Cofferdam Construction

What a cofferdam actually does

Every long river-spanning bridge with intermediate piers has, at some point during construction, sat dry inside a temporary metal box. The box is the cofferdam: a watertight enclosure driven into the bed of the river, lake, or sea, pumped out to expose the bed inside, and inside which a permanent foundation is built in dry conditions before the cofferdam is removed, flooded, or — for very large structures — left in place forever. The technique has existed since the Romans, who used double-walled timber cofferdams filled with clay puddle for bridge piers across the Danube. Modern cofferdams are driven steel sheet piles or cellular sheet-pile cells filled with granular material, but the fundamental idea has not changed.

Why bother? Because concrete cured underwater is no good. Rebar laid underwater is no good. Welding underwater is exotic and expensive. Bolts torqued underwater are uninspectable. Pretty much every quality and inspection standard collapses below water level. Building a 50 m × 20 m pier foundation requires welding, concrete cylinder testing, dimensional surveying, bolt torque verification, geotechnical sampling — all of which need a dry, well-lit, ventilated workspace. The cofferdam supplies that workspace by pushing the water back, even if only for the few weeks the foundation needs.

Cofferdam families

Type	Construction	Water depth	Cost driver	Where used
Earth-fill dam	Compacted earth or rockfill embankment around the work area	Up to 3 m	Cheap if fill is local	Small streams, drainage canals
Rockfill	Quarried rock dumped to form a containment berm	Up to 5 m	Rock haul distance	Shallow rivers, lake-edge foundations
Single-wall sheet pile	Steel Z- or U-piles driven in a closed ring, internally braced with walers + struts	3 to 12 m	Pile cost + driving rig	Bridge piers, small underwater foundations
Double-wall sheet pile	Two parallel sheet-pile walls with the cavity backfilled, no internal bracing	5 to 15 m	2× pile quantity	Deep water where strut-free interior is required
Cellular cofferdam (circular)	Straight-web sheet piles in a closed circle, filled with sand/gravel	15 to 25 m	Pile quantity, fill volume	Dam abutments, lock structures, deep ports
Cellular cofferdam (diaphragm)	Tangent circular cells connected by straight diaphragm walls — long enclosures	15 to 25 m	Pile + fill + connecting walls	Navigation locks, dry docks, dam stilling basins
Concrete caisson	Hollow precast concrete shell sunk into the bed, then dewatered and used as final foundation	10 to 30 m	Caisson fabrication and sinking	Deep bridge piers (e.g., Verrazzano-Narrows piers)
Cofferdam-free underwater foundation	Permanent precast shell floated and sunk to position, then filled with tremie concrete	30 m and deeper	Underwater placement complexity	Very deep piers (Akashi Kaikyō, Storebælt)

The sheet-pile cofferdam is the workhorse of medium-depth bridge construction; the cellular cofferdam is the workhorse of major navigation works in deep water. Choice is governed by water depth, soil at the bed, current and ice loads, available pile-driving equipment, and how long the cofferdam needs to remain in place.

The construction sequence

Inside a typical sheet-pile cofferdam build:

1. Site survey and bathymetry. Map the river bottom contours, identify the foundation footprint, sample soil with a borehole or vibrocore to determine soil type and depth to bearing stratum.
2. Drive a template. A floating barge with a steel guide template positions the first sheet piles at the corners of the planned enclosure. Each sheet pile is roughly 12 to 25 m long and 600 mm wide.
3. Drive the perimeter wall. A vibratory hammer or impact hammer drives each sheet pile through the riverbed soil to the design penetration depth — typically 2 × the maximum hydrostatic head. Piles interlock with adjacent piles via integral grooves, forming a continuous, low-permeability wall.
4. Install first level of bracing. A horizontal steel waler is welded or bolted around the inside perimeter, with struts spanning across the interior. This frame keeps the sheet piles from caving inward when dewatering starts.
5. Start dewatering. Submersible pumps inside the cofferdam lift water out faster than seepage leaks in. As the water level inside drops, the next layer of bracing is installed, and dewatering continues in stages.
6. Excavate to design grade. With the cofferdam dry, excavate the bed material inside down to the planned founding level — typically 3 to 10 m below the riverbed.
7. Pour a tremie seal. Often a 1 to 2 m thick concrete layer is poured underwater first (a tremie pour) to seal the bottom against uplift before final dewatering — particularly important in permeable soils or where bottom heave is a concern.
8. Build the foundation. Place rebar, set anchor bolts, pour the pier footing or pile cap, build the pier shaft upward.
9. Flood and remove. Once the foundation cures and rises above water level, open the cofferdam to refill, then extract the sheet piles with a vibratory hammer for reuse — or leave them as permanent scour protection.

