Construction methods · Shallow foundations, cofferdams and dewatering

Pile caps in water

The block that gathers a pile group into one pier. On land it is a footing in an excavation; in water there are two choices: a low cap cast below the bed inside a cofferdam, protected from scour and ship impact, or a high cap at or above water level cast in a suspended form, faster and cheaper but exposed.

How bridges get built · Shallow foundations, cofferdams and dewatering · Pile caps in water

Also called: high pile cap, low pile cap, cap in cofferdam

Pile caps in water — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Low cap: cofferdam, excavate below the bed, tremie seal, cut piles to level, cast the cap dry.
High cap: weld brackets to the piles, hang a steel or precast soffit form, pour above the tide or use a tremie for the bottom lift.
Either way: check pile head cut-off levels and embedment into the cap (typically 100-150 mm plus bars), and cast the pier starter with the cap.

Where it fits

Low caps: navigable rivers with ship impact, high scour, aesthetics of piers rising from the water. High caps: sea viaducts with many piers, tidal sites where speed matters, no ship impact.

Where it does not

High caps in the splash zone in aggressive waters without extra cover and coatings.

Plant, pace and money

PlantCofferdam or suspended formwork, cranes or barges, tremie equipment.

ProductivityLow cap 4-8 weeks per pier; high cap 2-3 weeks.

CostLow caps cost 2-3 times high caps because of the cofferdam and seal.

Risks and controls

What goes wrong

Scour exposing a high cap; ship impact; chloride attack in the tidal zone.

Quality assurance

Pile cut-off survey, cover meter checks, curing of the splash-zone concrete.

Examples

Sea viaducts in the Gulf and Hong Kong use high caps with precast shells; river bridges in Korea use low caps below the bed.