Cofferdams & Excavation Support — The UK Safety Rules for Trades (2026)

What Is a Cofferdam?

A cofferdam is a temporary watertight enclosure built within or next to water, or in waterlogged ground, which is then pumped dry so that work can take place inside it below the surrounding water level. Bridge piers, lock walls, intakes, outfalls, pump stations and deep foundations are all commonly built inside cofferdams: the enclosure holds back the surrounding water and soil, and pumps keep the inside dry enough to work in.

Cofferdams are usually formed from interlocking steel sheet piles driven into the ground, braced internally with horizontal frames of walings and struts. A caisson is a related structure — a watertight box or cylinder sunk into the ground or bed, often left in place as part of the permanent works. Both share the same defining feature: they hold back ground and water under significant load, and must be treated with the rigour that implies.

How a Cofferdam Differs From General Trench Support

A trench box or a sheeted-and-strutted excavation is also temporary works, and many of the same principles apply, but a cofferdam is a different order of risk. A standard trench support system holds back soil and limited groundwater in a shallow excavation. A cofferdam is engineered to resist substantial hydrostatic pressure from open water or a high water table, often to considerable depth, while the inside is actively kept dry by pumping.

That difference matters in three ways. The loads are larger and the consequences of failure are catastrophic — a sudden inundation can drown a working party in seconds. The structure depends on continuous pumping, so loss of power or pump failure is itself a hazard event. And a pumped-out cofferdam frequently meets the legal definition of a confined space. None of those apply in the same way to a shallow strutted trench, which is why cofferdams demand a higher level of design competence and supervision.

The CDM 2015 Framework and Temporary Works

Cofferdams and excavation support are temporary works, and the Construction (Design and Management) Regulations 2015 (CDM 2015) govern how they are planned and managed. The principal contractor and contractors must ensure the work is properly planned, managed and monitored, and carried out by people with the right skills, knowledge, experience and training.

Industry practice — set out in BS 5975, the code of practice for temporary works procedures — is to appoint a Temporary Works Coordinator (TWC) to oversee the process on site. The TWC makes sure a design brief exists, that a register of temporary works is kept, and that nothing is loaded or used until a formal permit to proceed has been issued. On larger or higher-risk schemes a Temporary Works Supervisor may also assist.

The design itself must be produced by a competent temporary works engineer and subjected to an independent design check proportionate to the risk. For a cofferdam, that check sits at the highest category — design checked by a different organisation or an independent team — and must account for ground and groundwater conditions, water pressures, surcharge loads from plant and stockpiles, and the sequence of installation and removal.

Design, Installation and Alteration Under Competent Supervision

CDM 2015 is explicit on this point: a cofferdam or caisson must be of suitable design and construction, of suitable and sound material and strength for its purpose, and properly maintained — and its construction, installation, alteration or dismantling may only be carried out under the supervision of a competent person.

In practice that means you do not improvise changes on site. If conditions differ from those assumed in the design — an obstruction during pile driving, a higher water level, a weaker stratum — work stops and the temporary works engineer reviews the design before anything is altered. The same applies to the order in which frames and walings are installed and removed: the propping sequence is part of the design and must be followed exactly. Removing a strut out of sequence has caused fatal collapses.

Inspection Requirements and Records

A cofferdam used as a place of work must be inspected by a competent person to confirm it remains safe. The regime mirrors that for excavations under CDM 2015: inspect at the start of every shift before work begins, after any event likely to have affected stability (heavy rain, flooding, a rise in water level, an impact from plant, or a change in loading), and after any accidental fall of rock, earth or other material.

Where the cofferdam is a place where a person could be buried, trapped or drowned, the competent person must prepare a written report before the end of the shift and keep it available on site. This is not box-ticking — it is the audit trail showing the structure was checked and found sound before each working party went in, and the first thing the HSE will ask for after an incident.

Confined Space Risks Inside a Cofferdam

A pumped-out cofferdam can be a confined space within the meaning of the Confined Spaces Regulations 1997, even though it is open to the sky. The legal test is whether there is a reasonably foreseeable specified risk — and in a cofferdam there are several: sudden inundation if the structure or pumping fails, oxygen deficiency, the build-up of toxic or flammable gases (hydrogen sulphide from disturbed silts, carbon dioxide, or vapours from contaminated ground), and engine exhaust from pumps and plant collecting at low level.

