All About Coffer Dam



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.

Types of Cofferdams

  • Braced
  • Earth-Type
  • Timber Crib
  • Double-Walled Sheet Pile
  • Cellular

Braced Cofferdams

  • Formed from a single wall of sheet piling
  • Drive into the ground to form a box around the excavation site
  • The “box” is then braced on the inside
  • Interior is dewatering
  • Primarily used for bridge piers in shallow water.

Earth-Type Cofferdams

  • Simplest Type of Cofferdam
  • Consists of an earth bank w/a clay core or vertical sheet piling enclosing the excavation.
  • Used for low-level waters with low velocity
  • Easily scoured by water rising over the top

Timber Crib Cofferdam

  • Cellular-Type Cofferdam
  • Constructed on land and floated into place
  • Lower portion of each cell matched with contour of river bed
  • Uses rock ballast and soil to decrease seepage and sink into place
  • Also known as “Gravity Dam”
  • Usually consists of 12’ x 12’ cells
  • Used in rapid currents or on rocky river beds
  • Must be properly designed to resist lateral forces such as:
    • Tipping / Overturning
    • Sliding

Double-Walled Cofferdam

  • Two-parallel rows of steel sheet piles driven into the ground
  • Tied together with anchors and wales then filled with soil
  • Three principle types:
    • Box: Consists of straight flush walls
    • Semicircular cells connected by diaphragms
    • Circular cells connected with tie-rods or diaphragms

Cellular Cofferdam

  • Two main types are circular and segmental.
  • Can be used on a temporary or permanent basic.
  • Force are resisted by the mass of the cofferdam.

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 Cofferdams

  • 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

Disadvantages of Cofferdams

  • Special equipment required
  • Relatively expensive
  • Typically very time consuming & tedious
  • If rushed, sheets can be driven out of locks or out of plumb
  • When in flowing water “log jams” may occur creating added stress on structure

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

Pictures of Pile Hammers

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Steel Sheet Piling Properties

  • Moderately watertight
  • High shear and bending strength
  • High interlock strength
  • Easy to install/remove
  • Reusable
  • Can be cantilevered but typically require additional structural member (i.e. wales and cross bracing)

Traditional Sheet Pile Shapes

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Steel Sheet Pile Interlocks

  • No industry Standard
  • Interlocks Should:
    • Provide relative water or earth-tight connections
    • Permit reasonable free sliding to connect sheets during installation
    • Provide minimum guaranteed pull strength
    • Allow minimum swing between locks in order to form a circle

Typical Types of Interlocks

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Braced Cofferdam Construction

  • Install Wale and Strut System for Framework / Template
  • Install Sheet piles using the Framework as Template

Tips for installing Sheet piles

    • Always set-up a template system
    • Rule of thumb: Crane Boom length should be twice that of the sheets
    • Drive the Sheets with the “male” interlock leading in order to avoid soil plugs
    • If the “female” interlock must lead, place a bolt or other object at the bottom to avoid debris filling the slot
    • Align and plumb the first two sheets and drive carefully and accurately
    • Drive sheets in pairs when possible placing the hammer in the center of the pair

Site before construction

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Excavation of terrain performed to avoid “sliding” while lock foundation was being excavated

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Slowed the water work

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Deflector and cells being constructed simultaneously

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Typical Cell Framework and Construction

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Typical pile driving operation using a vibratory hammer. Impact hammers were also used.

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Extremely good river conditions

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Typical cell filling operation

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Typical round Cell Construction

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Dewatering

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Dewatering after 40 days

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Completed Cofferdam, turned over to Lock Contractor

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Typical day of operations within the cofferdam

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Overview shot of project

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