Mud, water and wood: The system that kept a 1604-year-old city afloat

Most modern structures are built to last 50 years or so, but ingenious ancient engineering has kept this watery city afloat for more than 1,600 years – using only wood.

As any local knows, Venice is an upside-down forest. The city, which turned 1604 years old on March 25, is built on the foundations of millions of short wooden piles, pounded in the ground with their tip facing downwards. These trees – larch, oak, alder, pine, spruce and elm of a length ranging between 3.5m (11.5ft) to less than 1m (3ft)  – have been holding up stone palazzos and tall belltowers for centuries, in a true marvel of engineering leveraging the forces of physics and nature.

In most modern structures, reinforced concrete and steel do the work that this inverted forest has been doing for centuries. But despite their strength, few foundations today could last as long as Venice's. "Concrete or steel piles are designed [with a guarantee to last] 50 years today," says Alexander Puzrin, professor of geomechanics and geosystems engineering at the ETH university in Zurich, Switzerland. "Of course, they might last longer, but when we build houses and industrial structures, the standard is 50 years of life."

The Venetian piles technique is fascinating for its geometry, its centuries-old resilience, and for its sheer scale. No-one is exactly sure how many millions of piles there are under the city, but there are 14,000 tightly packed wooden poles in the foundations of the Rialto bridge alone, and 10,000 oak trees under the San Marco Basilica, which was built in 832AD.

"I was born and raised in Venice," says Caterina Francesca Izzo, environmental chemistry and cultural heritage professor at the University of Venice. "Growing up, like everyone else, I knew that underneath the Venetian buildings, there are the trees of Cadore [the mountain region next to Venice]. But I didn't know how these piles were placed, how they were counted and knocked down, nor the fact that the battipali (literally the 'pile hitters') had a very important profession. They even had their own songs. It is fascinating from a technical and technological point of view." 

The battipali would hammer down the piles by hand, and they would sing an ancient song to keep the rhythm – a haunting and repetitive melody with lyrics that praise Venice, its republican glory, its Catholic faith, and declare death to the enemy of the time, the Turks. On a more lighthearted note, a Venetian expression still in use today, na testa da bater pai (literally 'a head that is good to pound down the piles') is a colourful way of saying that someone is dull or slow-witted.

The piles were stuck as deep as possible, until they couldn't be pounded down any further, starting at the outer edge of the structure and moving towards the centre of the foundations, usually driving nine piles per square metre in a spiral shape. The heads were then sawn to obtain a regular surface, which would lay below sea-level.

Transverse wooden structures – either zatteroni (boards) or madieri (beams) – were placed on top. In the case of the bell towers, these beams or boards were up to 50cm (20in) thick. For other buildings, the dimensions were 20cm (8in) or even less. Oak provided the most resilient wood, but it was also the most precious. (Later on, oak would only be used to build ships – it was too valuable to stick in the mud.) On top of this wooden foundation, workers would place the stone of the building.

The Republic of Venice soon began protecting its forests to provide sufficient wood for construction, as well as for ships. "Venice invented sylviculture," explains Nicola Macchioni, research director at the institute for bioeconomy at Italy's National Council for Research, referring to the practice of cultivating trees. "The first official sylviculture document in Italy is indeed from the Magnificent Community of the Fiemme Valley [to the north-west of Venice], dating from 1111AD. It details rules to exploit the woods without depleting them."

According to Macchioni, these conservation practices must have been in use years before they were written down. "That explains why the Fiemme Valley is still covered by a lush fir forest today." Countries such as England, however, were facing wood shortages by the middle of the 16th Century already, he adds.

Venice is not the only city relying on wooden piles for foundations – but there are key differences that make it unique. Amsterdam is another city partially built on wooden piles – here and in many other northern European cities, they go all the way down until they reach the bedrock, and they work like long columns, or like the legs of a table.

"Which is fine if the rock is close to the surface," says Thomas Leslie, professor of architecture at the University of Illinois. But in many regions, the bedrock is well beyond the reach of a pile. On the shore of Lake Michigan in the US, where Leslie is based, the bedrock could be 100ft (30m) below the surface. "Finding trees that big is difficult, right? There were stories of Chicago in the 1880s where they tried to drive one tree trunk on top of another, which, as you can imagine ended up not working. Finally, they realised that you could rely on the friction of the soil."

