The Engineering Behind Bridges Over Water: How Cofferdams Let Engineers Work on “Dry Ground” Inside a River

Ahmed Adegbami

If you have ever watched a bridge being built across a river and wondered how concrete work happens in water, one common answer is this: engineers first create a temporary barrier that keeps water out.

That barrier is called a cofferdam.

What is a cofferdam?

A cofferdam is a temporary watertight enclosure built in a river, lagoon, or any water body so engineers can pump the water out and work on the foundation in a dry, controlled space.

You can think of it as creating a small “dry room” inside water, just long enough to build the foundation properly.

Why a cofferdam is useful

Water makes foundation work difficult because it:

1. Disturbs the soil during excavation
2. Washes cement and weakens fresh concrete
3. Makes reinforcement placement and inspection harder
4. Increases safety risk for workers and equipment

A cofferdam gives engineers control, and control improves quality.

When cofferdams are a better choice

Engineers often consider a cofferdam when:

1. The foundation must be built below water level
2. The riverbed needs excavation and preparation before concrete work
3. The project needs accurate, inspectable work rather than working blindly under water

In Nigeria, it is often better to carry out major river foundation work during the dry season, when water levels are lower and currents are calmer. That usually makes pumping easier and reduces the risk of flooding the work area.

How a cofferdam works, step by step

Here is the process in plain language.

Step 1: Mark the exact foundation location

Surveyors set out the pier position accurately, because a small error below can affect the whole bridge alignment above.

Step 2: Build a closed wall around the work area

A strong wall is installed in a loop around the foundation spot, forming an enclosure that limits water entry.

Step 3: Strengthen the enclosure

Water pressure and soil pressure can push the walls inward, so internal bracing is added to keep the enclosure stable.

Step 4: Pump water out

Pumps remove water inside the enclosure until the area is dry enough for safe excavation and concrete work. Small leaks are normal, the goal is to keep inflow lower than pumping capacity.

Step 5: Excavate and prepare the base

With water controlled, the team excavates to the required depth, removes weak material, and prepares a stable base. This is a major advantage of cofferdams, it allows proper ground preparation.

Step 6: Place reinforcement and cast the foundation

Steel reinforcement is placed and checked, then concrete is poured under controlled conditions, similar to working on land. This improves inspection, concrete quality, and foundation accuracy.

Step 7: Remove the cofferdam after the foundation gains strength

Once the foundation is strong enough, the cofferdam is removed and the pier construction continues above water.

Final word

Cofferdams are one of the most practical ways engineers temporarily create a dry work zone inside a river, so bridge foundations can be built with better control and quality. The key idea is simple: bridges over water are built more safely when the water is controlled first, and planning major foundation work for the dry season often makes that easier.