Forces on the cofferdam wall

The wall sees hydrostatic pressure that increases linearly with depth. For a 10 m water head with seabed at the cofferdam base:

Hydrostatic pressure at base:   p = ρ_w g h = 1000 × 9.81 × 10 = 98.1 kPa
Resultant water pressure per metre of wall:
  P_water = ½ p h = ½ × 98.1 × 10 = 490 kN/m   acting at h/3 above base

Plus active earth pressure from the soil outside, if backfill or river silt:
  P_soil  = ½ K_a γ' h_soil²   (typically additional 50 to 150 kN/m)

Plus current drag if river flows past:
  q_current ≈ ½ ρ_w v² × C_D   (≈ 1 to 5 kPa for v = 1 to 3 m/s, projected area)

Plus possibly ice load (for cold-climate rivers): up to 200 kN/m of perimeter

The sheet-pile wall must resist all of these as bending moment between bracing supports, plus the bracing must carry the resultant horizontal thrust. Conventional design tools (Coulomb earth-pressure theory, Caquot–Kerisel charts, Terzaghi piping criterion) handle the static calculation; for major projects, finite-element modelling captures soil-structure interaction more accurately.

Worked example: a 20 × 30 m sheet-pile cofferdam

Bridge pier in a river 8 m deep over a sandy riverbed. Foundation must reach 4 m below the bed. Required excavation depth inside cofferdam: 12 m below water surface.

Cofferdam plan:           20 m × 30 m (100 m perimeter)
Water depth:              h_w   = 8 m
Bed-to-base depth:        h_b   = 4 m
Max head at base:         h_max ≈ 12 m
Sheet pile total length:  L_p   ≈ water depth + bed-to-base + 2 × head
                                ≈ 8 + 4 + 2 × 12 = ~36 m
                          (often 25 to 30 m if good rock at depth)
Pile section:             AZ48 or PZC18 — steel Z-piles, ~600 mm width

Hydrostatic resultant per metre of wall:
  P_w = ½ × 10 × 12² × 9.81 = 706 kN/m

Number of bracing levels:
  3 to 4 levels of horizontal walers at vertical spacing 3 m

Pumping demand (estimated by Darcy seepage):
  Q ≈ k × i × A
   where k = 1e-4 m/s (medium sand), i ≈ 0.5, A = perimeter × pile depth
  Q ≈ 1e-4 × 0.5 × 100 × 24 ≈ 0.12 m³/s = 432 m³/h
  → provide 2 × submersible pumps each rated 300 m³/h plus standby

Piping check (Terzaghi):
  Critical gradient i_crit = (γ' / γ_w) ≈ 1.0 for typical sand
  Exit gradient at base = h_max / (2 × penetration depth)
                       = 12 / (2 × 12) = 0.5  →  FS = i_crit / i_exit = 2  ✓

Bottom heave check:
  Weight of soil plug between piles inside cofferdam vs upward water pressure
  → adequate with 2 m tremie seal poured before final dewatering.

Schedule:
  Pile driving: 8 weeks
  Dewatering and excavation: 2 weeks
  Tremie seal pour + cure: 2 weeks
  Foundation construction: 12 weeks
  Cofferdam removal: 2 weeks
  Total: ~26 weeks

The numbers are illustrative — real designs work to actual site conditions, AASHTO/Eurocode safety factors, and may use deep penetration where bedrock is shallow, or rely on a thick tremie seal where bedrock is deep and piping is the critical concern.

