Building a bridge over water requires solving one fundamental problem: creating a stable foundation on a surface that’s constantly moving, soft, or deep. Engineers have tackled this challenge for thousands of years, evolving from timber stakes hammered into riverbeds to pressurized underwater chambers and massive concrete platforms lowered to the seafloor. The methods vary depending on the depth of the water, the type of soil beneath it, and the loads the bridge needs to carry.
The Roman Approach: Concrete That Sets Underwater
Roman engineers were among the first to build durable stone and concrete structures in water, and their methods were surprisingly sophisticated. Their key innovation was a concrete mixture made from volcanic ash (called pozzolana), lime, and water, combined with chunks of rock as filler. This wasn’t ordinary concrete. When saltwater came into contact with the pozzolana and lime, a chemical reaction produced a rare crystal called tobermorite, which actually made the concrete stronger over time rather than weaker. Some Roman harbor structures have survived more than 2,000 years partly because of this chemistry.
To get the concrete into position underwater, Roman builders used two main techniques. The first was driving wooden stakes into the ocean or riverbed to form a box-shaped enclosure, then filling it with concrete. The second, more efficient method was building the boxes onshore, floating them out to the correct position, and sinking them. For larger projects, they constructed entire barges, floated them into place, sank them, and filled them with concrete. These approaches let workers do most of the construction on dry land, minimizing the amount of labor done in the water itself.
Cofferdams: Creating Dry Ground in the Middle of a River
For centuries, the most common technique for building bridge foundations in shallow to moderate water was the cofferdam. The concept is straightforward: build a watertight enclosure in the water, pump the water out, and then construct the foundation on the exposed riverbed as if you were working on dry land.
Early cofferdams were made from timber planks driven into the riverbed in a ring or rectangle, with clay packed between double walls to seal out water. Modern cofferdams use interlocking steel sheets that slot together to form a tight barrier. Once the interior is pumped dry, workers excavate the soil, pour concrete footings, and build the bridge pier upward from there. Cofferdams work well in relatively shallow water with manageable currents, but they become impractical in deep or fast-moving rivers where the water pressure is too great to hold back.
Pneumatic Caissons: Working Under Pressure
When bridges needed to reach bedrock far below a deep river, engineers in the 19th century developed the pneumatic caisson, a technology that made landmarks like the Brooklyn Bridge and the Eads Bridge in St. Louis possible. A caisson is essentially a massive hollow box, usually built from timber or iron, that’s floated out to the construction site and sunk to the riverbed.
The structure is divided into two halves. The lower half is the working chamber, and the upper half is the caisson proper, which will eventually become part of the bridge’s permanent foundation. Once the caisson rests on the bottom, water is pumped out of the working chamber and replaced with compressed air to keep the river from flooding back in. Workers enter through pressure-regulating airlocks and access shafts, then dig out the soil and rock beneath the structure. As material is removed, the caisson sinks deeper under its own weight. Additional tubes supply fresh compressed air, remove excavated material, and provide communication channels. The process continues until the caisson reaches solid bedrock or a suitable layer of firm ground.
This method was effective but dangerous. Workers in the high-pressure chambers suffered from what was then a mysterious illness, now known as decompression sickness or “the bends.” During construction of the Eads Bridge in the 1870s, the health of workers was severely affected by the compressed air, and several died as a result. Engineers at the time didn’t fully understand that returning to normal atmospheric pressure too quickly caused dissolved gases in the blood to form painful and sometimes fatal bubbles.
Driven Piles: Hammering Columns Into the Seabed
Not every bridge needs a massive foundation excavated to bedrock. For many structures, especially in marine environments with soft soil, the solution is to drive long columns deep into the ground beneath the water. These piles are typically made from steel, concrete, or timber, and they support the bridge by transferring its weight through friction and pressure into the surrounding soil.
The installation process uses powerful impact hammers mounted on floating barges or fixed platforms. Each hammer delivers repeated blows that embed the pile progressively deeper into the seabed. Compared to excavating and pouring concrete foundations, pile driving is relatively fast, which helps keep construction timelines short. The main limitation is rocky or uneven ground, where piles can hit obstructions, refuse to go deeper, or become misaligned. In those conditions, engineers typically switch to drilled shafts, where a hole is bored into the rock and filled with reinforced concrete.
Floating Bridges: Skipping the Foundation Entirely
Some bodies of water are simply too deep or too soft-bottomed for conventional foundations. In these cases, engineers build floating bridges, which rest on the water’s surface rather than anchoring to the bottom. The concept goes back to antiquity: one of the earliest recorded examples used nearly 400 ships tied together with heavy flaxen and papyrus ropes, weighted with anchors, to form a crossing over a strait. An opening was left in the middle so smaller vessels could still pass through.
Modern floating bridges use concrete or steel pontoons that sit in the water and support a roadway on top. Some of the largest carry heavy commuter and commercial traffic daily. The pontoons are anchored to the lakebed or seabed with cables to prevent drifting from wind and currents. Washington State has several of the world’s longest floating bridges, built across lakes that are hundreds of feet deep with soft sediment floors where traditional piers would be impractical.
Protecting What’s Already Built
Once a bridge stands over water, its foundations face ongoing threats from ship collisions, ice, and erosion. Engineers use several systems to protect bridge piers. Amber lights are installed on critical piers to outline the navigation channel and guide ship traffic safely through. Fender systems, large energy-absorbing barriers around the piers, slow or stop vessels that drift off course. Some bridges also use subsurface attenuation devices, essentially obstacles placed parallel to the channel edge at a depth that won’t interfere with normal boat traffic but will slow or stop a wayward ship before it reaches a pier.
Reducing Harm to Marine Life
Underwater construction, particularly pile driving, generates intense sound waves that can injure or disorient fish and marine mammals. To reduce this impact, engineers deploy air bubble curtains around the construction zone. These systems pump air through perforated hoses on the seafloor, creating a wall of rising bubbles that surrounds the work area. Because air and water have very different densities, the bubble curtain acts as an effective barrier to sound, absorbing and scattering the pressure waves before they travel far into the surrounding water. This relatively simple technology has become a standard protective measure on bridge and offshore construction projects in ecologically sensitive areas.