Caisson

A caisson is a watertight retaining structure lowered into the ground or water to give workers a dry, enclosed space for building foundations below the surface, used for bridge piers, dams and underwater repair work. The basic principle dates back to Roman and medieval engineers, who used early forms of watertight enclosures for underwater foundation and harbor work, but modern pressurized, or pneumatic, caisson construction emerged in the nineteenth century, most notably in the building of the Royal Albert Bridge in 1859, the Eads Bridge in 1874 and the Brooklyn Bridge in 1883. A caisson is sunk through soft sediment until it reaches a stable foundation, ideally bedrock, and the dry interior it creates makes it easier to place concrete and reduces the risk of settling in surrounding structures; workers who decompressed too quickly after working inside pressurized caissons suffered a painful and sometimes fatal condition then called caisson disease, now known as decompression sickness, which caused numerous deaths and permanent injuries during the Brooklyn Bridge's construction.