Eiffel 101: 6 Types of Caissons
Caissons are concrete, wood, or steel structures utilized for the purpose of dewatering as the foundations of bridges, ports, piers, and quay walls are constructed. Generally, caissons are permanent structures and become a part of the final product of deep foundations. This is a major difference between caissons and cofferdams, which are used for many of the same purposes but are often temporary structures. Caissons are watertight and sized to meet the needs of the project. They come in various shapes, sizes, and types, depending on the needs of the particular marine construction. They are often hollow until they are securely in place on clay or bedrock. Once in place, they are typically filled with concrete. Different pile types are often used including bearing piles and sheet piles.
Some of the benefits of caissons of all types are that they are fairly economical in comparison to other dewatering alternatives. Additionally, the significant height and weight of most caissons allows them to penetrate quite deeply and securely into the earth. A drawback to caissons is that workers must be trained and skilled in utilizing them to avoid potential hazards associated with their interiors.
As previously noted, there are various types of caissons suited to different purposes. Below are six of the most common types:
1. Box Caissons – These caissons are made up of rows of boxes. The prefabricated boxes each have four sides and a bottom. They are hollow before they reach their final location. They are floated to their final location of a foundation, and once they are in place, their hollow insides are filled with concrete. Box caissons are a popular option for bridge pier foundations, and they are a relatively affordable choice in comparison to other types of caissons. Box caissons can be made from various types of material, including steel sheet pile and other piling materials.
2. Open Caissons – Open caissons are very similar to box caissons, except their prefabricated structures do not have a bottom, and they merely consist of four sides. Open caissons are an excellent choice for use in soft ground areas (soft clay areas, riverbeds, etc.) They are not ideal for use in areas where there are sizeable ground obstructions. These caissons are common in foundation work involving piers, pump stations, tunneling, and deep manholes. Oftentimes, open caissons are constructed using steel sheet pile.
3. Pneumatic Caissons – Pneumatic caissons are made up of bottomless, top-sealed boxes. They are filled with air that has been pressurized. Once they are placed in an area of water, they are able to push water, mud, and sediment out through themselves. Workers are then able to relocate mud and sediment, as it is being pushed out. Airflow to a pneumatic caisson must be constant until all mud, rock, water etc. is removed and the caisson reaches bedrock. At which point, airflow is stopped, and the caisson boxes are filled with concrete in a dry environment.
4. Monolithic Caissons – These caissons are also sometimes called “monoliths.” They are the largest type of caisson. They are constructed within quay walls where extra resistance is needed against impacts of ships and other vessels. Quay walls are earth-retaining structures utilized to dock vessels and barges for the transport of materials and goods.
5. Excavated Caissons – These are caissons that are used in excavated sites. They are most often cylinder-shaped as opposed to box-shaped. Once they are floated in place, they are filled with concrete.
6. Floating Caissons – With box-shaped exteriors and cylinder-shaped cavities, floating caissons are a top choice for foundation projects for port construction, wharf construction, and berthing facility construction. Floating caissons are also sometimes referred to as “floating docks.”
Determining the best type of caisson for the job should take factors like the type of construction and natural environment into consideration. All of the caisson categories listed above come with unique benefits for particular applications.
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