Caisson drilling is a process by which open chambers are created under water or in the earth in order to reach soil or bedrock that is stable enough to support a massive structure above, such as a bridge or a large building. In this way, concrete (or other materials) can be used to fill in the opening, making for a more stable base than could be provided by, say, pilings, pads, or other foundational structures resting on earth that might shift or settle.
The history of caissons is long and storied, beginning more than a century ago, when caisson engineering was first used in the construction of massive bridges. Caisson engineering dates back to at least the 18th century, when it was used to anchor such famous structures as the Brooklyn Bridge, which finished its 14-year construction in 1883 and which is considered to be a hallmark of American innovation. Because the construction of this bridge was so well documented, much is known about how the early caisson process worked.
In this instance, airtight wooden boxes were erected, pinned to the bottom of the river with granite blocks, and then filled with pressurized air so that workers could go in and excavate the river bottom down to the bedrock. Unfortunately, little was known about a condition that today we call the bends (but was then referred to as caisson disease), and more than 100 workers were documented as suffering from it. It wasn’t until 1909 that the first caisson-safety laws were passed in New York to help protect workers from the many accidents and deaths that could occur in early caisson operations.
After the so-called “eighth wonder of the world” was successfully unveiled in 1883, it paved the way for further caisson projects. Because of the use of caissons (and other innovations in structural engineering), the Brooklyn Bridge was the tallest structure in the western hemisphere at the time of its unveiling, and for many years after.
In 1884, to prove the strength and stability of the bridge, P.T. Barnum decided to march 21 elephants across the Brooklyn Bridge. The mere fact that it still stands, over 130 years later, is a testament to the superior design of this iconic structure.
Diagrams of caissons date back even further than the Brooklyn Bridge, with early examples appearing before the 1850s, but the Brooklyn Bridge stands as one of the earliest and best examples of the use of this type of engineering. While bridges remained the most common use of caisson engineering, the process was subsequently used to build piers, offshore platforms, and eventually, structures on land.
Today, caisson drilling is one of the most effective means of supporting large structures (like skyscrapers, commercial buildings, and more) with reduced risk of shifting, settling, sinking, or other potential issues that accompany traditional, pad-type foundations. Not only is weight more evenly distributed on a grid of caissons, but drilling down to bedrock (when possible) ensures the greatest stability for large structures.
There were no humble beginnings for caisson engineering. However, this innovation has changed the way we engineer large structures, from bridges spanning massive waterways to buildings on a grand scale, and it looks to persevere long into the future.