A cofferdam is defined as a temporary structure which is constructed so as to remove water from an area and make it possible to carry on the construction work under reasonably dry conditions.
Following points should be remembered in connection with the construction of cofferdams:
1) The cofferdam should be reasonably watertight. It may either rest on impervious soil or be extended to impervious strata through pervious soils. Otherwise, a layer of concrete may be laid at the bottom of a cofferdam and this layer should be allowed to harden sufficiently before pumping of water is started.
2) It should be noted that absolute water tightness is not desired in a cofferdam. It is not only impracticable but expensive too. The design and layout of a cofferdam should therefore be such that the total cost of construction, maintenance and pumping is minimum.
3) The cofferdam should be designed for the maximum water-level and other destructive forces so as to make it stable against bursting, overturning and sliding.
4) The water to be excluded by a cofferdam may be either ground water or water lying above ground level. It may be deep or shallow and still or running.
5) The materials used in the construction of a cofferdam are earth, timber, steel and concrete.
6) The cofferdam is generally constructed at site of work.
Uses of Cofferdam:
Following are the uses of cofferdams:
1) to facilitate pile driving operation;
2) to place grillage and raft foundations;
3) to construct foundations for piers and abutments of bridges, dams, docks, etc. ;
4) to enclose a space for the removal of sunken vessels;
5) to provide a working platform for the foundations of buildings when water is met with; and
6) to provide space for carrying out the foundation work without disturbing or damping the adjoining structures such as buildings, pipelines, sewers, etc.
Types of Cofferdams:
A wide variety of different types of cofferdams is available. The factors which influence the choice of a particular type of cofferdam are as follows:
1) The area to be protected by a cofferdam, i.e. , a small area or large area.
2) The depth of water to be dealt with i.e., shallow depth or deep depth.
3) The possibility of overtopping by floods, tides, etc.
4) The nature of bed on which the cofferdam is to rest, i.e., a previous layer or an impervious layer.
5) The nature of velocity of flow i.e., water flowing with slow current or with swift current.
6) The chances of bed erosion due to reduction of water way caused by the construction of a cofferdam.
7) The materials available at site of work for the construction of a cofferdam.
8) The facilities available for the transport of equipment and materials required for the construction of cofferdam.
Following are the most common types of cofferdams:
2) Single wall cofferdams
3) Double wall cofferdams
4) Cellular cofferdams
5) Rock-filled crib cofferdams
6) Concrete cofferdams
7) Suspended cofferdams.