Caissons and Cofferdams

Caissons and cofferdams are temporary watertight enclosures used to allow construction below water level.
Work inside these structures is classified as confined space entry under the Confined Spaces Regulations 1997.
Cofferdams are typically formed from sheet piles or sandbags and are pumped dry to create a work area.
Caissons are sunk into position under their own weight or by excavation, often into riverbed or seabed material.
Flooding is the primary catastrophic risk — structural failure or overtopping can drown workers instantly.
The atmosphere inside deep caissons can become oxygen-depleted or contaminated with ground gases.
Working inside cofferdams combines confined space, excavation, working near water, and structural stability risks.
Temporary works designs must account for water pressure, soil conditions, and construction loading.
Emergency evacuation routes must be maintained at all times with rapid escape from rising water levels.
Continuous dewatering is required, with backup pumps and power supply to prevent sudden flooding.

Prevent drowning	Structural failure or pump loss floods the enclosure rapidly, trapping and drowning workers inside.
Confined space hazards	Deep caissons and enclosed cofferdams create toxic atmosphere and restricted escape route risks.
Structural collapse	Water pressure and soil loading can cause cofferdam walls to fail if the temporary works design is inadequate.
Legal compliance	Confined Spaces Regulations 1997 and CDM 2015 require permits, rescue plans, and competent supervision.

Do

Don't

Treat all entry into caissons and cofferdams as confined space work requiring a permit
Ensure the temporary works design is approved before construction or entry begins
Maintain backup dewatering pumps and a standby power supply at all times during work
Test the atmosphere for oxygen, toxic gases, and flammable gases before every entry
Establish and brief a rapid evacuation procedure in case of water ingress or flooding
Monitor water levels, pump performance, and structural condition continuously throughout
Ensure rescue equipment and a trained standby team are in place before entry
Inspect sheet piles, seals, and bracing daily for signs of movement or water seepage
Maintain clear escape routes from the lowest working level to safe ground at all times
Brief all workers on emergency signals and the evacuation plan before each shift

DON'T enter a caisson or cofferdam without a valid confined space entry permit
DON'T work inside without confirmed backup dewatering and standby power supply
DON'T assume the atmosphere is safe — test before entry and monitor continuously
DON'T allow more workers inside than the rescue plan and escape routes can accommodate
DON'T ignore signs of water seepage, soil movement, or bracing displacement
DON'T block escape routes with materials, plant, or equipment inside the enclosure
DON'T continue working if dewatering pumps fail — evacuate immediately and investigate
DON'T enter without a rescue team and equipment on standby at the entry point
DON'T exceed the design loading of the cofferdam with plant or material storage
DON'T rely on a single pump — always have redundancy for continuous dewatering

See also: Confined Space Entry Procedures | Temporary Dams and Cofferdams

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