A cofferdam is the space between two watertight bulkheads or sheet pile walls standing a relatively short distance apart — usually a few metres. The water between them is pumped out so that work can be carried out in the dry, or so that a double barrier against leakage is formed. Cofferdams are almost always temporary.
The double wall is what distinguishes it from an ordinary excavation support. If the outer wall leaks, the second wall and the space between it catch the water, where it can be pumped away. The soil between the walls contributes too: it makes the whole heavier and more stable.
Where it is used
- Laying pipes and cables in or beside water, for example a gas main crossing under a watercourse
- Recovering wrecks and objects from the bed, where the surrounding ground must stay stable
- Working next to existing buildings — the cofferdam buffers vibration and ground displacement from piling
- Temporarily diverting or damming a river or canal
- Screening a working area during works on the bed
Practical tips
- Expect uplift and seepage — a dry box in wet ground wants to float, and water finds its way under the wall; embedment depth, base sealing and the dewatering plan are the core of the design, not details
- Dewatering needs consent — abstracting groundwater requires notification or a permit from the water authority, and contaminated ground brings extra requirements
- Monitor the surroundings — record settlement and water levels at neighbouring buildings before, during and after the works; those records are also your evidence in any damage claim
- Keep the intermediate space accessible — a cofferdam you cannot inspect or pump out loses the very advantage it has over a single wall
- Plan the removal — extracting sheet piles causes vibration and ground loss, and belongs in the same risk assessment as installing them
Related terms
- Sheet pile wall
- Excavation support
- Dewatering
- Wellpoint dewatering
- Combi wall
- Flood defence
- Seepage
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