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

  • 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

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


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


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


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


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


Slowed the water work


Deflector and cells being constructed simultaneously


Typical Cell Framework and Construction


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


Extremely good river conditions


Typical cell filling operation


Typical round Cell Construction





Dewatering after 40 days


Completed Cofferdam, turned over to Lock Contractor


Typical day of operations within the cofferdam


Overview shot of project


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