What is a cofferdam, its types, and uses in the construction industry?
Besides the latest innovations and technological advances in the construction industry today, there are still various constraints that make construction seem impossible at some point. One of these is building structures over or under large bodies of water. With 71% of the earth’s surface being water, putting up major infrastructures can be really challenging. Fortunately, some methods for sea-borne construction have been developed in the earlier days. Hence, the development of cofferdams that are proving to be just as significant today as they were during ancient times.
A cofferdam, also called a coffer, is an enclosure built within or across a body of water to allow the enclosed area to be pumped out. This pumping creates a dry working environment so that the work can be carried out safely. Enclosed coffers are commonly used for construction or repair of permanent dams, oil platforms, bridge piers etc., built within or over water. These cofferdams are usually welded steel structures, with components consisting of sheet piles, walers, and cross braces. Such structures are usually dismantled after the construction work is completed.
There are various construction scenarios where a cofferdam is necessary:
Building a structure in a riverbed, seashore, or lake.
Building in an area of course-grained soil, where deep excavations are required.
Building below the groundwater table.
Building when trenches would likely collapse — typically during deep excavations.
To protect adjoining buildings or nearby structures.
It is formed from a single wall of sheet piling which is driven into the ground to form a “box” around the excavation site. The box is then braced on the inside and the interior is dewatered. It is primarily used for bridge piers in shallow water (30 - 35 ft. depth)
It is the simplest type of cofferdam. It consists of an earth bank with a clay core or vertical sheet piling enclosing the excavation. It is used for low-level waters with low velocity and easily scoured by water rising over the top.
3. TIMBER CRIB
Constructed on land and floated into place. Lower portion of each cell is matched with contour of river bed. It uses rock ballast and soil to decrease seepage and sink into place, also known as “Gravity Dam”. It usually consists of 12’x12’ cells and is used in rapid currents or on rocky river beds. It must be properly designed to resist lateral forces such as tipping / overturning and sliding.
4. DOUBLE-WALLED SHEET PILE
They are double wall cofferdams comprising two parallel rows of sheet piles driven into the ground and connected together by a system of tie rods at one or more levels. The space between the walls is generally filled with granular material such as sand, gravel or broken rock.
Cellular cofferdams are used only in those circumstances where the excavation size precludes the use of cross-excavation bracing. In this case, the cofferdam must be stable by virtue of its own resistance to lateral forces.
Working around water is going to be an ever-present hurdle for contractors. Fortunately, cofferdams are around to offer a practical, proven method for selectively and temporarily removing water from the work area. Cofferdam construction is a tried-and-true technique that has allowed us to build bridges to connect communities within and between countries.
Cofferdams are used to build dams to power our lives and allow commerce between international ports. The cofferdam continues to evolve with the use of different materials and construction methods. Understanding which type of cofferdam is best as well as knowing their strengths and limitations will allow you to complete your project safely and on time.