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Cofferdams and Caissons

Designers Knowledge Base


The CDM 2015 Regulation 23 makes specific reference to cofferdams and caissons. The regulation highlights that the units must be of suitable design & construction, properly maintained and appropriately equipped for emergency escape. The Regulation defines the conditions under which they may be used in construction and the inspection regime required.

When designing for the use of cofferdams the designer should be aware of, and assess the risks from, the following principal hazards.

Principal hazards and risks

  • falls from height from dam walls into excavation or into water
  • health hazards from contaminated water, hazardous gases in confined spaces, noise and vibration from piling operations
  • hazards from services such as cables in river bed, temporary power cables in water etc
  • contact with moving equipment such as collisions between boats, pontoons, crane barges and cofferdam. moving plant and slewed equipment in confined space
  • flooding caused by failure of cofferdam walls


Investigate the site to ascertain the underwater topography,water table, ground contamination and any obstructions. Consider whether cofferdam can be wholly incorporated into the permanent structure. Investigate surrounding environment in terms of plant and materials access and to anticipate impact on adjacent features or structures.

Some design options to reduce hazards

  • consider a temporary causeway instead of floating rigs
  • can prefabricated caissons towed into place be used instead of driven piled cofferdams to lessen the work in the water and reduce the noise?
  • consider the level of the working platform or causeway with respect to high tide and flood levels
  • provide for structural enhancement to respond to adverse monitoring reports
  • plan the shape of the cofferdam to suit working space and construction plant as well as final structure
  • consider the use of different construction or of remotely controlled techniques to design out the need for underwater working
  • incorporate lugs and holes in design to aid fixing of equipment

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