What is Cofferdam & Types of Cofferdam

What Is Cofferdam | Types of Cofferdam

A cofferdam is a temporary structure built to create a dry working environment in areas that are normally submerged in water. This technique is commonly used in the construction of bridges, dams, and other water-based infrastructure projects.

Cofferdams can be constructed using different materials such as steel sheet piles, concrete, and timber. There are several types of cofferdams, each with its own unique design and construction method. Understanding the different types of cofferdams is essential to select the appropriate one for a specific project.

In this context, this article will introduce the concept of cofferdams and provide an overview of the different types of cofferdams commonly used in construction projects.

What Is Cofferdam?

A Cofferdam is one type of temporary construction structure that is built in the river, lake, etc. to remove water from an area and make it possible to carry on the construction work under reasonably dry conditions.

Ohio River Cofferdam

Different types of Cofferdam are usually required for projects such as denis, docks, and the construction of bridge piers and abutments.

Types of Cofferdam

The following are various types of cofferdams mostly used in dam construction

  1. Earthfill Cofferdam
  2. Rockfill Cofferdam
  3. Rockfilled Crib Cofferdam
  4. Single Walled Cofferdam
  5. Double Walled Cofferdam
  6. Cellular Cofferdam
  7. Concrete Cofferdam
  8. Suspended Cofferdam

1. Earth-Filled Cofferdam

This is the simplest type of cofferdam. It essentially consists of an earthen embankment built around the area to be enclosed.

It is constructed in places where the depth of water is not much, say 1.3 to 1.8m, and the velocity of flow is low, the top of the embankment should be about 1m above the water.

Earth Fill Cofferdam
Earth Fill Cofferdam

The top width of the bank should not be less than 1m, and side slopes may be from 1:1(1/2) to 1:2.

The earth embankment is generally made from a mixture of clay and sand or clay and gravel. If the estimated quantity of clay is not easily obtainable, the bank may be constructed with central clay wall slopes of sand on either side.

When the construction of earth fill types of cofferdam is over, the water from inside is pumped out so as to leave a dry surface inside and the construction work of the foundation is started.

2. Rockfill Cofferdam

If the depth of water to be retained by the embankment of the dam is of the order of 1.8m to 3m, stone or rubble is used for the embankment.

This cofferdam is generally selected only if the stone is easily available in nearby areas. An impervious layer of the earth is laid on the outer face of the dam which makes it impervious.

Rock Fill Cofferdam
Rock Fill Cofferdam

The particles of earth area carried in the voids of rocks and a fairly water-tight structure is gradually formed.

For better construction, the core walls or steel sheet piles extending up to the impervious layer are to be provided. The core wall may be of clay or concrete.

3. Rockfill Crib Cofferdam

Rockfill Crib Cofferdam
Rockfill Crib Cofferdam

A rockfill crib cofferdam is comprised of timber cribs. Therefore, it is also known as timber crib cofferdam. A crib is a frame of wooden horizontal and cross beams laid in an alternate course.

The cribs are open at the bottom and are filled with rock or gravel or earth. This gives stability to the crib against overturning or sliding.

4. Single Walled Cofferdam

This type of cofferdam is used in places where the area to enclosed is very small and, the depth of water is more, say 4.5 to 6.0.

Timber piles are known as guide piles are first driven deep into the firm ground below the river bed.

Single Walled Cofferdam
Single Walled Cofferdam

Depending upon the velocity of the flow of water, the center-to-center spacing of the piles may vary between 1.8 m to 4 m.

longitudinal runners called wales are then bolted to the guide piles at a suitable distance apart sheet piles of steel or wooden are then driven into the river bed along the wales and are secured to the wales by bolts. The sheets on the two faces are braced by struts.

Half-filled bags of sand are stacked on the inside and outside faces of t sheets to increase the stability of the dam.

After the dam is constructed the water in the enclosed area is pumped out the construction work is taken up.

Read More: Pile Load Test – Method, Load on Pile & Causes of Pile Failure

5. Double-Walled Cofferdam

A double-wall cofferdam is come up with the larger enclosed working area. Its construction is essentially the same as that of a single-wall cofferdam, except that in place of one wall, a pair of walls with a gap in between is used all along the boundary of the space to be enclosed. These cofferdam can be used in the depth of water up to 12 m.

The double-wall cofferdams can be divided into two categories:

(a) Ohio river type wood sheeting the cofferdam.

(b) Wood or steel sheeting cofferdam wales and tie rods.

a) Ohio River Type Wood Sheeting Cofferdam:

Ohio River Types Cofferdam
Ohio River Types Cofferdam

These cofferdam were frequently used on the river Ohio in the USA, and hence it derives its name as Ohio River type cofferdam.

It is cheap and can be built rapidly. It can even be used on soft ground if properly protected against erosion. It is not suitable for deep water and for the flow of water with swift currents.

b) Wood or steel sheeting cofferdam with wales and tie rods:

This type of cofferdam is useful when the depth of water is about 6 meters to 10 meters. In the case of the ordinary and small-sized cofferdam, the wooden sheet piles are employed.

