Building Bridge Foundations Underwater

For millennia, constructing bridges across wide and turbulent waterways has presented civil engineers with their most perilous challenge: establishing solid foundations below the water's surface. In antiquity, Roman engineers pioneered the timber cofferdam, a temporary watertight enclosure constructed by driving interlocking wooden piles deep into the riverbed. Once the enclosure was secured, workers packed the gaps with clay and pumped out the trapped water using Archimedes screws or bucket waterwheels, exposing dry riverbed upon which stone masonry piers could be erected. While ingenious, this technique was limited by the structural strength of timber and the manual labour required for dewatering, restricting bridges to relatively shallow, slow-moving rivers with firm, accessible subsoils.

By the early nineteenth century, the expansion of railway networks demanded crossings over much deeper and faster-flowing rivers, driving the invention of the caisson. A caisson is essentially a massive prefabricated retaining structure that serves as a permanent foundation for a bridge pier. The earliest widespread variant was the open caisson, a hollow cylinder or box made of timber or wrought iron, open at both the top and bottom. Sunk into position, heavy excavating buckets scooped out the silt, sand, and gravel from within its walls. As internal material was removed, the caisson gradually sank under its own immense weight until reaching bedrock. However, open caissons frequently encountered boulders that jammed their descent or experienced uneven settling, causing the structure to tilt unpredictably.

To overcome these obstacles, engineers developed the pneumatic caisson, which transformed deep-water foundation engineering. This design added an airtight ceiling near the bottom of the structure, creating a pressurised working chamber below. Compressed air was continuously pumped into this chamber to balance the external hydrostatic water pressure, preventing water from entering and allowing labourers to excavate the riverbed by hand in dry conditions. Material and personnel entered and exited through specialised airlocks. However, working in these hyperbaric environments took a severe toll on the workforce. Labourers suffered from a mysterious affliction then known as caisson disease—characterised by joint pain, paralysis, and occasionally death. Only decades later was it discovered that rapid decompression caused nitrogen gas to form bubbles in the bloodstream, leading to the establishment of regulated decompression protocols.

In contemporary engineering, pneumatic caissons have largely been replaced by mechanised methods that eliminate the need for human workers to descend into high-pressure environments. The most prevalent technique is the drilled shaft, also known as the bored pile. Powerful rotary drilling rigs drive heavy steel casings deep into the riverbed to seal off surrounding water. As the drill bores into bedrock, a viscous bentonite clay slurry is pumped into the borehole. The hydrostatic pressure of this slurry prevents the borehole walls from collapsing before concrete can be poured. Once drilling reaches the target depth, a reinforcement cage of steel is lowered inside, and concrete is placed from the bottom up using a submerged pipe known as a tremie, displacing the lighter slurry.

For exceptionally deep marine crossings where drilled shafts are impractical, engineers frequently deploy floating caissons. These enormous hollow concrete structures, often resembling multi-storey buildings, are constructed in dry docks and towed to the bridge site by tugboats. Once precisely positioned using satellite positioning systems, internal compartments are flooded with water, causing the structure to sink onto a pre-levelled bed of crushed rock on the seabed. Once seated, the internal chambers are filled with gravel, sand, or concrete to provide permanent ballast. In recent decades, suction caissons have also been adapted from offshore oil extraction; these open-bottomed steel cylinders use water pumps to create internal negative pressure, rapidly sucking the caisson into soft sea sediments without heavy driving equipment.

Beyond the initial challenge of installation, submerged bridge foundations must withstand the relentless physical forces of their environment. The most hazardous of these is scour, a hydrodynamic phenomenon where fast-moving river currents or tidal flows excavate sediment from around the base of a pier. Scour undermines the foundation's stability and accounts for roughly half of all bridge failures worldwide. To mitigate this threat, engineers encircle vulnerable piers with riprap—large, angular boulders that resist displacement—or install collar deflectors that redirect downward water vortices away from the bed. Furthermore, because seawater causes severe corrosion in steel reinforcement, modern piers incorporate sacrificial zinc anodes or low-permeability concrete to extend their service life.

Looking ahead, bridge foundation engineering is becoming increasingly reliant on automated monitoring and advanced materials. Autonomous underwater vehicles fitted with sonar sensors can now inspect submerged piers for microscopic structural fractures and scour damage, removing the need for hazardous commercial dive operations. Concurrently, material scientists are refining alkali-activated cements and geopolymer concretes that actually gain structural integrity when cured in cold, mineral-rich saltwater while producing a fraction of the greenhouse gas emissions associated with traditional Portland cement. These innovations ensure that underwater foundations will be both more resilient and environmentally sustainable in the decades ahead.