A caisson foundation is a large, hollow structure that is sunk into the ground or through water to reach a stable layer of soil or rock, then filled to create a permanent deep foundation. Think of it as a massive watertight box or cylinder that gets pushed down through soft or waterlogged ground until it hits something solid enough to support the weight above. Caissons are used when surface soil is too weak for conventional foundations, and they’re capable of supporting extremely heavy loads like bridge piers, dams, and offshore structures.
How a Caisson Foundation Works
The basic principle is straightforward: instead of driving a pile into the ground from above, you build a large hollow shell, place it where the foundation needs to go, and remove the soil from inside. As the interior is excavated, the structure’s own weight causes it to sink. Once it reaches a layer of rock or firm soil that can bear the load, the hollow interior is filled with concrete to create a solid, permanent base.
What makes caissons distinct from other deep foundations like driven piles is their size and the way they’re installed. A driven pile is a relatively narrow column hammered into the earth. A caisson is a much larger structure, sometimes tens of feet across, that descends under controlled conditions. This gives engineers the ability to place enormous loads on deep, stable ground even when the surface conditions involve water, mud, or loose sand.
Three Main Types of Caissons
Open Caissons
An open caisson is a box or cylinder open at both the top and the bottom. It’s the simplest design. The structure is placed at the construction site (or built on a barge and floated into position), and soil is removed from inside using grab buckets that scoop material out through the open top. As the soil is cleared, the caisson sinks under its own weight until it reaches the bearing layer. Open caissons work well in sand or soft soil and can be sunk to great depths, making them a popular choice for bridge foundations in rivers.
The main drawback is that the bottom of the caisson can’t be thoroughly inspected or cleaned before it’s sealed, since the work happens underwater or through murky conditions. Engineers have to rely on indirect methods to confirm the caisson has reached solid ground.
Pneumatic Caissons
A pneumatic caisson is closed at the top and open at the bottom, creating a sealed working chamber at its base. Compressed air is pumped into this chamber to keep water and mud from flooding in, allowing workers to excavate in relatively dry conditions even deep underwater. This is particularly useful in depths over about 40 feet of water, or wherever rushing water and underwater obstacles would make open excavation impossible.
Pneumatic caissons produce better results because workers can directly inspect and clean the bottom before sealing, but they come with serious limitations. The compressed air environment is dangerous. Workers are subject to decompression sickness (historically called “the bends”), and human physiology sets a hard depth limit of roughly 115 feet. Beyond that, the air pressure required to hold back the water is more than the body can tolerate. These caissons also cost significantly more to build because of the airlocks, pressurization systems, and safety equipment involved.
Box Caissons
A box caisson is closed at the bottom and open at the top. It’s typically built on land, cured, then floated to the construction site and sunk into position by filling the interior with sand, gravel, or concrete. Because the bottom is sealed, box caissons don’t involve any underwater excavation. They simply sit on the ground surface beneath the water. This makes them the simplest and least expensive option, but they only work where hard, stable ground exists at a shallow depth, since there’s no way to dig down to a deeper bearing layer.
The Sinking Process
Sinking a caisson is a carefully controlled operation. The first step is building the initial section of the structure at ground level, fitted with a specially designed cutting ring at the leading edge. This cutting edge is slightly wider than the caisson walls above it, creating a narrow gap (called an annulus) between the sinking structure and the surrounding soil. That gap is filled with a lubricant, typically a thick clay-based fluid called bentonite, which supports the surrounding ground and reduces friction so the caisson can slide downward smoothly.
Excavation happens from within. Depending on ground conditions, this can be done “dry” using standard digging equipment or “wet” with the interior flooded to match the surrounding water pressure. For wet conditions, telescopic pole grabs or rope-operated digging buckets mounted on cranes remove soil from inside the flooded shaft, reaching depths of around 65 feet or more.
As the caisson descends, new sections are added at the surface. Hydraulic jacks positioned around a reinforced concrete collar at ground level control the rate and direction of sinking. Engineers continuously monitor the shaft’s alignment, making corrections with the jacks to keep it perfectly vertical. Even a small tilt at the surface translates to a large offset at depth, so this step is critical.
Once the caisson reaches its target depth, the lubricant in the annulus is replaced with cement grout in a single operation, locking the structure permanently into the surrounding soil. If the base was excavated wet, a concrete plug is placed at the bottom using underwater pumping methods and left to cure for at least five days before the water inside is pumped out.
Where Caissons Are Used
Caisson foundations show up in projects where the loads are massive and the ground conditions are challenging. Bridge piers sitting in rivers are the classic application: the riverbed is often soft silt or sand with bedrock far below, and the foundation needs to withstand not just the bridge’s weight but also lateral forces from water current and potentially ice or ship impacts. Many of the world’s major bridges sit on caisson foundations sunk through dozens of feet of water and soil.
Offshore structures, large dams, and heavy waterfront infrastructure also rely on caissons. In urban settings, caissons (sometimes called drilled shafts in this context) support skyscrapers and other heavy buildings where shallow foundations would settle unevenly or fail entirely. The technique is also used to build deep shafts for tunneling projects.
Safety Requirements
Pneumatic caisson work is one of the more hazardous activities in construction, primarily because of the compressed air environment. Workers entering and leaving the pressurized chamber must go through airlocks and follow decompression procedures to avoid decompression sickness, which can cause joint pain, neurological damage, or death. The Brooklyn Bridge’s construction in the 1870s famously killed or injured dozens of workers from what was then called “caisson disease” before the condition was well understood.
Modern regulations from OSHA set strict requirements for caisson operations. Pressure gauges must be maintained on both sides of every bulkhead and kept accessible at all times. Caissons larger than 10 feet in diameter must have a dedicated man lock and shaft used exclusively by workers. Staircases with landing platforms every 20 feet are required wherever space permits, and all compressed air work must follow detailed decompression protocols. Shafts are stamped on the outside to show the pressure they’ve been tested to withstand.
Advantages and Limitations
The chief advantage of caisson foundations is their ability to carry very heavy loads down to stable ground, even through deep water or extremely soft soil. They provide a solid, monolithic base that resists both vertical loads and lateral forces, which is why they’re the foundation of choice for bridges and other structures subject to sideways pressure. Because the caisson is built at or near the surface and sunk in place, engineers have more control over quality than with driven piles, where defects can go undetected underground.
The limitations are primarily cost and complexity. Caisson construction requires specialized equipment, skilled labor, and careful engineering. Pneumatic caissons in particular demand pressurization systems, safety infrastructure, and medical support for workers. Ground conditions can create unexpected problems during sinking: boulders, uneven rock surfaces, or sudden water inflows can stall or tilt the caisson. And for open caissons, the inability to inspect the bottom before sealing means engineers accept some uncertainty about the final bearing conditions. For smaller or lighter structures, simpler foundation types like spread footings or driven piles are almost always more practical and cost-effective.