Venice: Foundation Details of the Biggest Floating City in the World

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Venice, known as the floating city, lies inside a large lagoon in northeastern Italy. The city is characterized by the network of canals that crisscross this ancient town, as well as its fine palaces. Its unique canals and palaces set Venice apart from other European cities.

The historical city is composed of about a hundred small islands, where buildings arise close to one other. All the monuments and buildings of the city are constructed entirely on water. These buildings and monuments appear as if they are emanating from the water and give a feeling of a floating city to the tourist.

The construction of Venice city started back in 421 AD. The city was developed on marshy lands and an island was formed for the construction of buildings on it. The Barbarian invaders were the principal cause behind the formation of the island for the development of the Venice city, away from the mainland of Italy.

The people of northern Italy were afraid of the Barbarian invaders. Hence, the initiative of developing a city enclosed by water came to the mind of the people of northern Italy, so that the reach of the invaders could be restricted.

Venice city is a world heritage site and is considered as one of the most visited places by tourists. Tourists from around the world visit Venice city to get a glimpse of its canals, architectural landmark buildings, and art-packed churches. The entire Venice city was built on a unique foundation system, which was considered a marvel in civil engineering. 

In this article, we focus on the design details and the materials used for the construction of the foundation of the Venice city.

1. Geology of Venice City

One of the most amazing floating cities of the world was built on 118 islands in the center of Venetian lagoon of the Adriatic Sea. The following points describe the geology of Venice city:

1. Because of the tectonic movement in the Eurasian and African plates, Italy, Adriatic Sea and Alps mountains were formed during the Mesozoic Era. The features of the lagoon, on which the Venice city was built was formed due to the tectonic movement.
2. Due to the marine deposits, melting of ice, and multiple tectonic movements of the plate, various segments were deposited on the banks of the Venetian lagoon.
3. The depth of the Venetian lagoon is only 2 m above the water level. Thus, most of the land of Venice city is categorized as marshy land.
4. The foundation of Venice city consists of clayey soil, lime-clay with substantial organic matter, clay with remainders of molluscs, sandy lime soil, partially consolidated sandy-clay, sand and fine canal alluvium, limestone, clay and limey clay with Torba, marble, and sand inclusions.
5. From the pavement level (+0.90 m on MSL) to approximately 5.00 m depth, the soil consists of sandy-clayey silts (locally medium to fine sand with silt).
6. Masonry debris, trachyte blocks, wooden piles unreinforced concrete is present with variable thickness from few centimeters to 2.6 m.
7. Between 6-7 m depth from G.L., a layer of soft sandy clayey silt, silty clay with organic debris, and peat are found (as a general indication, the compressive strength of these soils is in the order of 1.0 to 2.0 MPa).
8. From 7 m to 10 m depth from G.L., medium to fine sand with medium to high compressive strength are found (values of compressive strength ranges from 7 to 15 MPa).
9. Beyond 10.0 m depth, an alternation of silty clay, clayey silts and silty sands are found.

2. Materials Used for the Construction of Venice City

At the beginning phase of the development of Venice city, two proposals were put up. The first one was by creating a base of the foundation using large marble stones. The second one was by driving timber in the foundation for creating a base for it.

The first idea was rejected due to unavailability of material and also the large blocks of stones could have caused large settlement of soil beneath the foundation. Timber was available in abundance and therefore it was used in the construction of the foundation of ancient buildings of Venice city.

The timber was brought from the mainland of northern Italy. Naturally water-resistant timbers were brought from the forest of the mainland, such as elm, oak, poplar, and mainly alder. The timber was used to create a platform for the foundation base. 

The timber was not only used for pile foundation but also for the construction of roofs, floors, doors, frames, and furniture. Extensive use of timber was found in almost all the architectural features. 

Other than timber, crushed stone and limestone were used in the construction of buildings. The crushed stone was used to fill the gap between the piles to avoid the deposition of sediments between the piles. The limestone blocks were used to create the platform above the timber piles. Bricks and stones were used in the superstructure of the buildings. 

3. Foundation Construction Methodology 

The following points describe the foundation construction methodology of the Venice city:

1. The first pile driving program was started in 421 AD. The piles were driven into the muddy sand till the bottom of the Venetian lagoon floor was reached.
2. Piles were formed in the shape of a pencil. The bottom end of the pile provided the point resistance to cut through the sediments of muddy sand.
3. The length of the pile varied between 4 to 8 m.  
4. On average, 6-12 piles were provided in 1 m² area. The diameter of each pile was in the range of 10 to 30 cm.
5. Different hammers were used to install the pile into the soil.
6. The length of piles was selected in such a way that the piles could easily penetrate the layer of loose muddy sand and also be able to reach the layer of compressed clay for better frictional resistance. Without the frictional resistance, the structures that would be built above the pile foundation would have sunk and collapsed.
7. For the construction of a single building, the installation of piles into the soil took more than one month. The task was very costly and labor-intensive.
8. Once the piles were installed at the top level, the piles were cut horizontally to form a leveled platform for the foundation base.
9. Crushed stones and stone debris were filled between the piles to prevent soil erosion. Further, the limestone blocks were layered on the top of the piles base. These blocks acted as a subfloor and provided support to resist the massive superstructure load.
10. The most significant pile driving program was performed for the construction of San Zaccaria, a church in the center of Venice. This church was built in mid-1400 and the piles were delivered in two stages for its construction because enough area was not able to stock the piles around the church site. Around 7000 piles were installed into the ground and 6000 wooden planks were used in the construction of the church. All the piles every installed at a very close spacing such that the top of all the piles formed a stable base itself.
11. Researchers have suggested that around 40,000 to 80,000 timber piles are still available beneath the two major islands (Venezia and Murano) of the city. The overall density of the piles in these islands is around 1.35 to 2.75 piles/m².
12. Area of the Venice city is around 5,909,000 m². Thus, on an average, if the pile density is taken around 2 piles/m², then there would be at least 11 million timber piles below the city.

4. Modern-Day Foundation Construction in the Venice City

The following points describe the modern-day foundation construction methodology used in the construction of buildings and monuments in Venice city:

1. Most of the buildings in Venice, that we see today, were constructed after the 16th century.
2. The buildings which were constructed before the 16th century, are now becoming unstable and uneconomical to maintain. Therefore, the new buildings were constructed by demolishing the existing buildings. However, the existing wooden pile foundation was reused.
3. Additional piles were added if the footprint of the new building changed or the substantial amount of superstructure load was added in the new building. 
4. By reusing the existing foundation, a large amount of money, material, and time was saved. 
5. San Marco tower of Venice is a perfect example of reusing the existing pile foundation. The tower was built on the existing building burnt in a fire in 1485. However, the piles of the building were intact and were able to support the San Marco tower without any remediation or rehabilitation. 
6. The existing piles are strong enough to withstand fires, hostile weather conditions, and massive superstructure load. 
7. The use of wooden foundation in the development of the 118 islands of Venice has turned the page in the construction industry. The development of the Venice city is a civil engineering marvel that has exhibited resilience over a thousand years and we are still learning from it. 
8. However, the foundation industry has progressed. The new buildings in Venice city are constructed using reinforced concrete piles. The reinforced concrete piles have greater compressive and uplift capacity than the timber piles for occupying the same surface area. 

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