Caissons and cofferdams are temporary enclosures used to create dry working areas below water level for construction of bridge foundations, quay walls, and underwater structures. Working inside these structures combines confined space hazards with water ingress risk, deep excavation, and heavy construction operations in restricted areas. This talk covers the specific controls for safe working within caissons and cofferdams.
Key Hazards
Flooding from cofferdam seal failure or water overtopping during tidal surges
Confined space conditions with poor ventilation and potential gas accumulation
Structural failure of the cofferdam walls under hydrostatic or soil pressure
Workers unable to evacuate quickly through limited access points during emergencies
Control Measures
Ensure the caisson or cofferdam is designed by a competent temporary works engineer for the conditions.
Treat the interior as a confined space and implement full entry permit and monitoring controls.
Monitor water levels inside and outside the structure continuously during all work activities.
Maintain dewatering pumps with backup capacity to manage water ingress at all times.
Provide multiple access and egress routes for emergency evacuation from within the structure.
Monitor the structural condition of walls, seals, and bracing throughout the working period.
Brief all personnel on the flood emergency procedure and evacuation signals before entry.
Carry out atmospheric monitoring for H2S, methane, and oxygen inside the enclosed structure.
Ensure rescue equipment is positioned at the access point ready for immediate deployment.
Emergency / Rescue
If water ingress accelerates or the cofferdam shows signs of failure, evacuate all personnel immediately using the designated escape routes. Sound the flood alarm, call 999, and do not re-enter until the engineer confirms it is safe.
Remember
The caisson or cofferdam must be designed by a competent temporary works engineer
Treat the interior as a confined space with full entry permit and monitoring controls
Monitor water levels continuously inside and outside the structure during all work
Maintain dewatering pumps with backup capacity to manage water ingress at all times
Multiple escape routes must be available for emergency evacuation from within the structure
Applicable Legislation: CDM Regulations 2015 · Confined Spaces Regulations 1997 · BS 5975:2019 (Temporary Works) · Health and Safety at Work etc. Act 1974