Cofferdams let crews build a bridge pier or footing in a river, lake, or other wet ground as if it were on land. Interlocking steel sheet piles form a temporary enclosure around the foundation site. Pumps lower the water inside, while bracing, seepage controls, and—when needed—an underwater concrete seal keep the excavation stable and prevent it from flooding from below.
What a cofferdam is and what it must do
A cofferdam is a temporary enclosure built around the location of a bridge pier, abutment, footing, pile cap, or similar foundation element. Its purpose is to create a controlled work zone in water or saturated soil so workers can excavate, install piles and reinforcement, and place structural concrete in the dry.
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Federal Highway Administration guidance describes cofferdams and foundation seals as common ways to construct bridge substructures in the dry. Steel sheet piling is the usual wall material. Each sheet has an interlocking edge; sheets are driven one after another into the riverbed or soil until they form a continuous ring around the planned foundation.
The wall performs several jobs at once:
- It separates the work area from the river or surrounding groundwater.
- Its embedded portion cuts through potential seepage paths and helps resist soil movement.
- Its exposed portion retains soil and withstands pressure from water outside the enclosure.
- With internal walers and struts, it transfers loads around the enclosure instead of allowing the sheets to bow inward.
A cofferdam is not normally expected to be perfectly watertight. Small flows through sheet-pile interlocks, seams, the surrounding soil, or the bottom are anticipated in the temporary-works design. Pumps remove manageable inflow, while seals or other seepage-control measures address conditions that pumps alone cannot safely handle.
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How the enclosure keeps river water out
Interlocking sheets create a cutoff wall
Crews drive the steel sheets into the bed in a planned sequence, usually working from a guide frame or template so the wall follows the designed footprint. The interlocks connect adjacent sheets and greatly restrict horizontal flow through the wall. The embedded length also provides resistance against the soil and water pressures acting on the enclosure.
Driving depth is not chosen simply by making the wall as tall as the visible water. The sheets must extend far enough into the bed to develop support and to lengthen or interrupt seepage paths. The required embedment depends on the soil, water level, excavation depth, wall geometry, and loads identified for the temporary works.
Pumping lowers the internal water level
After the wall is installed, pumps remove water from inside the ring. Lowering the internal level creates a dry or nearly dry working platform, but it also changes the pressure balance around the excavation. Pump capacity therefore has to account for expected seepage, rainfall, leakage at interlocks, and any inflow from the base.
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Bracing prevents collapse as excavation deepens
Water pressure is only one load. Removing soil from inside the enclosure removes support from the sheet piles, allowing the surrounding ground and outside water to push inward. Deep cofferdams therefore commonly use horizontal steel walers and struts. Walers collect load along the sheet-pile wall; struts span the enclosure and brace one side against the other.
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Bracing is installed progressively as excavation and water-level reduction advance. Installing it too late can allow excessive wall movement; installing it without room for the planned foundation work can obstruct piles, reinforcement, or concrete placement. The temporary-works design sets the locations and sequence.
Why the bottom does not simply flood from underneath
The most important distinction is between water leaking through the side wall and water pushing upward beneath the excavation. When the inside water level is lowered, the water pressure outside and below the cofferdam can become greater than the pressure inside. That difference can drive groundwater upward through the soil, causing piping, boiling, uplift, or a sudden inflow that pumps cannot control safely.
The underwater concrete seal
Where uplift or base inflow is a concern, engineers place a thick layer of concrete underwater on the soil inside the cofferdam before fully dewatering. This is called a concrete seal or foundation seal. It is commonly placed by tremie methods or by controlled pumping so that the concrete forms a continuous mass beneath the water without being washed apart.
The seal acts as a weighted and relatively impermeable plug. It resists the upward hydrostatic force and closes much of the flow path through the base. The enclosure remains flooded while the concrete is placed and while it gains sufficient integrity. Only then can crews pump the cofferdam down to expose the seal and continue with the permanent footing or pier base.
A seal is not automatically required for every cofferdam. A shallow excavation in favorable soil may be dewatered with pumping and seepage control alone. Deeper excavations, permeable deposits, high outside water, or artesian groundwater can make a seal or additional pressure-relief system essential. The decision is based on calculated uplift, seepage, and stability for the actual site.
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Typical bridge-foundation construction sequence
The exact method varies with the river, soil, foundation type, and temporary-works design, but the work generally follows this order.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problems- Survey and set out the foundation. Crews verify the pier location, riverbed levels, access, navigation constraints, and expected high-water conditions. A guide frame or template is installed to control the sheet-pile alignment.
- Drive the sheet piles. Interlocking steel sheets are installed around the planned footprint. The crew checks line, level, interlock engagement, and achieved embedment as driving proceeds.
- Install initial bracing. Walers and struts are fitted where the design requires them. For a deep excavation, bracing is added in stages rather than waiting until all soil and water have been removed.
- Excavate and manage seepage. Soil is removed from inside the enclosure while pumps control water entering through the wall, soil, or bottom. Water levels and wall movement are monitored continuously enough to detect a developing problem before it becomes unstable.
- Place an underwater seal when needed. If the uplift or base-inflow analysis calls for one, concrete is placed by tremie or controlled pumping on the prepared soil surface. The cofferdam stays flooded during placement and initial curing.
