Open caisson (well foundation)

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

Also called: well foundation, open well, sunk caisson

How it is done

Investigate the full sinking depth with borings at the corners and centre of the caisson: layers of dense sand, stiff clay or boulders control sinking, skin friction and bearing at the cutting edge.
Prepare the starting level: a sand island or the dry bank in shallow water, or a steel shell floated in and moored in deep water; set and level the cutting edge on a firm bed.
Cast or erect the first lifts (cutting edge, walls and dredging wells); floated shells are ballasted down onto the bed in a tide and weather window.
Excavate the wells symmetrically by grab, airlift or jet pump, keeping the cutting edge buried and the soil plug level; add lifts as the caisson sinks, keeping freeboard above water.
Control tilt and shift continuously: survey every shift, excavate selectively under the high side, and use kentledge, water ballast, jetting or bentonite lubrication of the outer wall to overcome friction; never over-excavate below the cutting edge.
At founding level, confirm the stratum (soundings, samples, divers or ROV), clean the base and place a tremie concrete plug in each well. Hold point.
Dewater only after the plug has cured and uplift has been checked; fill the wells with sand, water or concrete as designed and cast the top slab and pier base.

Key numbers

Plan size10–30 m for river piers; 50–95 m for the largest Chinese anchorages and pylon piers (Changtai 95 × 57.8 m)
Founding depth20–60 m common; about 73 m at San Francisco–Oakland Bay Bridge Pier W4 (1936)
Sinking rate0.3–1.5 m per day
Tilt and shift limits for Indian well foundations: 1 in 80 and 150 mm (IRC 78)
Sand islands for startingpractical in shallow, slow water; floated steel shells beyond that

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.

Choosing it

Choose an open caisson over a pile group where scour of 10–20 m or more, ship impact and long free pile lengths would make piles flexible and costly.
Choose it for very large anchorages and pylon piers on deep soil, where one massive block is simpler to design and protect (Jiangyin, Hutong, Changtai).
Prefer a pneumatic caisson where the base must be inspected directly or boulders stop the cutting edge; prefer bored piles where scour is modest and a jack-up or trestle can reach.
Prefer a floated steel shell over a sand island in deep or fast water, as on the Changtai and Taizhou Yangtze crossings.

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.

Origins

Masonry well foundations were used for river bridges in India for centuries and became the standard foundation of Indian railway bridges in the 19th century; steel and concrete open caissons later carried piers to record depths, such as Pier W4 of the San Francisco–Oakland Bay Bridge at about 73 m in the 1930s.

Examples

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

Case studies

San Francisco–Oakland Bay Bridge, Pier W4USA · 1936Central anchorage caisson about 28 × 43.5 m, sunk to about 73 m, a record depth for a bridge foundation at the time.

Jiangyin Yangtze River Bridge, north anchorageChina · 1999Open caisson 69 × 51 m sunk about 58 m (sinking around 1995), then among the largest caissons in the world.

Hutong (Shanghai–Nantong) Yangtze River Bridge, main pierChina · 2020Caisson 86.9 × 58.7 m in plan and about 115 m deep, built for a rail-road cable-stayed bridge with a 1,092 m main span.

Changtai Yangtze River Bridge, main pierChina · 2025Round-ended stepped steel-concrete caisson, 95 × 57.8 m at the base and 72 m high, divided into 36 compartments; prefabricated on shore, floated to site and sunk by staged excavation.