s nothing to build upon?
How was Venice built? Just looking at her, walking the narrow streets, and getting a Spritz at the bar, you may not reflect on it all that much. She looks very much like any other very old city, of which there are many all over Italy. But if you could see through the stone floor, through the pavement and the bricks, and observe what is under her, then you’ll quickly realize that she is different from any other medieval city. And it’s nothing less than a miracle that she is actually there at all.
The Venice lagoon is all mud. There is no rock to construct on, not even soil. So, if you want to build stone houses and palaces that weigh hundreds of tons you have to follow an ingenious and thorough strategy. And this strategy will have to be perfectly adapted for the very particular circumstances in the damp, muddy, and wet building place.
Venice was built by driving long pointed poles of wood; oak, larch, or pine, straight down into the seafloor. Two layers of horizontal planking were laid out. Over that, the old Venetians put layers of stone that made up the foundation of the city. This technique is ancient, and the method existed even as long ago as in the days of the Roman Empire. The Venetians just improved and developed it.
The genius of wooden piling.
The first settlers back in the 5th and 6th centuries didn’t use this technique because their houses were much more basic. They were mostly made from wood, and some even from reed and clay. These houses were lightweight and the simple sandbanks were in most cases enough to sustain them.
In the beginning, the settlers constructed where there was at least some ground or soil to build on. They constructed on the existing islands, on the sandbed. But those spots were few and far apart and twice a day they got flooded. As Venice increased its importance, the city needed more space, so they had to start building even where there was no land at all… They had to find a way to build directly on the water.
Even later on, the building technique went from wood and clay to stone and marble. The palaces became heavier and the foundation had to be stronger. All this made piling and drainage essential. The foundation had to withstand considerable pressure to prevent the buildings from simply sinking straight through the mud. The Romans piled too, but in Venice, the pilings had to be suitable for extreme conditions. So they gradually increased the length and number of the wood poles.
Is the so-called hard floor – Il Caranto.
But let’s dig further into the Venetian building strategy. A few meters down under the bottom there is a layer of hard clay, the so-called Caranto. This is a very fine-grained sediment that has undergone a process of over-consolidation in a sub-aerial environment. This is kind of technical language but it’s basically hard mud, and it withstands weight better than soft mud. So if you use woodpiles long enough to reach the Caranto it is obvious that down there, the resistance is greater than if you just pile down into the upper layer.
But here we need to disprove an old myth.
Because the Caranto is not a hard, flat bottom of stone-like clay on which the wooden piles stand. It is not a hard floor that the architect can use to firmly anchor the piles. That way of looking at it is a myth, even though many locals strongly believe that Venice is standing firmly on the Caranto.
The truth is that the Caranto is found at various levels. It can be as shallow as two meters and as deep as more than ten. And it’s not stone hard. It’s more like a very dense rubber-like material that holds the pile better than normal clay.
We can imagine that the construction engineer of the 9th and 10th centuries started with the piles and as work progressed he put more or less of them depending on the soil he was penetrating, in a spiral motion, from the perimeter and inward. He knew from the resistance if the clay was hard or soft. And if the Caranto wasn’t reachable, he simply put the piles closer to sustain the weight. Because it must have been very difficult to determine the structure of the material 5 meters down into the seabed, before starting the project.
Piling into the upper layer.
So, if the Caranto was reasonably shallow, the piling was done in a certain fashion, always in a spiral movement from the perimeter inwards. But if they couldn’t reach the Caranto, the technique changed into what is called soil compaction. This is a process where mechanical pressure is used to compress the soil thus making it more sustainable. It practically turns the soft mud into hard mud. In this case, the piles were planted over the entire surface, first closing the perimeter, with thick piling. Just as before, they placed the piling in a spiral movement inwards, but in this case, with no space between the poles.
As the pressure is much greater under the outer walls, that’s where the highest number of poles is if they reached the Caranto, or the longest and sturdiest in case of compaction. Many buildings in Venice stand on the Carranto, but others do not. And it was in no way required to construct a strong and solid foundation.
How to build a foundation that can support a city like Venice.
To sustain this fact, we just have to consider that for some of the bigger buildings in the city, the Rialto Bridge, Basilica della Salute, and others, the documentation is rather complete. At least the financial situation, and in the records we can find the orders of wood poles that the city made. There were many types of poles, each with its own nomination, and they should be around 5 or 6 meters (16 – 20 feet) in length… Some, are a bit shorter. Just the fact that they were ordered in a certain dimension before initiating the building site, shows that the project was in no way depending on the depth of the Caranto, or if it could be reached.
