A caisson is a large, watertight structure used in construction to build foundations underwater or in waterlogged ground. The word comes from the French for “box,” and that’s essentially what it is: a rigid enclosure sunk into the earth or riverbed so workers can build on stable ground, even beneath lakes or rivers. Caissons are permanent, becoming part of the finished structure itself. The term also gave its name to a once-mysterious illness, caisson disease, now known as decompression sickness or “the bends.”
How Caissons Work in Construction
Caissons solve a fundamental problem: how do you build a bridge pier, dam, or building foundation when the ground is submerged or too soft to support weight? The answer is to sink a massive hollow structure down through unstable layers of sand, loose rock, and mud until it reaches solid bedrock. Workers excavate material from inside the caisson while its own weight, sometimes aided by additional loading, drives it deeper. Once it reaches bedrock, the interior is filled with concrete to create a permanent foundation.
This distinguishes a caisson from a cofferdam, which serves a similar purpose but is temporary. A cofferdam is a barrier that holds back water so construction can happen in a dry area, then gets removed when the work is done. A caisson stays in place and becomes a structural part of the bridge, pier, or building it supports.
Types of Caissons
There are three main types, each suited to different conditions:
- Open caissons have no bottom and no pressurized air. They’re open at the top and bottom, sinking under their own weight as material is dredged from inside. These work well in softer soils where water intrusion can be managed with pumps.
- Box caissons are closed at the bottom and open at the top. They’re typically built on land, floated to the construction site, then sunk into position on a prepared foundation. They work best in shallower water where the riverbed or seabed has already been leveled.
- Pneumatic caissons are the most complex. They use compressed air to keep water out of the working chamber at the bottom, allowing laborers to dig in dry conditions even far below the waterline. This is the type that made history during major bridge projects in the 1800s, and the type that caused serious health problems for workers.
The Brooklyn Bridge and Caisson Disease
The connection between caissons and medicine dates to 1840, when a French engineer named Jacques Triger used compressed air to mine coal in the Loire Valley of France. He called his pressurized chambers “caissons.” During the project, Triger noticed something troubling: workers experienced temporary breathlessness, joint pain, and soreness about half an hour after returning to the surface. He called it “mal de caisson,” and his account became the first recorded description of what we now call decompression sickness.
The problem became far more visible during construction of the Brooklyn Bridge in the 1870s. Workers spent hours in pressurized caissons deep beneath the East River, digging through mud and rock to reach bedrock. When they returned to normal atmospheric pressure at the surface, many fell ill. Of the men working in the caissons, 86 documented cases of the illness were recorded by Andrew H. Smith, the physician assigned to their care. Symptoms included joint pain, headaches, itchiness, shortness of breath, paralysis, and vomiting.
Many of the stricken workers emerged from the caisson hunched over from the pain, walking in a bent posture that reminded onlookers of the “Grecian Bend,” a fashionable women’s posture from the 1820s. That’s how the condition got its lasting nickname: “the bends.” In 1873, Smith formally named it caisson disease in a speech to the College of Physicians and Surgeons in New York, publishing the term in a textbook that same year.
What Happens Inside the Body
The underlying problem is gas physics. When you breathe air under high pressure, whether in a pneumatic caisson or during a deep-sea dive, the nitrogen in that air gets pushed into your tissues. Under pressure, the nitrogen stays dissolved, much like carbon dioxide stays dissolved in a sealed bottle of soda. As long as the pressure holds, everything is fine.
The danger comes when pressure drops too quickly. If a worker leaves a pressurized caisson and returns to normal atmospheric pressure too fast, dissolved nitrogen comes out of solution and forms bubbles in the blood and tissues. These bubbles can block blood vessels, damage tissue, and trigger inflammation. The effect is similar to what happens when you crack open that soda bottle: the rapid pressure drop causes gas to fizz out all at once.
A slow, gradual return to normal pressure gives nitrogen time to leave the tissues safely, passing back into the lungs and getting exhaled. A rapid transition does not. This is why modern protocols for both divers and compressed-air workers emphasize controlled decompression, carefully staged reductions in pressure over set time periods.
Symptoms of Decompression Sickness
The most common symptom is joint pain, which typically appears within minutes to hours after leaving the high-pressure environment. The shoulders, elbows, and knees are frequent sites. Skin symptoms like itching and a mottled rash can also develop. These milder presentations primarily affect the muscles, joints, and skin.
More serious cases involve the nervous system. Symptoms can include numbness, tingling, muscle weakness, difficulty walking, dizziness, and vision changes. In rare but severe instances, nitrogen bubbles can affect the spinal cord or brain, causing paralysis or loss of consciousness. Breathing difficulties and chest pain can occur when bubbles accumulate in the lungs, a condition sometimes called “the chokes.”
Treatment and Recovery
The standard treatment is a hyperbaric oxygen chamber, which essentially reverses the process that caused the problem. The patient is placed in a sealed chamber where the air pressure is raised to about 2.8 times normal atmospheric pressure while they breathe pure oxygen. This combination forces nitrogen bubbles back into solution and floods the tissues with oxygen to help repair damage.
A typical treatment session follows a structured schedule of pressure changes, gradually stepping the patient back down to normal pressure. Most people with mild to moderate symptoms improve significantly after one session. Residual symptoms may require one or two additional treatments. Severe cases, particularly those involving the nervous system, sometimes need more. The key factor in recovery is how quickly treatment begins: earlier recompression generally leads to better outcomes.
Modern Safety Standards
Workers in compressed-air environments today are protected by strict regulations. In the United States, OSHA limits the maximum working pressure to 50 pounds per square inch (psi) under normal circumstances, with emergency equipment rated for 75 psi. Emergency airlocks must be large enough to hold an entire work shift and maintained at no more than 12 psi. Air supplies must be free of oil and carbon monoxide, with systems capable of pressurizing an airlock from zero to 75 psi within five minutes for rapid response.
These rules, combined with mandatory decompression schedules that dictate how long workers must spend transitioning back to normal pressure, have dramatically reduced the incidence of caisson disease compared to the 19th century. Pneumatic caissons are still used today for bridge piers, deep building foundations, and tunnel construction, but the dangerous working conditions that once defined them are largely a thing of the past.