When constructing foundations on flooded sites, cofferdams have long been the default solution.
A cofferdam is a temporary structure consisting of a watertight retaining wall held in place by piles driven into a river, lake or sea bed. In modern cofferdam construction these two elements are combined in sheet piles, corrugated, interlocking steel sheets that provide at once stable shoring and a watertight barrier CMILC.
Since the widespread adoption of sheet piling techniques after World War I, cofferdam construction has been guided by the ‘4 Ds’: drive, drain, dig and dismantle. First, sheet piling contractors are employed to drive sheet pile walls around the work site. Drainage systems are then employed to de-water the site before foundations are dug and laid. Once foundation construction and other works are complete, the site is filled and the cofferdam dismantled.
While, in the early days, workers within cofferdams faced significant risk of injury or death, these dangers have largely been overcome with more than 100 years of experience and innovation. During construction of the Brooklyn Bridge, for example, a new type of cofferdam was used that is known as a ‘pneumatic caisson’ (‘caisson’ being the French word for ‘box’). The pneumatic caisson solved the problem of constructing foundations deep underwater by sealing and pressurising the work site. While the pressurised work site effectively prevented flooding, it also gave rise to what came to be known as ‘caisson disease’ among the workers – a condition similar to decompression sickness in divers that contributed to the deaths of some 27 workers on this project.