A.D.I.T New V. V. Nagar Branch : Civil Engineering (5th semester) Subject : Soil Mechanics (2150609) Topic : Cofferdams Prepared by : Tina Naik (130010106025) Mohini Parmar (130010106029) Guided by : Prof. K. R. Trivedi sir

Foundation It is the lowest part of a structure below the ground level which is in direct contact with the ground and transmits all the loads to the ground.

Types of foundation : 3. Coffer dams Foundation Shallow Foundation D <= B 1. Spread footing 2. Combined footing 3. Strap footing 4. Raft or mat foundation 5. Grillage foundation Deep Foundation D > B 1. Pile foundation 2. Caissons or well foundation 3. Coffer dams

COFFERDAMS

COFFERDAMS A cofferdam is a temporary structure which is built in a river, lake etc. to remove water from an area and make it dry conditions for construction work. Cofferdams are usually required for projects such as dams, docks & construction of bridge piers and abutments.

Requirements of a cofferdam : The cofferdam should be watertight. It should be generally constructed at site of work. The design & layout of a cofferdam should be minimum. It should be stable against bursting, overturning & sliding, under the floods & waves.

Uses of cofferdams : To facilitate pile driving operations. To place grillage & raft foundations. To construct foundations of piers & abutments of bridges, dams, docks etc. To provide a working platform for the foundations when water is met with.

The selection of a cofferdams : The area to be protected by a cofferdam (small area or large area). The depth of water (shallow depth or deep depth). The possibility of overtopping by floods, tides etc. The nature of bed on which the cofferdam is to rest (pervious or impervious layers). Velocity of flowing water. The possibility of scour due to reduction of waterway caused by the construction of cofferdam. The availability of construction materials. Transportation facilities available.

3. Rock fill crib cofferdam Types of cofferdams : Cofferdams 1. Earth fill cofferdam 2. Rock fill cofferdam 3. Rock fill crib cofferdam 4. Single wall cofferdam 5. Double wall cofferdam 6. Cellular cofferdam

Earth fill cofferdam : It is the simplest form of cofferdam. It is constructed in places where the depth of water is 1.3m to 1.8m, & velocity of flow is low.

Rock fill cofferdam : This construction is adopted only if the stone is easily available in the nearby areas. It is constructed when the depth of water is 1.8m to 3m, stone or rubble is used. An impervious layer of earth is laid on the outer face of cofferdam, which makes it impervious.

Rock fill crib cofferdam : A rock fill crib cofferdam is comprised of timber cribs. A crib is a framework of wooden horizontal & cross beams. This cribs are open at the bottom and are filled with rock, gravel or earth. This type of cofferdam favourable where the bed of stream is hard, depth of water is high, water is swift.

Rock fill crib cofferdam Rock fill cofferdam Rock fill crib cofferdam In this cofferdam no use of timber or wood. Stone or rubble is used for the embankment. Earth is only used for outer layer. It is suitable where depth of water is 1.8m – 3m. It has not good stability against overturning or sliding. In this cofferdam use of timber cribs. In this crib filled with rock, gravel or earth. It is suitable where the depth of water is around 4m. It has a good stability against overturning or sliding.

Single wall cofferdam : This type of cofferdam is used in places where the area to be enclosed is very small & depth of water is more, 4.5m to 6m.

Double wall cofferdam : Double wall cofferdams are provided to enclosed a larger area & depth of water up to 12m.

Single wall cofferdam Double wall cofferdam In this cofferdam using one wall. According to depth, more, say 4.5m – 6m & enclosed area to be very small. After the cofferdam is constructed, the water in the enclosed area is pumped out. In the double wall cofferdam using a pair of walls. According to depth of water up to 12m & enclosed area to be larger. A pair of walls with a gap in between is used all along the boundary of the space to be enclosed.

Cellular cofferdam : The cellular cofferdam is made of steel sheet piles. It is mostly used for de-watering large areas in places where the depth of water may be 18m to 21m. They are two types: (a) Circular type (b) Diaphragm type

Diaphragm cellular cofferdam Circular cellular cofferdam In this, series of arcs are connected to straight cross walls. The radius of arc is generally made equal to the distance between the cross walls. In this, series of complete circles are connected by short arcs. The radius of the arc is generally 250cm & it makes an angle of 30 – 40 degree at the point of contact with the circle cell.

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