Construction methods · Shallow foundations, cofferdams and dewatering

Tremie concrete seal

A mass of concrete placed underwater through a tremie pipe on the base of a cofferdam, thick enough that its weight plus friction on the sheet piles resists the uplift when the cofferdam is pumped dry. It converts a wet hole into a dry working floor.

How bridges get built · Shallow foundations, cofferdams and dewatering · Tremie concrete seal

Also called: underwater concrete plug, cofferdam seal, tremie plug

Concept diagram (TheBridgeEng)

How it is done

Clean the base by airlift or clamshell; check the level with soundings.
Set tremie pipes (250-300 mm) at 3-5 m spacing with hoppers above water; start each pipe with a plug or a go-devil to keep water out.
Pour continuously, keeping the pipe tip 1-2 m below the concrete surface; use a flowing mix with retarder and anti-washout admixture.
After 3-7 days, dewater and remove laitance from the top.

Where it fits

Sheet-pile and double-wall cofferdams in sand and gravel where the base would boil; pile caps in water built inside cofferdams.

Where it does not

Thin seals in deep water: the seal thickness is often 1-3 m and can exceed the footing itself.

Plant, pace and money

PlantTremie pipes and hoppers, concrete pump or crane and skips, divers.

Productivity300-1,000 m3 per day per pour.

CostUSD 200-350 per m3, plus a lot of concrete you never see.

Risks and controls

What goes wrong

Pipe tip lifting out of the concrete and washing the mix; cold joints from an interrupted pour; seal cracking and leaking under uplift.

Quality assurance

Continuous pour log, soundings during placement, diver inspection of the surface, cube tests on the mix.

Examples

Standard under river pier cofferdams worldwide; also used to plug pile caps of the game's water piers when the cap is low.