Caisson

In engineering, a caisson is a retaining, watertight structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships. These are constructed so that the water can be pumped out so the working environment is dry. When piers are to be built using the open caisson and it is not practical to reach suitable soil, friction pilings may be driven to form a suitable sub-foundation. These piles are connected by a foundation pad upon which the column pier is erected.

Shallow caissons may be open to the air, while deep caissons to penetrate soft mud may be sealed at the top and filled with compressed air to keep water and mud out at depth. An airlock allows access to the chamber. Workers move mud and rock debris (called muck) from the edge of the workspace to a water filled pit, connected by a tube (called the muck tube) to the surface.

A crane at the surface removes the soil with a clamshell bucket. The water pressure in the tube balances the air pressure, with excess air escaping up the muck tube. The pressurized air flow must be constant to ensure regular air changes for the workers and the height of the water in the muck tube must be carefully regulated to prevent unnecessary overpressure or low pressure which could allow excessive inflow of mud or water at the base of the caisson.

The caisson will be brought down through soft mud until a suitable foundation material is encountered. While bedrock is preferred, a stable, hard mud is sometimes used when bedrock is too deep.

Caisson disease is so named since it appeared in construction workers when they left the caisson and had rapid decompression. It is caused by the same processes as decompression sickness in divers. The Brooklyn Bridge was constructed with the help of caissons, and several workers died of caisson sickness.

Caissons have also been used in the installation of hydraulic elevators where a single stage ram is installed below the ground level.

Types of caisson

The four main types of caisson are box caisson, open caisson, compressed-air caisson and monolith caisson.

Box caissons are pre-fab concrete boxes with sides and bottom. They are set down on a pre-prepared base. Once in place they will be filled with concrete to become part of the permanent works, for example the foundation for a bridge pier. One problem with box caissons is that hollow concrete structures do in fact float (see WWII concrete ships) and so they must be ballasted or anchored to prevent this until they can be filled with concrete.

Open caissons are similar to box caissons except that they do not have a bottom face. They are suitable for use in soft clays (e.g. in some river-beds) but not for where there may be large obstructions in the ground.

Compressed-air caissons have the advantage of providing dry working conditions which are better for placing concrete. They are also well suited for foundations for which other methods might cause settlement of adjacent structures.

Monoliths are as their name suggests larger than the other types but are similar to open caissons. They are often found in quay walls where resistance to impact from ships is required.