Types Of Caissons
All caissons feature the shape of a tube, often with a cylindrical contour but it may also be rectangular, elliptical, or some other form. Some caissons are open at both ends, some are open only at the top, and some are open only at the bottom. It depends on the way each type of caisson is to be used.
A caisson open at both ends might be used to lay down a pier for a new skyscraper. The caisson would be driven into the ground to a certain depth and the earthy material inside the caisson would be scooped out. Depending on the depth of the pier required one long open cylindrical caisson could be used or a sequence of shorter caissons could be laid down one on top of the other. When the caisson(s) have been inserted to the desired depth and all the soil within them removed they might be filled with concrete. The decision as to whether to remove the caissons themselves before adding concrete would depend on the surrounding soil's nature. If the soils were too unstable to hold their shape the caisson would be left in place. With stable soils the caisson could be removed.
A caisson closed at the bottom and open at the top is a floating caisson. This type of caisson often is used in the construction of bridgepiers. The caisson is constructed on land of concrete, steel, wood, or some other material and floated to its intended position in a river, lake, or other body of water. The caisson then is filled with gravel, concrete, or some other material and allowed to sink to the river bed. The filled caisson then becomes the lowest portion of the new bridgepier. A floating caisson can be used only if engineers can be assured that the soil beneath and around the filled caisson will not wash away.
One interesting application of the floating caisson is in the reclamation of land from the North Sea around the Netherlands. In the first stage of this process a series of floating caissons are moved into the ocean where they are arranged to form a new dike system. Ocean water trapped within the line of caissons is pumped out to form new farmland.
A caisson closed at the top and open at the bottom is a pneumatic caisson. This type of caisson generally is used in underwater construction projects. It can be used only if air is pumped in to produce a pressure greater than water pressure outside. Workers entering a pneumatic caisson must first pass through an intermediate chamber that allows their bodies to adjust from normal atmospheric pressure to the higher pressure within the caisson or vice versa. Pneumatic caissons can not be used at a depth of more than 120 ft (36.6 m). Beyond that point the air pressure needed inside the caisson to keep out water is too great for the human body to withstand.
See also Bridges.
How Things Work: Structures. Alexandria: Time-Life Books, 1991.
Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.
David E. Newton