Caissons are structures used in construction and engineering, providing a dry environment for workers and materials. Cofferdams keep water out of construction areas, while open caissons are used in shallow water and air box bodies in deep water. Caissons were used in the construction of early suspension bridges, causing caisson disease. The term caisson is also used in non-aquatic construction, navigation, architecture, and military purposes.
A caisson is a structure used in construction and engineering, especially underwater projects such as the construction of bridges or dams. Its purpose is to provide a dry and protected environment for workers and building materials. While water is kept out of the structure, air is let in, eliminating the need for scuba gear and the like. This structure has been used in construction since the 1800s.
Cofferdams are usually box-like structures with an open bottom and sometimes an open top as well. The facility keeps water out of the construction area while its open bottom allows workers to place foundations and piers into the seabed or river bed. In shallow water, an open caisson is used; her open top allows light and air to enter from above the waterline. For deep water construction, an air box body has a closed top; pressurized air is pumped in and personnel enter and exit through an airlock. Both types have a steeply sloped bottom edge, which allows the structure to be deeply rooted in the ground so that water cannot penetrate.
A skip must be securely mounted on a stable base for maximum safety and efficiency. Engineers prefer a rock layer underneath. If the bedrock is too much below ground, they sometimes rely on impacted mud or earth or create an artificial foundation.
With pneumatic caissons, mud, rock and other unwanted materials are brought to the surface through the “manure tube,” which connects to the top of the caisson. Overground cranes haul this material away from the construction site. The manure tube also helps stabilize the air pressure by releasing excess air. When construction within the structure is complete, the structure can be repositioned to allow work to continue in another location.
The caissons were used in the construction of the first suspension bridges in the United States such as the Eads Bridge in St. Louis, Missouri and the Brooklyn Bridge in New York. Many workers, including Brooklyn Bridge chief engineer Washington Roebling, suffered from what was then called caisson disease; some died. This disease was later found to be a decompression sickness caused by the human body’s inability to adjust to rapid changes in external pressure. Modern caisson workers compensate for this by gradually returning to the surface, just as deep-sea divers do.
The term caisson is also used to refer to various other engineering and architectural structures. The caisson foundation can be embedded deep underground in non-aquatic construction. In navigation, caissons filled with water carry ships from one rise to another in passages such as canals. In architecture, a caisson is a box-like structure used in the construction of domes or ceilings and is sometimes called a coffer.
The military definition of a caisson also involves a box, one that carries ammunition during battle and the coffin during a military funeral. Though different in engineering type, all these devices share a similar shape.
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