How did pneumatic caissons allow bridge foundations to be built at greater depths?

Pneumatic caissons revolutionized bridge engineering by enabling construction at depths that were previously inaccessible due to water and soil infiltration.

Unlike open caissons, which rely on dredging from the surface, a pneumatic caisson features an airtight bulkhead positioned above its cutting edge. This creates a pressurized "working chamber" at the bottom of the structure. By maintaining air pressure within this chamber, engineers can effectively counteract the external hydrostatic pressure of the surrounding water and soil. This pressure prevents water from flooding the workspace, allowing laborers to work directly on the riverbed or seabed to excavate material safely. As the soil is removed, the caisson sinks deeper into the ground under its own weight until it reaches a stable bearing stratum, at which point the chamber is typically filled with concrete to form a permanent, solid pier foundation.123