Most modern buildings have been built to survive for 50 years, but unique ancient techniques that only use wood have kept Venice city floating on the water for more than 1,600 years.
Venice is built on a dense network of wooden piles. Image: Emmanuel Lafont/ BBC
All local residents know Venice is a upside down forest. The 1604 -year -old city on March 25 was built on the foundation made from millions of low wood, closed into the ground with pointed downwards. Wood plants such as deciduous pine, oak, python, python and travel from nearly one meter to 3.5 m have supported high stone buildings and bell tower in centuries, becoming a technical challenge of physical and natural force, according to BBC.
In most modern buildings, reinforced concrete undertakes the work that this upside -down forest has been carried out for hundreds of years. However, despite their durability, very few foundations can exist as long as in Venice. “Currently, concrete – steel piles are designed to exist for 50 years,” Alexander Puzrin, a professor of local engineering and geodetic system at EHT University in Zurich, Switzerland. “Of course, they can last longer, but when we build houses and industrial buildings, operating standards are 50 years.”
The pile technology in Venice is very interesting because of the shape, durability over the centuries and the large scale. No one knows exactly how many millions of piles below the city, but there are 14,000 wooden piles arranged in the foundation of Rialto Bridge and 10,000 oak trees under the San Marco Basilica church, built in 832.
Workers will use a deep pile hammer to be as deep as possible until it is impossible to mix the hammer further, starting from the outer edge of the building and gradually towards the center of the foundation, usually with a density of 9 piles/m2 in spiral shapes. The head of the pile is then saw to create a flat surface and below the sea level. Wooden structures are horizontal like planks or beams arranged on the pile. In the case of building a bell tower, board or beam up to 50 cm thick. For other works, the size is 20 cm or thinner. Oak wood is the most durable but also the most valuable wood. On the wooden foundation, workers will arrange stones to build houses.
Venice is not the only city to use wooden piles as a foundation but there are key differences that make their method unique. Amsterdam is another city based on a wooden pile. Here and many other European cities, the pile is closed until it reaches the original stone layer, acting as long columns or table legs.
“It is a good thing if the rock is near the ground,” said Thomas Leslie, a professor of architecture at the University of Illinois. But in many areas, the root stone layer beyond the reach of the pile. On the banks of Michigan lake in the US, where Leslie lives, the original rock can be up to 30 m above the ground and find such a big tree is very difficult. Instead, workers are based on friction of the soil.
The principle is based on the idea of reinforcing the soil, inserting many piles as much as possible, increasing the friction between the pile and the soil. The term of this method is hydrostatic pressure, which means the soil “cling” if there are many piles inserted dense in a position. In fact, the wooden piles in Venice operate this way. They are too short to reach the original stone layer and support the house above thanks to friction.
After more than 1.5 millennia submerged underwater, the foundation in Venice proved special durability. However, they are not immune to damage. 10 years ago, a research team from Padova and Venice checked the city’s foundation status, starting from the Frari church tower built in 1440 on python wood piles. The Frari bell tower sinks 1 mm per year from construction, so far a total of 60 cm. Compared to the churches and buildings, the bell tower has a large weight distributed on a smaller area, thus sinking deeper and faster.
The research team of Caterina Francesca Izzo works in the field, drilling, collecting and analyzing wooden samples from below the church, bell tower and along the canal. They found that in the works they checked, the wood was damaged but the water system, mud and wood still maintained the foundation. According to them, the wood below the city is not not rotten due to being in an anaerobic environment. Bacteria still attack wood even without oxygen but their activity is much slower than mushrooms and insects in oxygen. Moreover, water filled the gaps that bacteria chiseled, allowing wooden piles to maintain shape. Therefore, even when the wooden pile is destroyed, the entire wood, water and mud system is still connected under high pressure and maintaining durability for centuries.