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Cofferdam

Temporary watertight enclosure to dewater an area for in-the-dry construction

Also calledcoffer damsheet pile cofferdamcellular cofferdamtemporary water barrier
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CODES
IS 9527
Definition

A cofferdam is a temporary structure built to enclose and exclude water (and sometimes soil) from a working area so that foundation, pier or substructure work can be carried out 'in the dry'. Common types are single/double-skin sheet-pile cofferdams, braced sheet-pile boxes, cellular (steel sheet-pile cell) cofferdams for large river works, and earthfill bunds for shallow situations. They are the standard enabling works for bridge piers, intake structures, dock and marine foundations and deep pump stations.

Design — guided by IS 9527 and bridge/marine practice — must address external water and earth pressure, internal bracing/strutting, seepage and base heave/piping, dewatering capacity, and the construction sequence and removal. Cofferdams are temporary works with a real failure history (flooding, base blow-out, strut buckling), so they require a competent temporary-works design, instrumented monitoring, and a defined flooding/contingency plan; economy comes from right-sizing the enclosure and the dewatering system to the ground and water regime.

Where used
  • Bridge pier + abutment foundation construction
  • Intake, outfall + pump-station substructures
  • Dock, jetty + marine foundation works
  • Deep excavation below the water table
  • River + canal in-the-dry construction
Acceptance / threshold
Designed as temporary works per IS 9527 / bridge-marine practice for water + earth pressure, seepage, base stability + bracing, with dewatering capacity, monitoring and a flooding/contingency plan; constructed + removed per the approved sequence.
Frequently asked
What is a cofferdam used for?
To form a temporary watertight enclosure that excludes water (and soil) from a work area — typically bridge piers, intakes, marine and deep substructures — so the permanent works can be built in the dry.
What are the main design concerns for a cofferdam?
External water and earth pressure, internal bracing/strut design, seepage and base heave/piping, dewatering capacity, and a safe construction/removal sequence with monitoring and a flooding contingency plan.
Related terms
Sheet Pile
Interlocking steel/concrete piles forming a continuous wall — used for retaining temporary excavations and waterfront.
Bored Cast-in-situ Pile
Pile constructed by drilling a hole and filling with concrete + reinforcement. Diameter 300-2400 mm, depth up to 60 m.
Pile Cap
Reinforced concrete cap connecting tops of piles to the column. Designed as a deep beam or strut-and-tie.
Pile Load Test
Field test verifying a pile's safe capacity by applying test load + measuring settlement
Negative Skin Friction (Downdrag)
Downward soil drag on a pile when surrounding soil settles more than the pile
Under-Reamed Pile
Bored pile with one/more enlarged bulbs — the standard remedy for expansive soil
Raft Foundation
Single thick slab covering the entire building footprint — for soft soil, heavy loads, or when isolated footings overlap.
Well Foundation
Hollow shaft sunk through soft soil to firm strata, filled with concrete — used for major bridge piers in rivers.
Stepped Footing
Footing with stepped/tapered profile — saves concrete on large footings while maintaining bearing pressure distribution.
Combined Footing
Single footing supporting two or more columns — used when columns are close (<2 m) or property line restricts.