- Dewater after the seal is reliable. Pumps lower the water level only after the seal has developed sufficient integrity. Any remaining inflow is collected and discharged through the designed system.
- Build the permanent foundation. Workers install piles, pile reinforcement, footing reinforcement, or other structural steel, then cast the footing, pile cap, or pier base in the dry work area.
- Remove or adapt temporary works. Depending on the design, sheet piles and bracing may be extracted, cut off below a specified level, or left in place and incorporated as a form or protection element. The project documents determine the final treatment.
Loads engineers check before choosing a cofferdam
There is no single cofferdam arrangement that works for every bridge site. Design cases include normal hydrostatic pressure, soil loads, river flow, and high-water events. Engineers also consider how construction activities change those loads over time.
| Site or construction factor | What it affects | Design response |
|---|---|---|
| Water depth, current, and flood exposure | Outside pressure, drag, debris impact, and the chance that a high-water event overtops or bypasses the enclosure | Wall elevation, sheet capacity, bracing, access planning, and emergency pumping or evacuation provisions |
| Soil permeability and seepage paths | How much water enters through the wall or excavation base and whether soil particles could move with the flow | Sheet embedment, pumping capacity, filters or other seepage controls, and possible use of a concrete seal |
| Required excavation depth and footprint | Wall bending, bracing clearance, excavation stability, and the volume of water and soil to remove | Sheet-pile section and embedment, number and level of walers and struts, and staged excavation |
| Uplift and artesian groundwater | Upward force beneath the excavation and the risk of base heave or uncontrolled inflow | Underwater concrete seal, pressure-relief measures, and a controlled dewatering sequence |
| Pump capacity and discharge route | Whether inflow can be kept below the working level without erosion or unacceptable discharge | Redundant or staged pumps, monitored water levels, and approved discharge controls |
| End-of-work requirements | Whether temporary steel can be removed without damaging the new foundation or riverbed | Extraction, cut-off, or incorporation of sheeting and bracing as specified for the project |
USACE maintains a dedicated manual for sheet-pile cellular cofferdams and retaining structures, EM 1110-2-2503, dated June 11, 1990. Separate USACE technical guidance addresses construction-phase dewatering and seepage-control systems. These documents are design references; they do not replace site-specific calculations, permits, or the engineer’s temporary-works drawings.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What makes some cofferdams especially difficult
Deep water and strong current
Deep water increases the pressure difference across the wall and makes installation, surveying, and access harder. Current adds horizontal force and can scour the riverbed around the enclosure. Flood exposure must be treated as a design case rather than an unlikely interruption, particularly when the cofferdam remains in the channel for an extended construction period.
Permeable or layered soils
Gravel, sand, fractured layers, or an irregular bed can provide rapid seepage paths beneath or between sheet piles. Pumping harder may lower the water level temporarily while accelerating soil movement outside the enclosure. Engineers may respond with deeper sheets, staged drawdown, a concrete seal, or other seepage-control and pressure-relief measures.
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Contaminated ground
Contaminated sediment adds environmental controls to ordinary dewatering. Pumped water may require treatment or controlled handling, and excavation can mobilize contaminated material. FHWA identifies contaminated conditions as among the situations in which sheet-pile installation, seal placement, or dewatering can become particularly challenging.
Artesian groundwater
Artesian conditions can push water upward even when the river surface appears calm. The pressure comes from groundwater beneath the site, so lowering the inside level without addressing that pressure can destabilize the base. A seal, pressure-relief system, or a different excavation strategy may be required.
How crews know the system is working
Construction monitoring turns the cofferdam from a static wall into a controlled temporary system. Useful observations include:
- Water levels inside and outside the enclosure.
- Pump flow and changes in the amount of inflow.
- Sheet-pile movement, bracing loads, and visible distortion.
- Seepage that carries soil, cloudy discharge, boils, or sand movement at the base.
- River level, current, weather, and upstream conditions that could signal a high-water event.
A rising pump rate, new cloudy seepage, wall movement, or an unexpected water-level change warrants a controlled response under the project’s emergency plan. Depending on the condition, that may mean stopping excavation, reducing the drawdown rate, adding bracing, installing relief measures, or evacuating the enclosure before reassessing the design.
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The cofferdam is temporary, but its removal is still an engineered operation. Pulling sheets can disturb the riverbed, transmit vibration, or damage newly placed concrete if done too soon. Some designs require the sheets to be cut off at a specified elevation or left as a protective outer form. Bracing is removed in an order that preserves stability until the permanent foundation and surrounding backfill can carry the required loads.
The final method is determined before construction and coordinated with environmental, navigation, and structural requirements. It is not safe to assume that every sheet pile will be extracted or that every cofferdam will remain around the finished pier.
The practical answer to “How do cofferdams keep water out?”
They do it through several layers of control rather than one watertight barrier: interlocking steel sheets limit lateral flow and retain soil; embedment interrupts seepage paths; pumps remove water that still enters; internal bracing keeps the walls stable as excavation deepens; and an underwater concrete seal, when required, resists the upward pressure that could flood the excavation from below. The construction sequence is deliberately staged so each measure is in place before the next reduction in water level or excavation support is attempted.
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