What are caissons and how are they used to build bridge foundations in water?

In engineering, a caisson is a boxlike, hollow structure used as a foundation for bridge piers in water. These structures are typically rectangular or circular and are sunk into the riverbed or sea bottom to provide a stable base for heavy loads.123

There are three primary types of caissons used in bridge construction:

• Box Caissons: These are open at the top and closed at the bottom. They are constructed on land, floated to the desired location, and sunk onto a prepared foundation. Once in place, they serve as a shell for the pier.1
• Open Caissons: These are open at both the top and bottom and feature a cutting edge. As material is excavated from the interior through dredging wells, the caisson sinks into the ground. Once the required depth is reached, the bottom is sealed with concrete.1
• Pneumatic Caissons: Similar to open caissons, these include airtight bulkheads. The "working chamber" at the bottom is pressurized to keep out water and soil, allowing workers to excavate the foundation manually.1

Ultimately, these structures are filled with concrete to become a permanent, load-bearing part of the bridge foundation.123