Sheet-pile cofferdam

Also called: steel sheet piling, braced cofferdam, single-wall cofferdam

Concept diagram (TheBridgeEng)

How it is done

Drive sheet piles from a barge or a temporary platform with a vibro-hammer, using a guide frame to keep them plumb and interlocked; embed 1.5-2 times the excavation depth below the bed in sand.
Excavate inside in stages, installing walings and struts at each level as the water is lowered.
Where the base would boil or the sheet piles cannot reach a cut-off, pour a tremie concrete seal, then dewater.
Build the footing or cap, backfill, and extract the sheet piles or cut them off below the bed.

River and estuary piers in water to 8-10 m, alluvial beds, temporary works that must be removed.
Single-wall for narrow footprints; braced frames for depths over 4 m.

Boulders and hard rock heads that refuse the piles or split the interlocks.
Fast rivers in the flood season: the cofferdam narrows the channel and raises the flood level.
Deep water beyond about 10 m, where a double-wall cofferdam or caisson is safer.

PlantSheet piles (Z or U profiles, often 12-20 m), vibro-hammer on a crane or barge, walings and struts, pumps.

ProductivityA pier cofferdam of 15 x 25 m is driven in 5-10 days, excavated and sealed in another two to three weeks.

CostUSD 300,000-1,000,000 per river pier including seal and dewatering; the game's temporary works rates follow this.

Flooding in a flash flood (the game's cofferdam flood event), interlock declutching, base heave or piping, strut failure during excavation. Cofferdam failure is the most common temporary works accident on river bridges.

Interlock and plumbness checks, strut loads monitored, base inspected by divers before the seal, flood watch procedure with pumps on standby.

Nearly all river bridges in Korea, Japan and Europe with piers in shallow water.