Construction methods · Shallow foundations, cofferdams and dewatering

Precast and floating cofferdam boxes

Instead of driving sheet piles, a steel or precast concrete box is floated to the pier, lowered onto pre-installed piles or the bed, and pumped dry. It doubles as the permanent pile-cap formwork and removes divers and cofferdam bracing from the critical path.

How bridges get built · Shallow foundations, cofferdams and dewatering · Precast and floating cofferdam boxes

Also called: floated-in cofferdam, precast concrete cofferdam, steel box cofferdam

Precast and floating cofferdam boxes — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Fabricate the box in a dry dock or on a slipway; tow it to the pier.
Lower it over the pile group on strand jacks or by ballasting, seat it on brackets welded to the piles or on a prepared bed.
Seal the pile penetrations with grout collars or a tremie plug, dewater, and cast the cap inside; the box becomes lost formwork.

Where it fits

Sea and estuary piers with pile groups in 8-25 m of water, where sheet piles cannot be driven or would take too long.

Where it does not

Sites without a fabrication yard or towing route; small piers where a sheet-pile cofferdam is cheaper.

Plant, pace and money

PlantYard, tugs, strand jacks or ballast systems, survey for setting.

ProductivityOne box every few weeks from a yard; setting in one or two tidal windows.

CostUSD 1-4 million per box, offset by faster caps.

Risks and controls

What goes wrong

Towing and setting in weather windows; seal failure at pile penetrations.

Quality assurance

Leak test after seating, level and position survey, grout collar inspection.

Examples

Hong Kong-Zhuhai-Macau, Incheon Bridge and many Gulf sea bridges used precast cap shells; the game's high pile cap uses this rate.