Caisson
Caisson
A caisson is a hollow structure made of concrete, steel, or other materials that can be sunk into the earth. It used as the substructure for a bridge, a building, or other large structures. Caissons come in many sizes and shapes depending on their future use. The one shared feature of caissons is that their bottom edges are sharp so they easily can be sunk into the ground. These sharp edges are known as the cutting edges of the caisson.
General principle
The purpose of using a caisson in construction is to provide a temporary structure from which the earth (such as soil and bedrock), water, and other materials can be removed and into which concrete or some other fill material can be placed. For example in the construction of a bridge it may be necessary to burrow into the soil at the bottom of a river until bedrock is reached. One way of doing this is to sink a caisson filled with compressed air into the river until it reaches the river bottom. Workers then can go into the caisson and dig soil out of the riverbed until they come to bedrock. As they remove soil, it can be transported upward out through the caisson. During this process, the caisson continues to sink more deeply into the riverbed until it reaches bedrock. At that point concrete may be poured into the caisson to form the lowest section of the new bridge pier.
Caissons may consist of a single unit looking like a tin can with both ends cut out. On the other hand, they may be subdivided into a number of compartments similar to a honeycomb. One factor in determining the shape of the caisson is the area it must cover. The larger the size of the caisson the more necessary it may be to subdivide it into smaller compartments.
Types of caissons
There are four types of caissons: box, open, compressed-air, and monolith. Box caissons (also called floating caissons) are pre-fabricated box-like containers with sides and a bottom. Open caissons are box caissons without a bottom. Compressed-air caissons (also called pneumatic caissons) are caissons in which the caisson is closed at the top but open at the bottom, and compressed air is used. Monolith caissons are similar to large open caissons, but are built to resist large impacts.
All caissons feature the shape of a tube, often with a cylindrical contour but it may also be rectangular,
KEY TERMS
Bedrock— A portion of Earth’s mantle made of solid rock on which permanent structures can be built.
Cutting edge— The bottom edge of a caisson that has sharp edges and sinks into the earth of its own weight.
Pier— One of the vertical structures on which a bridge, skyscraper, or other structure is supported.
Reclamation— The process by which seawater is pumped out of an area in order to create new land for farms, homes, and other human habitation.
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 bridge piers. 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. It is then allowed to sink to the riverbed. The filled caisson then becomes the lowest portion of the new bridge pier. 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 is generally 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 cannot be used at a depth of more than about 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.
Resources
BOOKS
Burg, R.G. High-strength Concrete in Massive Foundation Elements. Skokie, IL: Portland Cement Association, 1999.
How Things Work: Structures. Alexandria: Time-Life Books, 1991.
Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.
David E. Newton
Caisson
Caisson
A caisson is a hollow structure made of concrete , steel , or other materials that can be sunk into the earth. It used as the substructure for a bridge, a building, or other large structures. Caissons come in many sizes and shapes depending on their future use. The one shared feature is that their bottom edges are sharp so they easily can be sunk into the ground. These sharp edges are known as the cutting edges of the caisson.
General principle
The purpose of using a caisson in construction is to provide a temporary structure from which earth, water , and other materials can be removed and into which concrete or some other fill material can be placed. For example in the construction of a bridge it may be necessary to burrow into the soil at the bottom of a river until bedrock is reached. One way of doing this is to sink a caisson filled with compressed air into the river until it reaches the river bottom. Workers then can go into the caisson and dig soil out of the river bed until they come to bedrock. As they remove soil it can be transported upward out through the caisson. During this process the caisson continues to sink more deeply into the river bed until it reaches bedrock. At that point concrete may be poured into the caisson to form the lowest section of the new bridgepier.
Caissons may consist of a single unit looking like a tin can with both ends cut out. Or they may be subdivided into a number of compartments similar to a honeycomb. One factor in determining the shape of the caisson is the area it must cover. The larger the size of the caisson the more necessary it may be to subdivide it into smaller compartments.
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.
Resources
books
How Things Work: Structures. Alexandria: Time-Life Books, 1991.
Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.
David E. Newton
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Bedrock
—A portion of Earth's mantle made of solid rock on which permanent structures can be built.
- Cutting edge
—The bottom edge of a caisson that has sharp edges and sinks into the earth of its own weight.
- Pier
—One of the vertical structures on which a bridge, skyscraper, or other structure is supported.
- Reclamation
—The process by which seawater is pumped out of an area in order to create new land for farms, homes, and other human habitation.
caisson
1. Watertight chamber in which underwater construction work takes place.
2. Device for sinking foundations under water or in water-logged conditions, in the form of an air-tight box the size of the pier to be built, which is sunk to bedrock, or other surface on which it is to remain, then filled with concrete.
3. Coffer in ceilings, cupolas, soffits, and vaults.
caisson
cais·son / ˈkāˌsän; ˈkāsən/ • n. 1. a large watertight chamber, open at the bottom from which the water is kept out by air pressure and in which construction work may be carried out under water. ∎ a floating vessel or watertight structure used as a gate across the entrance of a dry dock or basin.2. hist. a chest or wagon for holding ammunition.