The length of the poles also varies through the centuries. When Venice started out in the 800 and 900, some poles were as short as a meter or a meter and a half. This obviously causes less stability to the building on top of them.
The collapse of the Saint Mark’s Bell Tower.
When the Saint Mark’s Bell Tower came down one tragic morning in July 1902 it opened for the possibility of investigating the foundation of one very old building. The tower had been renovated and overhauled on various occasions but the foundation was, and partly still is, from the 10th century.
What they found was that the length of the poles was 4 meters. The horizontal planks and the stone layers mounted to around 3,5 meters. The foundation was largely intact and it was not the settling of the ground that had caused the collapse of the tower. The wood was mineralized and had practically turned into stone.
Most of the underlying wood-piled ground in Venice today is from the 15th and 16th centuries.
So how did they put them in place?
Without hydraulic pumps and steel cranes, getting the poles down into the mud probably was a bit of a problem. And this probably was one of the reasons why they were so short in the beginning. This was a job mainly done by hand, at least in the first centuries. Building large mechanical machines wasn’t only difficult because we are in the early days of the Middle Ages… But also because there wasn’t any ground to put them on. Remember that in the beginning the piling often had to be done where there was absolutely nothing to stand on.
The method was to just stand one on each side of the top of the pole with a heavy thingy in between hammering on top of the wooden pile. It was a tiresome and dangerous job. As the years went by they invented heavier and more automatized machines that could pile deeper and faster. But it was still labor for those without a family name, background, and education.
And it wasn’t only piling that was time-consuming. The whole process of building with draining, and digging, was dangerous and hard work. But time and cheap labor wasn’t an issue in the Middle Ages.
Still, there is not one building in Venice standing straight up. Most medieval cities do not have a perfectly horizontal foundation but none is as wavy as Venice. It makes sense if you consider that she’s actually floating on top of the water. If you ride the ferry boat from Tronchetto to Lido and watch the bell towers as you pass through the Canale della Giudecca, you will notice that they are all leaning. Some dangerously so.
How was Venice built? The floating city
This picture is from an excavation in Torcello. The foundation is from the 6th century and you can see the wooden piles are all there, more or less intact. This was the “fondamenta”, the quay, and it has slightly more poles than the surrounding buildings. If they are left in the open, they will decompose in a very short time.
On top of the piles, there were two horizontal layers of thick, cross-plied wood planking, and on top of that the bricks and stones. The part of the foundation standing against the water of the canal is made of Istrian stone, a dense type of impermeable limestone from the peninsula of Istria on the Croatian side, opposite Venice.
The outer wall is also conical to better withstand the weight of the building. The Istrian stone reaches from the canal bottom to above the highest level of high water to protect the inside of the ground from direct contact with the water.
So, why don’t the wood piles rot?
Because they’re stuck into the mud. And inside the sludge, the air doesn’t have access. The wood has no contact with oxygen and the microorganisms doing the decomposition just can’t work. Instead, the minerals from the humidity make the wood harden. Something that became obvious at the Saint Mark’s Bell Tower collapse in 1902., The wood underneath was in more or less perfect condition… after a thousand years. And they had turned into stone.
Actually, it was decided to leave the original ground when building the new tower and enlarge the pilings with 3100 new poles around the base. The central part of the foundation under the tower has been there for more than 1000 years and it’s still perfect.
How Venice was built is no less than a miracle.
The miracle of the city floating in the lagoon comes from a combination of ingenious workmanship, a natural environment, and hard work.
But it’s not a fixed technique. Throughout history, Venice has frequently come up with adjustments to the building technique. Some were improvements, while others were not. And research is ongoing. New ways to build and maintain the palaces in Venice are invented on a daily basis. We have better knowledge today and more precise instruments. We lack cheap labor though and some old knowledge is getting lost. The economic factors are increasingly important. Very often it’s a balance between doing what’s best and paying the least. Like the cleaning of the canals. That is nowadays made in a much cheaper way, but a way that is less thorough.
Venice is still floating, though, Her nose tip is still above the waterline, even if the problems with humidity, cracks, settling, and the ever more frequent high waters, are threatening her.
Will she still be here a thousand years from now? I hope so. But even though the immediate problem with high water could have been solved, for now, by the MOSE lock gates, there still is one big future threat that doesn’t have anything to do with the wood planks, the Istrian stone, or the piling… A threat that is scary and not in any way tied only to the future of the lagoon city. The rising sea levels.