Coffer belongs to a trunk, casket or chest in which temporary structure is constructed to surround and cover an area that is prepared for excavating of the foundation of the construction. Cofferdams are applicable for structural foundation in Open Water & Open Land like bridge piers, docks, locks, dams and high ground water table.
A cofferdam is a wall with water resistance capacity that is constructed around the boundary of the projected excavation to resist the water flow into the excavation to keep the basement in dry condition.
Given below the lists of common types of coffer dams :
1) Earth Coffer Dams 2) Rockfill Coffer Dams 3) Single-Sheet Pile Coffer Dams 4) Double-wall sheet piling coffer dams 5) Braced Coffer Dams 6) cellular Coffer dams
Categories of cofferdam:
1) Earth Coffer Dams: Earth Coffer Dams are mainly built up in an area where the velocity of the current and depth of the water remains at 13 to 18 in and it is lifted 1 m over the water level. It is built up with a by amalgamating clay and sand. The Side Slopes of the bank on the water side should have been sloped with rubble Boulder to get rid of embankment from scouring. Once the coffer dam is finished, the water is pumped to wipe the interior surface. Sand bag can be utilized in an urgency.
2) Rockfill Coffer Dams: Rockfill coffer dams are constructed with rockfill. The site is often surrounded with dewatered. To safeguard against wave action, the crest and the upper portion of the impermeable membrane are supplied with rip rap. The slopes of rockfill cofferdam is built as steep as 1 horizontal to 1.5 vertical.
3) Single-Sheet Pile Coffer Dams: Single Sheet Coffer Dam is built in a very small area and it’s depth of water surpasses.5 to 6m. Initially First Guide Piles alias Timber Piles are inserted into the firm Ground that is underneath water bed. On the basis of the velocity of the Current in water, the Longitudinal runners spacing fluctuates and it is called as wales which are bolts to a timber piles at a requisite space.
Steel or Wooden Sheet is placed into the River Bed together with wales which are fastened to the wales through bolts. To make the walls stronger against the water pressure, the sheets on the two faces arc bolstered with trussed arrangement of struts and Half-filled bags of sand piled on the internal and the external faces of the sheets. Once the cofferdam is built up, the water in the surrounded area is poured out and the construction work starts.