A cofferdam is a temporary watertight enclosure built in a river, harbor, or shallow coastal area so construction can happen below the waterline. It works like a dry box in the water, keeping outside water back while pumps remove water from the inside. Cofferdams are used to build bridge piers, foundations, tunnels, dams, docks, and other marine structures.
They matter because many strong structures must connect to solid ground that is normally covered by water.
A common cofferdam is made from interlocking steel sheet piles driven into the riverbed or seabed to form a sealed wall. After the enclosure is braced, pumps lower the water level inside, and crews excavate mud or sediment until they reach a stable working surface. The walls must resist water pressure, soil pressure, seepage, and construction loads.
Engineers design the shape, depth, bracing, and pumping system so the dry work area stays safe until the permanent structure is finished.
Key Facts
- A cofferdam is temporary, but it must be designed to safely resist water and soil forces.
- Water pressure increases with depth: P = rho g h.
- The total horizontal force on a flat vertical wall grows with the square of depth: F = 1/2 rho g h^2 A/h for a rectangular wall of area A, or F per meter width = 1/2 rho g h^2.
- Sheet piles are interlocking steel panels driven into the ground to form a mostly watertight barrier.
- Pumps remove water from inside the cofferdam, but seepage can continue through soil or small gaps.
- Internal braces, wales, or struts spread wall forces so the sheet piles do not bend inward.
Vocabulary
- Cofferdam
- A temporary watertight enclosure that lets crews work on a dry area below the surrounding water level.
- Sheet pile
- A long interlocking steel, wood, or concrete panel driven into the ground to form a retaining wall.
- Seepage
- The slow movement of water through soil, joints, or small openings into the cofferdam.
- Bracing
- A system of beams, struts, or frames that supports the cofferdam walls against inward pressure.
- Dewatering
- The process of pumping water out of an excavation or enclosure to create a dry work area.
Common Mistakes to Avoid
- Assuming the wall feels the same pressure at every depth. This is wrong because water pressure increases with depth according to P = rho g h.
- Forgetting seepage after the cofferdam is pumped out. Even a strong wall may allow water to enter through soil or joints, so pumps and filters are often still needed.
- Thinking a cofferdam is the permanent foundation. The cofferdam is usually removed or left inactive after the permanent pier, footing, or tunnel structure is completed.
- Ignoring braces in the design. Sheet piles alone may bend or fail under water and soil pressure unless struts, wales, or tiebacks help carry the load.
Practice Questions
- 1 A cofferdam wall holds back river water that is 4.0 m deep. Using rho = 1000 kg/m^3 and g = 9.8 m/s^2, what is the water pressure at the bottom of the wall?
- 2 For a straight cofferdam wall 1.0 m wide with water 3.0 m deep on the outside and dry conditions inside, calculate the total horizontal water force per meter width using F = 1/2 rho g h^2.
- 3 A cofferdam has sheet piles and pumps, but water is still slowly entering the dry work area. Explain two possible reasons this could happen and one engineering response for each.