Cofferdams in Building and Construction

Cofferdams

• A temporary structure designed and constructed to keep water and/or soil out of an excavation area in which a bridge pier or other structure or structural component is constructed.
• A cofferdam is a temporary enclosure built within or in pairs across a body of water, constructed to allow the enclosed area to be pumped out, creating a dry work environment for major work to proceed.

Types of Cofferdams

• Single skin cofferdams: consist of a single row of sheet piles forming an almost watertight box.
• Double skin cofferdams: self-supporting gravity structures constructed by using two parallel rows of sheet piles with a filling material placed in the void created.
• Braced cofferdams: formed from a single wall of sheet piling driven into the ground to form a box around the excavation area, braced on the inside, and dewatered.

Advantages of Cofferdams

• Allow excavation and construction of structures in otherwise poor environments.
• Provide a 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.

Limitations of Cofferdams

• Special equipment required.
• Relatively expensive.
• Typically very time-consuming and tedious.
• Steel sheet piles sometimes driven out of lock.

Installation Requirements

• Pile driving hammer; vibratory or impact.
• Crane of sufficient size.
• Steel sheet piles.
• H piles and/or wide flange beams for wales and stringers.
• Barges may be required.

Steel Sheet Pile Properties

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

Typical Cofferdam Components

• Sheet piling.
• Bracing frame.
• Concrete seal.
• Bearing piles.

Typical Cofferdam Construction Sequence

• Pre-dredge to remove soil or soft sediments and level the area of the cofferdam.
• Drive temporary support piles.
• Temporarily erect bracing frame on the support piles.
• Set steel sheet piles, starting at all four corners and meeting at the center of each side, and drive sheet piles to grade.
• Excavate inside the grade or slightly below grade, while leaving the cofferdam full of water.
• Drive bearing piles.
• Place rock fill as a levelling and support course.
• Place tremie concrete seal.