A caisson is a large, hollow structure that gets sunk into the ground or underwater to create a stable foundation for bridges, piers, and other heavy structures. Think of it as a giant box or cylinder that’s pushed down through soft soil or water until it reaches solid ground, then filled with concrete to become a permanent foundation. The basic principle hasn’t changed much since the 1800s: you build a rigid shell, sink it to the right depth, seal the bottom, and fill it so it becomes an immovable base.
The Three Main Types of Caissons
Caissons come in three designs, each suited to different ground conditions and depths.
A box caisson is the simplest version: a prefabricated box with sides and a sealed bottom, open at the top. It’s built on land or in a dry dock, floated to the construction site, and lowered onto a flat, prepared surface on the riverbed or seabed. Once in position, it’s filled with ballast (usually concrete or sand) to anchor it in place. Box caissons work well for bridge piers in relatively shallow, calm water where the ground surface has already been leveled.
An open caisson looks similar but has no bottom. Its lower edge is sharpened into what engineers call a “cutting shoe,” angled outward to help the structure slice downward through soil. Workers or machines excavate material from inside, and the caisson sinks under its own weight as the ground beneath it is removed. Open caissons are commonly used for deep manholes, pump stations, and launch pits for tunneling operations. They perform best in soft clays, like those found in riverbeds, but struggle where large buried obstructions like boulders could block the cutting shoe.
A pneumatic caisson is the most complex. It’s a bottomless box sealed at the top, with compressed air pumped inside to push out water and mud. This creates a dry workspace at the bottom where workers can dig and pour concrete, even far below the waterline. An airlock system lets workers enter and exit the pressurized chamber. Pneumatic caissons are particularly useful when the foundation work could cause nearby structures to settle, since the controlled air pressure stabilizes the surrounding soil.
How a Caisson Gets Sunk Into the Ground
Sinking a caisson is a carefully staged process that relies on the structure’s own weight to push it downward. Construction typically begins at ground level. Workers pour the cutting shoe first, along with the first section of the caisson wall. For a large project, this initial wall section might be 9 feet of a 3-foot-thick wall. Even at this early stage, the caisson can begin sinking under its own weight as the soil beneath gives way.
From there, the process is repetitive: build another wall section on top, excavate soil from inside, and let gravity pull the whole assembly deeper. Each new lift of concrete adds thousands of pounds. On one project in Grand Forks, North Dakota, a caisson began sinking during the second wall pour, dropping about 2 feet below grade on its own. Workers continued adding 17-foot wall sections while excavating interior soil until the caisson reached its target depth.
Getting the depth exactly right is tricky. Caissons can over-sink if the soil is softer than expected. On the Grand Forks project, the caisson dropped about 15 inches past its intended depth. The crew compensated by adding extra height to the final wall section and correcting for any tilt that developed during sinking. Once the caisson reaches depth, contractors inject cement grout through pipes embedded in the walls, filling the gap between the outer wall and surrounding soil to lock everything in place.
The final step is sealing the bottom. A thick reinforced concrete slab, sometimes 6 feet deep, is poured inside the caisson to create a watertight base. This base slab turns the caisson from an open cylinder into a solid, load-bearing column anchored in the earth.
Suction Caissons for Offshore Work
A newer variation skips the slow, gravity-driven sinking process entirely. Suction caissons are open-bottomed steel cylinders used primarily offshore. Instead of relying on weight, they use vacuum pressure. After the cylinder is lowered to the seabed, water is pumped out through a hole near the top. This creates a pressure difference that effectively sucks the cylinder into the seafloor.
These caissons are popular for anchoring floating oil platforms and offshore wind turbines, especially in soft clay seabeds. They can be positioned with precision and installed relatively quickly compared to traditional methods. They also resist large uplift forces, which matters for structures that pull upward on their anchors during storms. The main limitation is dense sand or gravel, which resists the suction effect and makes installation difficult.
The Brooklyn Bridge and Compressed Air
The most famous caisson project in history is the Brooklyn Bridge, built in the 1870s. Engineers sank massive pneumatic caissons to anchor both towers. On the Brooklyn side, the caisson reached 44.5 feet below the river. On the Manhattan side, where bedrock was deeper, it went down 78.5 feet. Workers inside these pressurized chambers dug through river mud in compressed air, one of the first large-scale uses of this technology.
The project also revealed a deadly hazard nobody fully understood at the time. Workers who spent hours in the high-pressure environment experienced severe pain, difficulty breathing, and joint problems after returning to the surface. Six men died as one caisson neared its maximum depth of 93 feet. The condition became known as “caisson disease,” now called decompression sickness or “the bends.”
Why Compressed Air Is Dangerous at Depth
The problem is nitrogen. At normal atmospheric pressure, nitrogen dissolves harmlessly in your blood. But as pressure increases inside a caisson, more nitrogen gets forced into your bloodstream and tissues. If you return to normal pressure too quickly, that nitrogen comes out of solution and forms bubbles, like opening a carbonated drink. Those bubbles can block blood vessels, damage joints, and in severe cases cause paralysis or death.
Early caisson projects operated at pressures up to 3.5 times normal atmospheric levels. Workers on these projects reported joint pain, itching, muscle aches, and shortness of breath. French physicians B. Pol and T.J.J. Watelle were among the first to document these symptoms in the 1850s. Later, French physiologist Paul Bert confirmed the mechanism through experiments with rapid decompression in animals, and recommended slow decompression to let nitrogen escape gradually through the lungs.
The real breakthrough came from Scottish physiologist John Scott Haldane, who developed the concept of staged decompression. His method brought workers to half the pressure they’d been working at, then reduced pressure in gradual steps. Modern dive tables and decompression protocols are far more sophisticated, incorporating advances in gas physiology and computer modeling, but they’re built on Haldane’s core insight: come up slowly, in stages, and give your body time to clear the dissolved gas.
Where Caisson Foundations Make Sense
Caissons aren’t the right choice for every project. They’re typically used when surface soil is too weak to support a structure on its own and the foundation needs to reach deeper, more stable ground. The ratio between a caisson’s depth and its diameter generally falls between 1 and 6, meaning a caisson 10 feet wide might be sunk anywhere from 10 to 60 feet deep, depending on where solid bearing material is found.
Soil type determines both the caisson design and how much load it can support. Most research on caisson bearing capacity has focused on clay soils in wet conditions, which is where caissons are most commonly used. In soils that contain both clay-like cohesion and sand-like friction, engineers calculate bearing capacity using depth factors that account for the caisson’s width, how deep it’s embedded, and the internal friction angle of the soil. The interface between the caisson wall and the surrounding soil is assumed to be fully rough, meaning the soil grips the outside of the caisson and adds to its stability.
Pneumatic caissons have a particular advantage in urban areas. Because the pressurized air stabilizes the soil around the excavation, they’re less likely to cause the ground to shift beneath neighboring buildings. This makes them a preferred option when building foundations close to existing structures that can’t tolerate even small amounts of settling.