Cofferdams تعني المنشأت المؤقته التي تصمم لحفظ الماء او التربة بعيدا عن الحفريات مثل دعامات الكباري= الجسور او السدود او اي منشأت اخرى يرغب في بناؤها .
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- Cofferdams are temporary enclosures to keep outwater and soil so as to permit dewatering andconstruction 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 itsinstallation, 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
- good design
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
- supervisorsTypes of cofferdam:
Double-Walled Sheet Pile
- 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).Cofferdam Design Considerations
- Scouring or undermining by rapidly flowing water
Stability against overturning or tilting
Upward forces on outside edge due to tilting
Stability against vertical shear
Effects of forces resulting from:
Ice, Wave, Water, Active Earth and Passive Earth Pressures.
Advantages of Cofferdam
- Allow excavation and construction of structures in otherwise poor environment
Provides safe environment to work
Contractors typically have design responsibility
Steel sheet piles are easily installed and removed
Materials can typically be reused on other projects
Items needed for installation
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
Barges may be required
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