The principle is based on the idea of reinforcing the soil, by sticking in as many piles as possible, raising substantial friction between piles and soil. "What's clever about that," says Leslie, "is that you're sort of using the physics… The beauty of it is that you're using the fluid nature of the soil to provide resistance to hold the buildings up." The technical term for this is hydrostatic pressure, which essentially means that the soil "grips" the piles if many are inserted densely in one spot, Leslie says.

Indeed, the Venetian piles work this way – they are too short to reach bedrock, and instead keep the buildings up thanks to friction. But the history of this way of building goes back further still.

The technique was mentioned by 1st-Century Roman engineer and architect Vitruvius; Romans would use submerged piles to build bridges, which again are close to water. Water gates in China were built with friction piles too. The Aztecs used them in Mexico City, until the Spanish came, tore down the ancient city and built their Catholic cathedral on top, Puzrin notes. "The Aztecs knew how to build in their environment much better than the Spanish later, who have now huge problems with this metropolitan cathedral [where the floor is sinking unevenly]."

Puzrin holds a graduate class at ETH that investigates famous geotechnical failures. "And this is one of these failures. This Mexico City cathedral, and Mexico City in general, is an open-air museum of everything that can go wrong with your foundations."

After more than a millennium and a half in the water, Venice's foundations have proved remarkably resilient. They are not, however, immune to damage. 

Ten years ago, a team from the universities of Padova and Venice (departments ranging from forestry to engineering and cultural heritage) investigated the condition of the city's foundations, starting from the belltower of the Frari Church, built in 1440 on alder piles.

The Frari belltower has been sinking 1mm (0.04in) a year since its construction, for a total of 60cm (about 24in). Compared with churches and buildings, belltowers have more weight distributed on a smaller surface and therefore sink deeper and faster, "like a stiletto heel", says Macchioni, who was part of the team investigating the city’s foundations.

Caterina Francesca Izzo was working on the field, core drilling, collecting and analyzing wood samples from underneath churches, belltowers and from the side of the canals, which were being emptied out and cleaned up at the time. She said that they had to be careful while they were working on the bottom of the dry canal, to avoid the wastewater sporadically gushing from the side pipes.

The team found that throughout the structures they investigated, the wood was damaged (bad news), but the system of water, mud and wood was keeping it all together (good news).

They debunked the common belief that the wood underneath the city doesn't rot because it's in an oxygen-free, or anaerobic, condition – bacteria do attack wood, even in absence of oxygen. But bacteria action is much slower than the action of fungi and insects, which operate in the presence of oxygen. Furthermore, water fills up the cells that are emptied out by bacteria, allowing wooden piles to maintain their shape. So even if the wooden piles are damaged, the whole system of wood, water and mud is held together under intense pressure, and is kept resilient for centuries.

"Is there anything to worry about? Yes and no, but we should still consider continuing this type of research," says Izzo. Since the sampling 10 years ago, they hadn't collected new ones, mainly because of the logistics involved.

It's not known for how many more hundreds of years the foundations will last, says Macchioni. "However, [it will last] as long as the environment remains the same. The foundation system works because it is made of wood, soil and water." The soil creates an oxygen-free environment, the water both contributes to that and maintains the shape of the cells, and the wood provides friction. Without one of these three elements, the system collapses.

In the 19th and 20th centuries, wood was completely replaced by cement in foundation construction. In recent years, though, a new trend of building with wood has gained increased interest, including the rise of wooden skyscrapers. "It's kind of the cool material right now, and for really good reasons," comments Leslie. Wood is a carbon sink, it's biodegradable and thanks to its ductility, it's considered among the most earthquake-resistant materials.

"We can't of course build entire cities on wood nowadays because we are too many on the planet," adds Macchioni, but it's undeniable that without artificial materials and without motors, ancient builders just had to be more ingenious. Venice is not the only city with wooden foundations, but it is "the only one [where the friction technique was used] en masse that is still surviving today and is so insanely beautiful", adds Puzrin. "There were people out there who didn't study soil mechanics and geotechnical engineering, and yet they produced something we can only dream about producing, which lasted so long. They were amazing, intuitive engineers who did exactly the right thing, taking advantage of all these special conditions." 

* The illustrations in this story are for artistic purposes only and are not a true representation of the timber pile foundations under Venice, which are tightly packed and do not have branches. 

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