Where a confined space exists, avoid entry if the work can be done another way; if entry is unavoidable, work to a safe system of work with atmospheric testing before and during entry, continuous personal gas detectors, forced ventilation where needed, a clear means of raising the alarm, and suitable emergency arrangements. These are baseline controls, not optional extras.

Water Management, Pumping and Sudden Inundation

The whole purpose of a cofferdam is to keep water out, so water management is central to its safety. Pumps remove seepage and any water that overtops or leaks through the sheet piles, sized to a design pumping capacity. Loss of that capacity — a pump failure, a power cut, a blocked sump — allows the water level inside to rise, so standby pumps, backup power and a rising-water alarm are standard precautions.

The most feared event is sudden inundation: rapid, uncontrolled flooding caused by piping or boiling of the ground (water forcing up through the base), overtopping during a flood, or a structural failure of the support frame. Anyone working below water level must have a planned, rehearsed escape route and a way to get out fast. River and tidal sites add the hazard of changing levels, so flood and tide forecasting and a trigger level for evacuation form part of the safe system of work.

Sheet Piling, Frames and Waling Support

Most cofferdams rely on interlocking steel sheet piles to form the watertight wall, driven to a toe depth that resists both lateral pressure and base instability. Inside, horizontal frames of walings (the beams running along the face) and struts (the props spanning the excavation) brace the piles against the water and soil load, with additional frames installed at design levels as the dig goes deeper.

The integrity of this system depends on following the design sequence: frames in the correct order and at the correct levels, props of the specified section properly seated, and nothing cut, removed or repositioned without the temporary works engineer's sign-off. Overloading a waling with surface plant or spoil, or knocking a strut with an excavator bucket, can compromise the structure — which is exactly why supervision and inspection matter so much.

Ground and Groundwater Assessment

No cofferdam can be designed safely without a proper understanding of the ground. A ground investigation should establish the soil profile, strength parameters, the position and variability of the water table, and the permeability of the strata — all of which feed the geotechnical design. In the UK, geotechnical design is carried out to BS EN 1997 (Eurocode 7), which sets out how ground and groundwater actions are assessed and how safety factors are applied.

Particular attention goes to base failure: in fine sands and silts a high upward water gradient can cause boiling or quick conditions at the base, undermining stability even when the walls look sound. Contaminated ground and buried services must also be identified before driving piles. Where the investigation reveals conditions worse than assumed, the design must be revisited rather than worked around.

Emergency and Rescue Planning

The worst-case events in a cofferdam — inundation, collapse, a gas incident — develop fast and leave little time to react, so emergency and rescue planning is a legal and practical necessity. A rescue plan must be in place before anyone enters, must not rely solely on the emergency services, and must be specific to the structure and the failure modes identified in the risk assessment.

That means trained rescuers on or immediately available to site, the right equipment positioned and maintained, a reliable way to raise the alarm, and a means to recover a casualty from depth — including breathing apparatus where the atmosphere may be unbreathable. Rehearse the plan: a rescue arrangement that has never been practised is not a plan, it is a hope.

Pre-Start and Inspection Checklist

Use a structured checklist at the start of every shift and after any event that could affect stability. The table below summarises the key items a competent person should confirm before the working party enters. Adapt it to your specific design and record the result.

Standards and Guidance to Reference

Build your procedures around the recognised UK standards and guidance rather than custom and practice. The key documents are:

• CDM 2015: the legal duties for planning, managing and supervising construction work, including the specific requirements for cofferdams, caissons and excavations.
• BS 5975: the code of practice for temporary works procedures, covering the TWC role, design brief, design check categories and the permit-to-proceed system.
• BS EN 1997 (Eurocode 7): geotechnical design, including ground and groundwater actions and the treatment of base stability.
• HSE guidance on excavations and temporary works: practical advice on inspection, support, water management and confined space controls.
• Confined Spaces Regulations 1997: where a pumped-out cofferdam meets the definition of a confined space.

Keep copies of the design, design check, inspection reports and rescue plan on site, and make sure the people doing the work understand the sequence they must follow.

The Bottom Line for Groundwork Contractors

A cofferdam is not a job you scale up to from trench boxes by instinct. Get the right competent design and design check, appoint a Temporary Works Coordinator, follow the propping sequence exactly, inspect and record every shift and after any event, and have a rehearsed rescue plan ready before anyone goes below the water line. This article is general guidance only and does not replace a competent temporary works design for your specific site — always work to your engineer's design and the current standards.