Cofferdams are temporary water barriers which are created to allow for dry construction to occur in a specified area. The water barrier most typically consists of sheet pile and internal bracing. Single wall cofferdams which have sheet pile sufficiently embedded into the soil to resist the outside water pressure are called a cantilever design.
FIGURE 1: Example of constructed cofferdam.
Where internal lateral support is provided to assist to resist the water load, the cofferdam is considered a braced cofferdam. Where the retained water gets quite high, cellular cofferdams are used. Cellular cofferdams are connected cells, or sheet pile bins, which are filled with soil to provide dead weight and resistance against the external water pressure.
Once the cofferdam enclosure is completed it is pumped out and the specified construction can begin. Figure 1 shows photographs of some examples of the constructed cofferdams. Figure 2 is a photograph of a cellular cofferdam.
FIGURE 2: Photo of Cellular Cofferdam. Photo from C.J. Mahan Construction Company.
More information on cofferdam design can be found in: Update #47: Updated Improved Methods of Cofferdam Design