Caissons are open bottomed boxes or cylindrical casings of various sizes that are progressively dug into soils. Upon installation at their final service elevation, caissons most commonly serve as the base formwork for the bottom-most element of a concrete foundation (such as a pier). They can be sealed at the top and pressurized with air to work underwater or below the water table, if necessary. Some sealed caissons contain an airlock which would allow personnel to enter the caisson for inspection or hand digging around obstructions encountered during placement. Within an enclosed caisson, the open bottom face maintains air pressure by sealing itself against the soils that are gradually being removed from within the caisson’s side walls. One of the most famous early uses of pressurized air caissons was during the construction of the Brooklyn Bridge. Many workers returning from work in the caissons developed decompression sickness, also known as “the bends”, a newly discovered phenomenon at that time.
Shallow placement caissons can be unsealed, and left open on the top and bottom faces while they are dug into place. These may look similar to a braced excavation from the surface, but differ in their ability to be gradually repositioned to lower elevations.
Dredging or digging soils from within the caissons are the most common methods of material removal. Dredge pipes can be positioned within the caisson’s perimeter at key locations to remove enough supporting soil for controlled placement. Dredge spoil pipes attached to a pump system carry the waste soils to the surface. Digging by hand or with construction equipment is another common method. In any case, the removal of material within the inside base perimeter of the caisson allows it to incrementally sink lower into the soil.
Common challenges of caisson operations include hitting obstructions or inconsistent sub-grade materials during their placement, either of which can cause tilting of the leading edge and cause delays and additional costs. Another concern arises when digging the caisson through very cohesive soils such as soft clays. It is possible for the caisson to generate enough skin friction moving through cohesive soils that it becomes stuck in place.