Concrete piles are a type of building foundation system often designed and specified when the sub-grade soils required to support a structure are located too deep below the project’s grade elevation for less expensive foundation designs to be considered. The final tip elevation of concrete pile foundations are generally installed many times deeper than the bottoms of traditional concrete footings. Piles of all material construction, lengths and cross-sections are considered either to be friction piles or end-bearing piles. Friction piles, as their name implies, get most of their support strength through the frictional force that develops between the surface area of each pile face and the sub-grade soils that the pile is placed in.

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.