Definition - What does Caisson Shaft mean?
Caisson shafts are watertight enclosed structures designed to keep water out of the construction zone, therefore, keeping the work environment dry. They are most commonly used during construction activities in water or wet ground, for example, bridge pier foundations, dam construction, or even ship repairs.
Caissons are typically prefabricated hollow boxes or cylinders and may be constructed from timber, steel, reinforced concrete or masonry.
Trenchlesspedia explains Caisson Shaft
Caissons are especially effective in the construction of underwater foundations in deep open waters. They are designed to be strong enough to withstand a number of loads including water pressure, wave loading, and buoyancy.
Some of the most common types of caissons are:
Box caissons are watertight prefabricated caissons in the shape of a box with a closed bottom and an open top. The caisson is constructed on land where it is then shipped to the desired location and sunk into place using sand, concrete or gravel to provide extra weight. These types of caissons are most suitable for sites where the bearing strata are level and little to no excavation is required.
An open caisson is a prefabricated box structure with an open top and open bottom. The walls of an open caisson are usually thick and heavy enough to facilitate sinking. Ballasts or hydraulic jacks may also be used to help weigh down the structure to assist in the sinking. Open caissons are most suitable for softer soil strata such as clays and silts. In addition to bridge construction, open caissons may also be used for reception/launch pits for microtunnelling and pipe jacking applications.
Pneumatic caissons are boxy or cylindrical structures that are open on the bottom but closed at the top. Compressed air is typically used to force water out of the caisson to create a dry working environment to allow construction work to be carried out. Pneumatic caissons are used in situations where it is not possible to perform excavations in the open.