But, the size of the cofferdam is big or where it is not possible to drive guide piles due to more depth of water, the steel sheet piles with suitable bracings are employed in the construction of the cofferdam.

6. Cellular Cofferdam

These are made of steel sheet piles. It is mostly used for des watering large areas in places where the depth of water may be of the order of 18 m to 21m. They are two types,

(a) Circular type of cellular cofferdam

(b) Diaphragm Type Cellular Cofferdam:

Cellular Cofferdam
Cellular Cofferdam

In the case of diaphragm cellular cofferdam, a series of arcs connected to straight right cross walls. The radius of arcs is generally made equal to the distance between the cross walls.

With this arrangement, the tension in the arcs and cross walls remains equal. After the cells are driven to the required depth, they are filled with earth, sand, or gravel. It is necessary to fill adjacent cells at approximately the same rate.

In the case of a circular cellular cofferdam; a series of complete circles are connected by short arcs as shown in the figure. The radius of the arc is generally 250 cm, and it makes an angle of 30° to 45° at the point of contact with the circular cell.

Advantages of Circular Cellular Cofferdam

  1. Each cell may be filled up independently of other cells without any distortion of cells. Hence, the construction work for cells may be started simultaneously from several points. The arcs are installed after the completion of cells.
  2. Each circular cell is a self-supporting unit.
  3. For the construction of cellular cofferdam with circular cells, less quantity of steel per running length of cofferdam with diaphragm cell.

7. Concrete Cofferdam

Concrete coffer dams are mainly small-sized concrete dams and they have been used economically on many jobs. The framework usually consists of pre-cast R.C.C. piles and sheers. The pre-cast R.C.C. sheet piles are provided with proper edges and are driven similarly to steel sheet piles.

In cofferdam types, the design of different units must be properly done. The main disadvantages of concrete is that it is costly. But when it is incorporated as part of the structure, it proves to be economical.

This cofferdam also proves to be useful under the following conditions,

  1. The headroom limitation prevents the driving process of piles.
  2. It is necessary to avoid vibrations from the process of pile driving.
  3. The ground through which the piles are to be driven contains boulders that would spill steel sheet piles or cause them to come out of the interlock.

8. Suspended Cofferdam

Sometimes, a suspended cofferdam is designed in such a way a single unit of it is used several times. The cofferdam as such is lifted, floated, and placed in another position as soon as its purpose o served. A such cofferdam is also known as a movable cofferdam

They are suitable for the following situations,

  1. The construction work is of such types that a number of identical cofferdams are required. Hence if a single unit of the cofferdam is used several times in such a case it may result in an economy.
  2. A cap is to be provided on the piles or caissons below low water and the depth of the river bed is more. In such a case, the movable cofferdam will be found very much suitable.

Requirements of a Cofferdam

  1. It should be reasonably watertight.
  2. It should be generally constructed at the site of work.
  3. The design and layout of a cofferdam should be such that the total cost of construction, maintenance, and pumping is minimum.
  4. It should be stable against bursting, overturning, and sliding, under the floods and waves.
  5. The water to be excluded by a dam may be either groundwater or water lying above ground level. They may be deep or shallow and still or running.
  6. The materials used in the construction of a cofferdam are earth, timber, steel, and concrete.
  7. It should be so planned as to facilitate any dismantling and reuse of materials.

Read More: Caisson Foundation, Its Types & Application

Necessity of Cofferdam

The cofferdams are required in the following situations,

  1. When it is required to construct a structure in the river bed.
  2. When the structure is to be constructed on a seashore.
  3. When it is required to construct a structure one a bank of the lake or inside the lake.
  4. When deep excavations are carried out in coarse-grained soil.
  5. When excavation is carried out below groundwater table.
  6. During deep excavations, when sides of the trenches are likely to collapse.

Uses of Cofferdam

Following are the uses of cofferdam,

  1. To facilitate pile driving operations.
  2. To place grillage and raft foundations.
  3. To construct foundations of piers and abutments of bridges, darns, locks, 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.
  6. To provide space for carrying out the foundation work without disturbing or damaging the adjoining structures such as buildings, sewers, pipelines, etc.

Selection of Cofferdam Types

The cofferdam types are selected on the basis of the following factors,

  • The area to be protected by a cofferdam, i.e. a small area or a large area.
  • The depth of water to be dealt, with, i.e. shallow depth or deep depth.
  • The overtopping possibility by floods tides etc.
  • The characteristics of a bed on which the cofferdam is constructed.  Is i.e. a pervious layer or an impervious layer.
  • The velocity of flowing water.
  • The possibility of scour due to the reduction of waterway caused by the Construction of cofferdam.
  • The availability of construction materials in the vicinity of the site of work.
  • Transportation facilities are available.

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