A Guide to Types of Cofferdams for Contractors and Engineers

Flood management, underwater construction, and deep excavations near water all pose the same fundamental problem: how do you keep a work area dry? Cofferdams are the answer, and sheet piling plays a central role in how many are constructed.

This guide outlines the main types of cofferdams used in UK construction, how each works, and when to use them.

Table of Contents

1. What Is a Cofferdam?

2. When Are Cofferdams Used?

3. Types of Cofferdams

   • 3.1. Braced Cofferdams

   • 3.2. Cellular Cofferdams

   • 3.3. Rockfill Cofferdams

   • 3.4. Earthen Cofferdams

   • 3.5. Single-Wall and Double-Wall Cofferdams

   • 3.6. Inflatable Cofferdams

   • 3.7. Sheet Pile Cofferdams

4. Comparing Cofferdam Types

5. Design Considerations

6. FAQs About Cofferdams

7. Conclusion

1. What Is a Cofferdam?

A cofferdam is a temporary structure designed to exclude water from a particular area so that construction work can be carried out in dry conditions. Cofferdams are most commonly built using interlocking steel sheet piles that form a watertight barrier.

They are commonly used for bridge piers, flood defence work, dock and harbour upgrades, pipeline installations, and culvert construction.

2. When Are Cofferdams Used?

Cofferdams are used when:

• A dry work environment is needed in or near water

• Foundations, pipelines, or culverts need to be placed below the water table

• Work is taking place in tidal or flood-prone areas

• There is a need to isolate the excavation area from surrounding groundwater or river flow

3. Types of Cofferdams

3.1. Braced Cofferdams

Braced cofferdams are made from sheet piles driven into the ground and braced internally with waling beams and struts. These are commonly rectangular or circular and are used for deep excavations where ground pressure must be resisted.

Best for: Deep excavations with limited external access.

Sheet piles: Yes. Often primary structural element.

Example: Circular shaft excavations for pump chambers or valve pits.

Internal braced cofferdam used to provide safe working access for gas pipeline connection project.

3.2. Cellular Cofferdams

Cellular cofferdams are large, circular or diaphragm-shaped cells formed using steel sheet piles filled with granular material. The individual cells are interconnected for stability. They rely on mass and internal friction for stability, not internal bracing.

Best for: Large marine or riverine projects.

Sheet piles: Yes. Used to form the cell walls.

Example: Harbour works or large bridge piers in tidal rivers.

3.3. Rockfill Cofferdams

These are temporary dams made by placing rockfill in a trapezoidal profile across a river or stream. An impermeable membrane may be placed on the upstream face.

Best for: Small rivers or low-head water control.

Sheet piles: No. Typically loose rock.

Example: Diversion dams for water treatment works.

3.4. Earthen Cofferdams

Constructed from compacted earth, sometimes with a clay core. These are generally used for shallow, low-risk works and may require internal drainage systems.

Best for: Shallow water and short-term works.

Sheet piles: No. Often lined with geotextile or membrane.

Example: Stream diversions or shallow channel isolations.

3.5. Single-Wall and Double-Wall Cofferdams

• Single-wall cofferdams consist of a single row of interlocked sheet piles, typically used for small excavations or limited access.

• Double-wall cofferdams have two parallel rows of sheet piles with granular fill in between. They provide greater stability and water-tightness.

Best for: Urban sites and culvert installations.

Sheet piles: Always.

Example: Pipeline crossings or basement waterproofing.

3.6. Inflatable Cofferdams

Made of fabric tubes filled with water or air. These temporary barriers are quick to deploy and are mainly used in shallow or low-risk water control.

Best for: Emergency works or short-term flood isolation.

Sheet piles: No.

Example: Stream crossings or protection during minor road culvert upgrades.

3.7. Sheet Pile Cofferdams (General)

Most cofferdams used in infrastructure works across the UK are built using interlocking steel sheet piles. These offer fast installation, excellent water-tightness, and the flexibility to shape complex excavations.

Steel sheet pile cofferdams may be used with:

• Internal bracing or frames

• Waler beams and tie rods

• Silent or vibratory piling rigs

Best for: Projects with tight access, flood risk, or below-ground infrastructure.

4. Comparing Cofferdam Types

5. Design Considerations

When choosing a cofferdam type, engineers should consider:

• Water depth and flow rate

• Soil conditions and permeability

• Excavation size and depth

• Nearby structures or environmental sensitivities

• Duration of the works

• Equipment access and removal

For example, where silent installation is required near structures, a pressed-in sheet pile cofferdam is often the best approach.

6. FAQs About Cofferdams

Can sheet pile cofferdams be reused? Yes. In many cases the piles are extracted after works and reused or returned via a buy-back scheme.

Do I need a permit to install a cofferdam in a watercourse? Yes. You must usually consult the Environment Agency or local authority and obtain consent for works in or near water.

What’s the difference between a cofferdam and a caisson? A cofferdam is temporary and keeps water out during works. A caisson is usually permanent and forms part of the finished structure.

Can cofferdams be used in tidal zones? Yes. Cellular or braced sheet pile cofferdams are commonly used in tidal zones, with dewatering systems and monitoring in place.

7. Conclusion

Cofferdams play a critical role in modern infrastructure projects, allowing teams to carry out safe and dry excavations in wet or tidal environments. From compact braced shafts to expansive cellular structures, each cofferdam type offers different benefits based on the site conditions.

If you’re planning works near water or below ground, speak to a specialist piling contractor early to ensure the right cofferdam method is chosen.

Steel Piling Solutions offers full design, supply and installation of temporary sheet pile cofferdams for all types of infrastructure and water-based works across the UK.