A caisson is a large, watertight structure used in construction to build foundations underwater or deep underground. Think of it as a giant box, usually made of steel, concrete, or timber, that gets sunk into a riverbed, ocean floor, or soft ground so workers can excavate inside it and build a stable foundation. Caissons are the reason we can anchor bridges, skyscrapers, and docks in places where the ground is submerged or too soft to build on directly.
How a Caisson Works
The basic idea is straightforward. A caisson is built either on-site or onshore, then moved into position and sunk into the ground or waterbed. As it descends, workers or machines excavate material from inside, and the caisson’s own weight (sometimes increased with added water ballast) pushes it deeper. Once it reaches a layer of soil or rock firm enough to support the planned structure, the interior is filled with concrete to create a permanent foundation.
What makes caissons distinct from cofferdams, which serve a similar-sounding purpose, is permanence. A cofferdam is a temporary wall built to keep water out of a work area, then removed when the job is done. A caisson becomes part of the finished structure. Cofferdams also top out at about 18 meters below water level, while caissons can go much deeper.
Types of Caissons
There are three main types, each suited to different conditions.
Open Caissons
An open caisson has no bottom and sometimes no top either. It’s essentially a hollow shaft that gets pushed or sunk into the ground while material is dug out from inside. These are common for building bridge piers, pump stations, tunnels, and manholes. They’re the simplest design but only work when ground conditions allow excavation without excessive water flooding in.
Box Caissons
A box caisson is a four-sided structure with a sealed bottom but an open top. Because the bottom is closed, it can be built on land, floated to the construction site, and then sunk into position. This makes box caissons a popular choice for port construction, wharves, and berthing facilities. Some designs use quay walls that double as retaining structures for docks, providing resistance against ship impacts.
Pneumatic Caissons
Pneumatic caissons are the most complex type and the one with the most dramatic history. These are sealed, airtight chambers where compressed air is pumped in to keep water from flooding the workspace. The principle is simple physics: if the air pressure inside the caisson equals or exceeds the water pressure pushing in from outside, the water stays out. Workers enter through airlocks and excavate the bottom in a dry (or nearly dry) environment, even dozens of feet below a river.
The tradeoff is that humans working under high air pressure face serious physiological risks, which limited how deep pneumatic caissons could practically go. Modern projects have pushed to extreme depths, including 190 feet for subway construction in Russia and Seattle, and 225 feet for a tunnel in the Netherlands. Beyond about 165 feet, workers need specialized breathing gas mixtures to avoid nitrogen narcosis and oxygen toxicity.
The Brooklyn Bridge Caissons
The most famous caissons in history are the ones Washington Roebling used to build the Brooklyn Bridge in the early 1870s. The Brooklyn side required a massive timber caisson measuring 168 feet by 102 feet. Workers inside excavated downward until they hit dense, rocky soil at 44.5 feet below the river surface, where Roebling decided the ground was firm enough to support the tower.
The New York side was even more challenging. That caisson was slightly larger, 172 feet by 102 feet, and weighed 3,250 tons. The bedrock was deeper on the Manhattan side, so workers had to dig to 78 feet 6 inches before reaching sand and gravel dense enough for a foundation. The deeper the caisson went, the higher the air pressure needed to keep water out, and the more dangerous the work became.
Caisson Disease
The Brooklyn Bridge project helped reveal one of the most significant occupational health discoveries of the 19th century. Workers in the compressed-air caissons began experiencing mysterious symptoms after returning to the surface: joint pain, muscle soreness, breathlessness, and in severe cases, paralysis or death. The condition became known as “caisson disease,” now recognized as decompression sickness (the same “bends” that affects scuba divers).
The problem occurs when workers breathe air at high pressure for extended periods. Nitrogen dissolves into the blood and tissues under pressure. If the person returns to normal atmospheric pressure too quickly, that nitrogen forms bubbles in the body, similar to the fizz that erupts when you open a carbonated drink. The French engineer B. Triger documented some of the earliest cases in the 1840s, describing joint pain and soreness appearing about 30 minutes after workers returned to the surface. Minor cases often resolve on their own, but severe cases can be fatal without emergency treatment.
Washington Roebling himself was debilitated by caisson disease during the Brooklyn Bridge construction and spent much of the remaining project supervising from his apartment through a telescope.
Modern Safety Limits
Today, compressed-air work inside caissons is heavily regulated. Workers go through controlled decompression when leaving pressurized environments, gradually returning to normal atmospheric pressure so dissolved gases can leave the body safely. One common method is surface decompression, where workers exit to an intermediate pressure of about 18 psi, then enter a decompression chamber to finish the process in a controlled setting.
The practical limit for standard compressed-air work sits around 165 feet. Deeper projects require alternative breathing mixtures and significantly longer decompression times. Union rules in many countries cap pressurized work shifts at four hours, which creates a real constraint: the deeper you go, the longer decompression takes, and the less actual work time remains in a shift. This tension between depth, safety, and productivity shapes how modern engineers decide whether a pneumatic caisson is the right tool for a project.
Where Caissons Are Used Today
Bridge foundations remain the classic application, but caissons support a wide range of infrastructure. They’re used to anchor skyscrapers in cities with soft or waterlogged ground, to build deep shafts for subway systems, and to create foundations for docks and port facilities. Open caissons are standard for pump stations and large manholes. Floating caissons (a variation of box caissons) are transported by water to remote construction sites for port and wharf projects.
In many cases, modern caisson work has shifted from human labor inside pressurized chambers to automated or remotely operated excavation, reducing the health risks that made 19th-century caisson work so dangerous. But the fundamental engineering concept, a watertight structure sunk into the ground to create a stable foundation, remains unchanged from the principle Roebling used 150 years ago.