Construction methods · Caissons and marine foundations

Floating (box) caisson

A concrete or steel box built in a dry dock, towed to the pier, ballasted down onto a prepared bed or a pile group, and filled. It moves the foundation work into a yard and leaves only the seating to the sea: the pylon foundation of choice for straits where dredging or piling in place is slow.

How bridges get built · Caissons and marine foundations · Floating (box) caisson

Also called: floating caisson, gravity caisson, float-in foundation

Concept diagram (TheBridgeEng)

How it is done

Prepare the seabed: dredge soft material, place and level a gravel bed or install piles with a template.
Build the caisson in a dock or on a slipway; float out, tow with tugs, and hold on a mooring spread at the site.
Ballast down in a weather window; seat within tolerance; fill with sand or concrete; pump out and cast the pylon base inside.

Where it fits

Straits and estuaries with firm seabeds or rock, pylon foundations, water 15-60 m deep, sites with a yard within towing distance.

Where it does not

Soft seabeds without piles (settlement and bearing failure), sites with no weather windows for towing.

Plant, pace and money

PlantDry dock or slipway, tugs, mooring spread, ballasting systems, survey and positioning.

ProductivityMonths in the yard; days at sea for seating.

CostUSD 20-150 million for a large pylon caisson.

Risks and controls

What goes wrong

Loss during tow, seating out of tolerance, bed scour after seating, ship impact before the pylon is built.

Quality assurance

Bed survey before seating, seating tolerance survey, ballast records.

Examples

Akashi Kaikyo, Great Belt East, Oresund, Rion-Antirion (on improved ground), Busan-Geoje.