Tremie concrete seal

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

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

Tremie concrete seal — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Design the seal for uplift (its weight plus bond to piles and sheets), for bending between piles, and for the construction stage; specify a cohesive, highly workable mix with retarder and, for long flow distances, an anti-washout admixture.
Excavate to grade and clean the base and the pile and sheet surfaces by airlift, jetting or divers with brushes, since bond to the piles carries much of the uplift; sound the base on a grid to confirm level.
Set tremie pipes of 200-300 mm with watertight flanged joints on a gantry at 3-6 m spacing, marked for depth, with hoppers above water.
Start each pipe with its tip on the base and a plug or go-devil separating concrete from water; lift slightly to release the charge and keep the tip at least 1-1.5 m inside fresh concrete from then on.
Pour continuously, raising the surface evenly and moving pipes in a planned sequence; sound the surface regularly. If a pipe loses its seal, withdraw, re-plug and restart; never let concrete fall through water.
Leave the seal to gain strength for 3-7 days, then dewater in stages, watching for leaks at pile and sheet interfaces and repairing them before going lower.
Remove laitance and high spots down to sound concrete before casting the footing; core the seal where quality is in doubt.

Key numbers

Seal thickness1-3 m typical; a seal relying on its weight alone needs about 0.45-0.5 times the water head at its underside.
Tremie pipes200-300 mm diameter at 3-6 m spacing; tip embedment at least 1-1.5 m.
Mixslump about 150-230 mm (ACI 304R), maximum aggregate usually 20 mm.
Placing rateabout 50-150 m3/h on large seals; the pour must be continuous.
Wait before dewatering3-7 days.

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.

Choosing it

Choose a seal over pumped dewatering when the base is in permeable sand or gravel and the sheets cannot reach an impermeable layer, so pumping would cause boiling or very large inflows.
Omit the seal when the sheets toe into clay or rock that cuts off seepage and the base passes the heave check: the seal concrete, often thicker than the footing, is otherwise wasted.
Prefer a precast or steel box with a sealed floor when a seal would stand above the bed in a navigation channel or scour zone, or where divers cannot clean the base.
Where foundation piles pass through the seal, a thinner seal anchored by bond to the piles is cheaper than a gravity seal; check bond and punching.

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.

Origins

Placing concrete underwater through a tremie (French for hopper) dates from the 19th century; anti-washout admixtures, in use since the 1970s and 1980s, made wide, thin and self-levelling underwater placements reliable.

Examples

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

Case studies

Sikorsky BridgeUSA · 2003A tremie seal was considered and rejected because it would have stood above the mudline in the navigation channel and needed later removal; a precast 'bathtub' inside the cofferdam replaced it.

Kabinakagami River Bridge replacementCanada · 2008The geotechnical report recommended a tremie plug or mud mat at the base of the abutment cofferdam; none was placed and the soft base heaved.

Akashi Kaikyo Bridge (main tower foundations)Japan · 1998Steel caissons about 80 m in diameter were set on the excavated seabed and filled with anti-washout underwater concrete in the strong tidal currents of the strait.