Construction methods · Caissons and marine foundations

Open caisson (well foundation)

A concrete or steel shell with a cutting edge, built up in lifts on the bed or on a sand island and sunk under its own weight while the soil inside is dredged out. It ends as a massive block founded far below scour depth: the traditional foundation of large river bridges in India, China and the Mississippi, still unmatched for deep scour and ship impact.

How bridges get built · Caissons and marine foundations · Open caisson (well foundation)

Also called: well foundation, open well, sunk caisson

Concept diagram (TheBridgeEng)

How it is done

Build a sand island or a staging in shallow water; cast the first lift with the cutting edge and pump wells or dredge pockets.
Excavate inside with grabs or airlifts; the caisson sinks; add lifts as it goes; kentledge or water ballast helps in stiff layers; jetting and lubricating with bentonite reduce skin friction.
At founding level, clean and plug the base with tremie concrete, fill the wells with sand or concrete, cast the cap.

Where it fits

Rivers with deep scour (10-20 m), navigable rivers with ship impact, ground where piles would need to be very long, cohesionless beds that dredge easily.

Where it does not

Boulder beds and rock where the cutting edge stops (divers or pneumatic work needed), sites where tilt during sinking cannot be tolerated.

Plant, pace and money

PlantGrabs and cranes, airlifts, concrete supply, jetting pumps, survey for tilt and shift.

ProductivitySinking 0.3-1.5 m per day; a 40 m caisson takes months.

CostUSD 5-30 million per major pier; competitive only where scour and depth rule out piles.

Risks and controls

What goes wrong

Tilting and shifting during sinking (the classic failure), sand blows and sudden drops, cutting edge damage, base blow-in when dewatered.

Quality assurance

Tilt and position surveyed every shift, excavation kept balanced, dredging logs, base inspection by divers before the plug.

Examples

Howrah (Kolkata) and Mahatma Gandhi Setu, Jiangyin and Yangtze crossings, older Mississippi bridges; the game offers it for big water piers.