These are essentially temporary structures designed to support the surrounding ground and to exclude water from an excavation. However, any inflow is removed using reasonable pumping facilities in such a way that soil particles are not also removed. Remember that changing existing groundwater levels can affect surrounding land and any buildings supported by it.
Cofferdams must be designed by an engineer with adequate experience in temporary works of this kind. They are constructed in a wide variety of types and material and the choice and design will have been based on a thorough knowledge of the soil and full details of the groundwater conditions.
Large cofferdams for civil engineering works require special consideration at the design stage.