As every resident of Venice knows, their city is essentially an upside-down forest. The 1,604-year-old city rests on millions of short wooden piles driven into the ground, points facing downward. These trees – larch, oak, alder, pine, spruce, and elm, ranging in length from 11.5 feet to less than three feet – have been supporting stone palaces and tall bell towers for centuries, representing a true engineering feat that harnesses the forces of physics and nature.
In most modern structures, reinforced concrete and steel perform the work that this inverted forest has accomplished for centuries. Despite their strength, few foundations today could last as long as Venice's wooden foundation system. "Nowadays, concrete or steel piles are designed with a 50-year warranty," explains Alexander Puzrin, professor of geomechanics and geosystems engineering at ETH Zurich in Switzerland. "Of course, they could last longer, but for residential and industrial construction, the standard lifespan is 50 years."
The Venetian pile technique is fascinating for its geometry, centuries-old resistance, and sheer scale. No one knows precisely how many millions of piles lie beneath the city, but there are 14,000 wooden posts packed tightly together in the foundations of the Rialto Bridge alone, and 10,000 oak trees under St. Mark's Basilica, built in 832 AD. The workers who drove the piles into the silt were known as battipali, or pile beaters, and they used songs to help them keep rhythm while they worked.
The construction method was remarkably systematic. The piles were driven as deep as possible, starting from the outer edge of the structure and working toward the center of the foundation, typically at a rate of nine piles per square meter in a spiral pattern. The tops were then sawed off to create a level surface located below sea level. Wooden cross-structures called zatteroni (planks) or madieri (beams) were laid on top. On these wooden foundations, workers placed the building stones.
The Republic of Venice quickly began protecting its forests to provide sufficient wood for construction, as well as for ships. "Venice invented forestry," says Nicola Macchioni, research director at the Institute of Bioeconomy of the Italian National Research Council, referring to the practice of cultivating trees. Venice isn't the only city to use wooden piles for its foundations, but essential differences make it unique.
Amsterdam is another city partially built on wooden piles. There, as in many other Northern European cities, the piles extend down to bedrock and function like long columns, or table legs. "This is acceptable if the rock is close to the surface," explains Thomas Leslie, professor of architecture at the University of Illinois. On the shores of Lake Michigan in the United States, where Leslie is based, bedrock could be 100 feet deep. "Finding trees that tall is difficult, right? The story goes that in Chicago in the 1880s, they tried driving one tree trunk on top of another, which, as you can imagine, proved unsuccessful. Eventually, they understood they could rely on soil friction."
The principle relies on the idea of reinforcing the soil by planting as many piles as possible, thus significantly increasing friction between the piles and the soil. The technical term for this is hydrostatic pressure, which essentially means the soil "grips" the piles if several are densely inserted in the same location, Leslie explains. Indeed, Venetian piles work this way: too short to reach bedrock, they keep buildings standing through friction.
This construction technique dates back even further in history. The method was mentioned by the 1st-century Roman engineer and architect Vitruvius; Romans used submerged piles to build bridges, also near water. In China, water gates were also built with friction piles. The Aztecs used them in Mexico City until the Spanish arrived, who demolished 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 did, who now have huge problems with that metropolitan cathedral [where the ground is settling unevenly]."
Puzrin leads a graduate course at ETH that studies famous geotechnical failures. "And this is one of those failures. That Mexico City cathedral, and Mexico City in general, is an open-air museum where you discover everything that can go wrong with foundations." The wood, soil, and water combine to give Venice's foundations remarkable strength.
After more than a millennium and a half underwater, Venice's foundations have shown remarkable resilience. However, they are not immune to damage. Ten years ago, a team from the universities of Padua and Venice (departments ranging from forestry to engineering and cultural heritage) studied the state of the city's foundations, starting with the bell tower of the Frari church, built in 1440 on alder piles.
The Frari bell tower has been sinking at a rate of 1mm per year since its construction, totaling 24 inches. Compared to churches and buildings, bell towers have more weight distributed over a smaller area and therefore sink deeper and faster, "like a stiletto heel," explains Macchioni, who was part of the team studying the city's foundations. The team found that in all structures studied, the wood was damaged (bad news), but the water-mud-wood mixture held everything together (good news).
They debunked the common belief that wood under the city doesn't rot because it's in an anaerobic, or oxygen-free, environment. Bacteria do attack the wood, even in the absence of oxygen. However, bacterial action is much slower than that of fungi and insects, which act in the presence of oxygen. Moreover, water fills the cells emptied by bacteria, allowing the wooden piles to maintain their shape.
"Is there cause for concern? Yes and no, but we should still consider continuing this type of research," says the researcher. Since the sampling done ten years ago, they haven't taken new samples, mainly due to logistical constraints. It's unclear how many more centuries the foundations will last, Macchioni specifies. "However, [they will last] as long as the environment remains the same. The foundation system works because it's composed of wood, earth, and water. The soil creates an oxygen-free environment, water contributes to this and maintains cell shape, and wood provides friction."
In the 19th and 20th centuries, wood was completely replaced by cement in foundations. In recent years, however, a new trend of wooden construction has generated growing interest, particularly with the rise of wooden skyscrapers. "It's the trendy material of the moment, and for very good reasons," Leslie comments. Wood is a carbon sink, it's biodegradable, and thanks to its ductility, it's considered one of the most earthquake-resistant materials.
Venice isn't the only city with wooden foundations, but it's "the only one [where the friction technique was used] on a massive scale that still survives today and is incredibly beautiful," adds Puzrin. "There were people who hadn't studied soil mechanics or geotechnics, and yet they produced something one can only dream of, and which has lasted so long." The illustrations accompanying this story are for artistic purposes only and are not a faithful representation of the wooden pile foundations under Venice, which are tightly packed and have no branches.
