Construction methods · Caissons and marine foundations

Pneumatic caisson

An open caisson with a sealed working chamber at the bottom kept dry by compressed air, so workers (now often remote-controlled excavators) dig in the dry through boulders and onto rock. Precise founding, no tilt surprises, and historically dangerous; modern Japanese practice uses unmanned excavation and pressures beyond 0.3 MPa with mixed-gas decompression.

How bridges get built · Caissons and marine foundations · Pneumatic caisson

Also called: compressed-air caisson, pneumatic well

Concept diagram (TheBridgeEng)

How it is done

Build the caisson with a working chamber roof, airlocks and shafts; pressurise the chamber to balance the water head.
Excavate the chamber (unmanned excavators in Japan), lowering the caisson under its weight; add lifts.
At founding level, inspect the base directly, place concrete in the chamber under pressure, then fill the shafts.

Where it fits

Founding on rock or in boulder beds under 20-35 m of water and soil, where direct inspection of the base is required; urban sites with tight footprints (Tokyo expressways).

Where it does not

Depths beyond about 35 m (air pressure over 0.35 MPa), sites without specialist contractors.

Plant, pace and money

PlantAirlocks, compressors, decompression chambers, remote excavators, medical lock and hyperbaric support.

Productivity0.3-1 m per day.

CostHighest of the caisson family; USD 10-40 million per pier.

Risks and controls

What goes wrong

Decompression illness, blow-outs, fire in the chamber, base heave when the pressure drops.

Quality assurance

Pressure logs, base inspection records, air quality monitoring.

Examples

Brooklyn and Eads bridges historically; modern Tokyo Metropolitan Expressway piers and some Chinese river bridges.