Cofferdam

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Cofferdam


          “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.”
 Cofferdams are temporary enclosures to keep out water and soil so as to permit dewatering and construction of the permanent facility (structure) in the dry.
A cofferdam involves the interaction of the structure, soil, and water.  The loads imposed include the hydrostatic forces of the water, as well as the dynamic forces due to currents and waves.
Because cofferdams are typically constructed under adverse conditions in a marine environment, and because significant deformations of elements may occur at various stages of construction, it is difficult to maintain close tolerances.  Ample provisions must be made for deviations in dimensions so that the finished structure may be constructed according to plan.
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.
Removal of the cofferdam must be planned and executed with the same degree of care as its installation, on a stage-by-stage basis.  The effect of the removal on the permanent structure must also be considered.  For this reason, sheet piles extending below the permanent structure are often cut off and left in place, since their removal may damage the foundation soils adjacent to the structure.

In cofferdam construction, safety is a paramount concern, since workers will be exposed to the hazard of flooding and collapse.
Safety requires:
good design
proper construction
verification that the structure is being constructed as planned
monitoring the behavior of the cofferdam and surrounding area provision of adequate access light and ventilation, and attention to safe practices on the part of all workers and supervisors.

Types of cofferdam:
Braced
Earth-Type
Timber Crib
Double-Walled Sheet Pile
Cellular
Braced Cofferdams
Formed from a single wall of sheet piling
Driven into the ground to form a box around the excavation site
The "box" is then braced on the inside
Interior is dewatered

Primarily used for bridge piers in shallow water (30 - 35 ft depth)
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