A Caisson is a watertight structure used for retaining water in order to work on the foundations for bridges, piers, and other structures. Generally speaking, the water is pumped out to create a completely dry work environment. Some caissons can be open-air caissons, whereas others may use compressed air to keep the mud and water out. Installation involves pushing it into the mud, until it reaches clay or another solid foundation. Due to the pressures inside of caissons, there are many risks to the workers.
There are four main types of caissons:
Box caissons are put into place and filled with concrete, and may make up the foundation for other structures. These caissons must be anchored to allow them to remain in the proper position until they have been filled with concrete.
Open caissons do not have a bottom, and are driven into the soft ground to create a bottom. Therefore, they are not suitable for uses where there is a lot of debris on the floor of the body of water, or where the ground is not soft enough to drive in the caisson. Sometimes, it may be necessary to drive piles from the surface to act as load bearing walls or anchors.
Compressed air Caissons
This type uses the power of compressed air to create a dry environment which is more adequate for the laying of concrete.
This type are a “scaled up” version of the open-air caisson, and are mainly used because they are able the resist the forces of the possible impact of ships.
Caissons are generally used as the means to complete new construction, and may act as a cofferdam of sorts while the construction is ongoing. Cofferdams however, are not part of the permanent structure, and are typically employed to perform repairs or maintenance on the structure. Cofferdams may be used in new construction as well.
Cofferdams offer a less permanent, and more cost effective solution for dewatering a work area. The cofferdam is set up, and the water is pumped out of the work area to allow work to procede in a dry environment. Cofferdams have many applications such as: flood protection, environmental remediation, shoreline protection, pipeline building and repair, boat ramp repair, as well as bridge repair.