COFFERDAM

OVERVIEW

INTRODUCTION OBJECTIVE TYPES OF COFFERDAM METHADOLOGY FACTOR AFFECTING DESIGN OF COFFERDAMADVANTAGESCONCLUSIONFUTURE ASPECTS

INTRODUCTION The word "cofferdam" comes from "coffer"

meaning box, in other words a dam in the shape of a box.

A cofferdam is a temporary structure designed to keep water and/or soil out of the excavation in which a bridge pier or other structure is built.

A cofferdam involves the interaction of the structure, soil, and water and the loads imposed include the hydrostatic forces of the water, as well as the dynamic forces due to currents and waves.

INTRODUCTION CONT….

The loads imposed on the cofferdam structure by construction equipment and operations must be considered, both during installation of the cofferdam and during construction of the structure itself.

OBJECTIVE The main objectives of temporary structure

“COFFERDAM” are :-

Pile driving operation.Place grillage & raft foundation.Construct new structures.Enclose space for removal.Constructing without disturbances

TYPES OF COFFERDAM1. Braced: It is formed from a single wall of sheet piling which is driven into the ground to form a “box” around the excavation site.

2. Earth-Type: It is the simplest type of cofferdam. It consists of an earth bank with a lay core or vertical sheet

piling enclosing the excavation.

3. Timber Crib: Constructed on land and floated into place. Lower portion of each cell is matched with contour of river.

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 rod sat one or more levels.

TYPES OF COFFERDAM CONT……

5. Cellular: 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.

METHADOLOGY1 Fixing of alignment for cofferdam : Alignment of cofferdam shall be fixed on the ground with the help of total station as per the relevant drawing. 2 Construction of rock bund : Construction of rock bund with 4m top width and 1:1 side slopes. Rock bund will start by filling of boulders from the one end of diaphragm wall.3 Temporary earth fill : Filling of temporary earth fillfor cofferdam and entrance gate will start parallel to the rock bund.

METHADOLOGY CONT….4 Interlocking sheet piles : Driving of interlocking sheet piles will start from the east side the south dock with the help of crane with vibro hammer on completed portion of temporary bund.5 Capping beam : On completion of top plugging excavation along the interlocking piles will be carried out for construction of capping beam.6 Anchoring works : The main application of geotechnical ground anchors is to prevent horizontal movement and ensure stability of retaining structures including sheet pile, bored concrete pile and steel tubular pile walls.

Working inside a cofferdam

FACTORS AFFECTING DESIGN OF A COFFERDAM

Hydrostatic head of waterDimensions of area.Sub-soil condition.Fluctuations of outside water level.Possibility of erosion.Floating logs.Presence of ice.

ADVANTAGESPoor environment.Safe environment.Design responsibilityEasy work.Re-usability of materials.

CONCLUSION

Cofferdams are temporary structures and used in cases where the plan area of foundation is very large, depth of water is less and for the soft soils, where soils allow easy driving of sheet piles.

FUTURE ASPECTS

While significant progress and achievements have been made in the field of geotechnical finite element mechanics, further advancements are necessary. On the practical side, as computational power increases it is expected that three dimensional software programs will be come increasingly popular and more affordable. Hence, human judgment is by far the most important aspect of finite element analyses of cofferdam excavations.

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