Double-wall and cellular cofferdams

How bridges get built · Shallow foundations, cofferdams and dewatering · Double-wall and cellular cofferdams

Also called: cellular sheet pile cofferdam, double-wall steel cofferdam, circular cell cofferdam

How it is done

Select the form: a double wall (two parallel sheet lines tied with rods and filled) for long straight runs; circular cells joined by connecting arcs, or diaphragm cells, for higher heads; floated double-wall steel shells for deep, fast rivers.
Prepare the foundation: dredge soft material along the cell line; on rock, level the surface or use templates so the piles can seat evenly.
Thread the flat-web piles of each cell around a ring template supported on spud piles and close the ring before driving any pile to full depth; then drive in stages around the circle.
Fill each cell promptly with free-draining sand or gravel by clamshell or pumping, and provide weep holes or drains so the fill on the dry side stays unsaturated.
Add an inside berm where sliding or overturning governs, then dewater the enclosure in stages while surveying cell deflection and watching seepage and interlock distress.
Build the foundation inside on a strut-free floor; keep pumps and a flood plan, since overtopping can wash out the cell fill.
Remove by excavating the fill and extracting the piles, or leave the cells as permanent pier protection or island walls.

Key numbers

Retained headcommonly 10-20 m of water.
Equivalent width of a cellular cofferdamabout 0.7-1.0 times the retained height.
Circular cell diametertypically 10-25 m with flat-web sheet piles.
Flat-web interlock strengthabout 3,000-6,000 kN/m depending on section and grade; interlock tension designed with a factor of about 2.
Cell fillfree-draining sand or gravel with low fines content (below about 5-10 %).

Where it fits

Piers in 10-20 m of water, large footprints for pylons, sites where internal struts would obstruct the work.

Where it does not

Very soft foundation beds where the filled wall would slide; rock heads that prevent embedment.

Choosing it

Choose cells or double walls over a braced cofferdam when water depth exceeds about 10-12 m, the footprint is large, or struts would obstruct pylon, lock or cap construction.
Prefer a braced sheet-pile cofferdam for small pier footprints: cells need a width of about 0.7-1.0 times the head on every side, which a small pier site cannot spare.
Prefer a caisson or a floated box when the bed is too soft for a filled wall to resist sliding, or the water is deeper than about 20 m.
In deep, fast rivers where sheets cannot be driven through the bed, floated double-wall steel shells sunk by flooding the annulus are the usual choice for deep-water pile caps.

Plant, pace and money

PlantSheet piles, templates, vibro and impact hammers, filling plant, barges.

ProductivityWeeks to months per cofferdam; a large cellular cofferdam can take half a year.

CostUSD 2-10 million per major pier.

Risks and controls

What goes wrong

Cell rupture during filling, sliding under flood, interlock tension failure.

Quality assurance

Interlock tension design checked, fill density monitored, deformation surveyed daily during dewatering.

Origins

Steel cellular cofferdams developed in the United States in the early 20th century with flat-web sheet piling; their design method was set out by Terzaghi (1945) and in the Tennessee Valley Authority's 1957 monograph on cellular cofferdams on rock, drawn from its dam programme.

Examples

Lock and dam cofferdams on the Ohio and Mississippi; the artificial-island walls of the Hong Kong-Zhuhai-Macau link; double-wall steel cofferdams for deep-water pile caps on large rivers.

Case studies

Hong Kong-Zhuhai-Macau Bridge (tunnel-portal islands)China · 2011The walls of the two artificial islands were formed by 120 steel cylinders about 22 m in diameter and 40 m long, each weighing over 500 t, vibrated in by eight synchronised vibratory hammers; the island walls were completed in under seven months.

Olmsted Locks and Dam, Ohio RiverUSA · 1990s-2002The twin 366 m lock chambers were built in the dry inside a large cellular sheet-pile cofferdam before the dam itself was built in the wet from precast shells.