Cofferdam
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During the construction of bridges , dams or any other
structure where the foundation part of the structure is
most likely to lie underwater, we have to opt for
underwater construction
Construction in water poses many difficulties especially
in the places where there the depth is considerable.
During underwater construction our main objective is
to create dry and water free environment for working in
such a manner that the structural stability of the
structure is not compromised.
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UNDERWAT
ER
CONSTRUC
TION
Constructi
on
technique
s
Methods
of
Placing
of
concrete
Caissons
Cofferda
ms
Tremie
method
Pump
method
Toggle
bags
Bag
works
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Caissons:
Watertight retaining structure .
Permanent in nature.
Used to work on foundation of bridge pier,
construction of concrete dam or for the repair of ships.
Constructed in such a manner so that the water can be
pumped out.
Keeps working environment dry.
Working inside a caisson
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TYPES OF CAISSONS
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Control post
Open caisson
grab
Ground
water
level
Circumferential
gap
shoe
pontoon
Ballast water
Box caisson
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PNEUMATIC CAISSON
•Large inverted box on which pier is built.
•Compressed air is used to keep the water
and mud out.
•Used to work on riverbed or quicksand's.
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Suction caisson
• More suitable for off shore construction.
• Upturned bucket embedded in marine sediment.
• Embedment achieved by pushing or by creating negative
pressure.
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ADVANTAGES OF CAISSONS
Economic.
Slightly less noise and reduced vibrations.
Easily adaptable to varying site conditions.
High axial and lateral loading capacity.
Minimal handling equipment is required for placement
of reinforcing cage.
Placement is sometimes possible in types of soil that a
driven pile could not penetrate
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Dangers encountered:
Caisson disease is so named since it appeared in
construction workers when they left the compressed
atmosphere of the caisson and rapidly re entered
normal (decompressed) atmospheric conditions.
Construction of the Brooklyn Bridge, which was built
with the help of caissons, resulted in numerous
workers being either killed or permanently injured by
caisson disease during its construction, including the
designer's son and Chief Engineer of the project.
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Cofferdams:
 Temporary structure
 Built within or in pairs across a body of water
 Allows the enclosed space to be pumped out, creating a dry
work environment .
 Enclosed coffers are commonly used for construction and
repair of oil platforms, bridge piers and other support
structures built within or over water.
 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.
 Used for shallow constructions having depth less than 10m.
Working inside a cofferdam
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DEPT. OF CIVIL ENGINEERING GCET JAMMU
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Components of cofferdam:
 Sheet piling
Sheet piling is a manufactured
construction product with a
mechanical connection
“interlock” at both ends of the
section. These mechanical
connections interlock with one
another to form a continuous
wall of sheeting.
 Brace piling
 Concrete seal
Sheet piles
Bracing frame
COFFERDAM CONSTRUCTION SEQUENCE:
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1. Pre-dredge to remove soil or soft sediments and level the area of the
cofferdam
2. Drive temporary support piles.
3. Temporarily erect bracing frame on the support piles
4. Set steel sheet piles.
5. Drive sheet piles to grade.
6. Block between bracing frame and sheets, and provide ties for sheet
piles at the top as necessary
7. Excavate inside the grade or slightly below grade, while leaving the
cofferdam full of water
8. Drive bearing
9. Place rock fill as a levelling and support course.
10. Place Tremie concrete seal .
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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.
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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.
 Caissons are permanent structures and becomes economical in cases where
the plan area of foundation is small, large depth of water and for loose
soils.
 Suction caisson anchors are gaining considerable acceptance in the
offshore industry.