What is Cofferdam?

by Swati 30/10/2021

Cofferdam:

A cofferdam is a temporary structure designed to keep water and soil out of the excavation in which a bridge pier or other structure is built.
When construction must take place below the water level, a cofferdam is built to give workers a dry work environment.
Sheet piling is driven around the work site, seal concrete is placed into the bottom to prevent water from seeping in from underneath the sheet piling, and the water is pumped out.
The word “cofferdam” comes from “coffer” meaning box, in other words a dam in the shape of a box.

Types of cofferdam:

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.

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.

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.
Cellular cofferdams are mostly used in construction of Marine structures like dams, locks, wharfs etc.
It is constructed by driving straight steel sheet piles, arranged to form a series of inter connected cells.
The cells are constructed in various shapes and styles to suit the requirements of site.
Finally the cells are filled with clay, sand or gravel to make them stable against the various forces to which they are likely to be subjected to.

If stone is abundantly available near the site rockfill cofferdam can be preferred which can retain depth of water upto height of 3 m.
The stones are assembled in the required shape of the cofferdam and the voids and filled partially with soil/earth and with stone ships.
The slopes of the embankment are to be protected on the upstream face by pitching or riprap.

The success of any piling scheme requires satisfactory completion of the following stages:

As with all site operations the relevant legislation and guidance on matters pertaining to safety must be strictly adhered to.
Items needed for installation are pile driving hammer (vibratory or impact), crane of sufficient size, steel sheet piles are typically used, H-piles and/or wide flange beams for wales and stringers.
In many cases barges may be required for efficient installation of cofferdams.