So how do you build a city in the swamp?

Seriously, How was Venice built? The Venice lagoon is all mud. There is no rock to construct on, so to build stone houses that weigh hundreds of tons you have to follow an ingenious and thorough strategy. And the strategy is to drive long pointed poles of wood; oak, larch or pine, straight down into the bottom and then construct houses on top. This is a very ancient technique. The method existed even as long ago as in the days of the Roman empire. The Venetians just improved and developed it.

the Venice lagoon from airplaneThe first settlers didn’t use this technique because their houses were much more basic. They were made from wood, reed and clay and they where lightweight. They just constructed on the existing islands, on the sand. As Venice increased its importance, they needed more space. So they had to build where there was no land, directly on the water. And later on the palaces became heavier and the foundation had to be stronger. In fact, most of the underlying woodpile ground, that nowadays covers most of Venice is from the 15th and 16th century. Before that piling wasn’t all that predominant.

The hard floor?

A few meters down under the bottom there is a layer of hard clay, the so-called Caranto. This is very fine-grained sediment that has undergone a process of over-consolidation in a subaerial environment. Kind of technical but it basically hard mud, and it withstands weight better than soft mud. So if you reach the Caranto it is obvious that the resistance is greater than just piling down into the upper layer.

foundation of a palaca in veniceBut here we need to dismantle a myth.

It is not like we have a hard, flat bottom of stone-like clay on which the woodpiles stand. 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. The construction engineer of the middle ages probably started with the piles and as work progressed he put more or less of them in a certain way. He knew from the resistance if the clay was hard or soft and he adjusted the numbers and placement accordingly. Normal precaution told him to put more piles under the outer walls where most of the pressure is. Because there was no way to determine the structure of the material 5 meters down into the seabed, in advance. And this is how Venice was built: Without too much technology but with a lot of craftsmanship.

Actually, there is not one building in Venice standing straight up. Most medieval cities do not have a very solid foundation but none is as wavy as Venice. It makes sense if you consider that she’s sitting on top of the water, and how Venice was built. If you ride the ferryboat from Tronchetto to Lido and you watch the bell towers as you pass on the Canale della Giudecca, you will notice that they are all leaning. Some dangerously so.

escavations at Torcello VeneziaThe floating city.

This picture is from an excavation in Torcello. The foundation is from the 6th century and you can see the wood 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.

Continuing with the building technique… On top of the piles there are 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 of Venice. The outer wall is also conical to better withstand the weight of the building. The Istrian stone reaches from the canal bottom to over the highest level of high water to protect the inside of the ground from direct contact with the water. 

So why doesn’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. When the belltower in Saint Marks square collapsed in 1902, the piles underneath where still in more or less perfect conditions… after a thousand years. And they had turned into stone…

So the answer to the question: How was Venice built? lies in a combination of ingenious workmanship, natural environment, and hard work. I sometimes wonder how many houses cracked, fell over, and sunk into the mud before they had perfected the building technique to today’s standard.

So if the old ways of construction still work perfectly, maybe Venice will still be there in another thousand years from now..? Or will it?