Cross-section, drawn

SHEET-PILE COFFERDAM (cross-section, looking along river)

     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  ← river water surface
                  ↓ hydrostatic pressure
   ║▓▓║                                          ║▓▓║
   ║▓▓║                  AIR                     ║▓▓║   ← dewatered interior
   ║▓▓║          (cofferdam interior)            ║▓▓║
   ║▓▓║━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━║▓▓║  ← bracing level 1
   ║▓▓║                                          ║▓▓║
   ║▓▓║                                          ║▓▓║
   ║▓▓║━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━║▓▓║  ← bracing level 2
   ║▓▓║                                          ║▓▓║
   ║▓▓║          ┌──────────────────┐            ║▓▓║
   ║▓▓║          │   PIER FOOTING   │            ║▓▓║   ← foundation being built
   ║▓▓║━━━━━━━━━━┴──────────────────┴━━━━━━━━━━━━━║▓▓║  ← tremie seal
   ║▓▓║   ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓║▓▓║
   ║▓▓║   ▓▓▓▓▓▓▓▓ excavated bed material ▓▓▓▓▓▓▓║▓▓║   ← original riverbed level
   ║▓▓║   ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓║▓▓║
   ║▓▓║   ▓▓▓▓▓▓▓▓▓▓▓▓▓▓ NATIVE SOIL ▓▓▓▓▓▓▓▓▓▓▓▓║▓▓║   ← piles extend
   ║▓▓║   ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓║▓▓║     ~12 m below excavation
   ║▓▓║   ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓║▓▓║     to control seepage
   ║▓▓║   ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓║▓▓║

           seepage path follows perimeter pile wall,
           up under the cofferdam, exit gradient at the base must
           stay below the critical piping gradient.

The cellular cofferdam — for big water

Sheet-pile cofferdams hit their limit around 12 m of water. Beyond that, hydrostatic pressures get high enough that thick bracing eats the interior workspace, and seepage failure becomes hard to control with a single line of piles. The answer is the cellular cofferdam: a closed-perimeter ring (or tangent cells of rings) made of straight-web sheet piles, with the interior filled with sand or gravel. Cell diameter is typically 15 to 25 m; each cell is a free-standing gravity structure whose stability comes from the weight of its fill and the shear friction within the fill mass.

Cellular cofferdams handle 25 m of water and were extensively used in the construction of the Panama Canal and most major U.S. river locks. Their failure modes are different from sheet-pile: cells can shift sideways under unbalanced loads, the fill can drain through unsealed joints causing cell collapse, and high-current flow around the perimeter can scour at cell toes. Cellular cofferdams are also expensive — fill plus piles plus driving — and are reserved for situations where the deep-water sheet pile is no longer viable.

Real-world cofferdam projects

• Roman bridges across the Rhine and Danube (1st century BCE). Julius Caesar's 50 km/h Rhine crossing in 55 BCE described in De Bello Gallico used timber cofferdams to drive piles for bridge piers in 6 m of fast water.
• Brooklyn Bridge piers (USA, 1869–1883). Massive caisson cofferdams 51 × 31 m, sunk to 24 m and 14 m below water for the East River piers. Workers inside suffered decompression sickness ("caisson disease"), killing several including chief engineer Washington Roebling's father John.
• Three Gorges Dam temporary cofferdams (China, 1994–2003). 130 m tall concrete-faced rockfill cofferdams diverted the Yangtze River for 6 years while the permanent dam was built behind them — the largest cofferdams ever constructed.
• Hoover Dam diversion cofferdams (USA, 1931–1935). Pair of earth-and-rockfill cofferdams 30 and 20 m tall diverted the Colorado River through tunnels while the arch dam rose between them.
• Akashi Kaikyō tower foundations (Japan, 1988–1998). The world's longest suspension bridge's tower foundations sit on circular caisson cofferdams 80 m in diameter, sunk into 45 m of water on the seabed of the Akashi Strait.
• Modern highway-river bridges, worldwide. Almost every new interstate highway bridge over a navigable river uses sheet-pile cofferdams for each river pier — typically 20 × 30 m, 25 to 30 m of pile, complete with bracing and tremie seal.

When cofferdams fail

• Sand boils. High exit gradient at the base inside the cofferdam fluidises the sand, and a literal "boil" of moving sand and water erupts from the floor. Foundation excavation is buried under metres of liquefied sand within hours. The 1928 Bonnet Carré Spillway construction saw a series of sand boils that nearly stopped work. Modern designs use deeper piles and relief wells to manage this.
• Bracing failure. A single waler or strut breaking under load can trigger progressive collapse of the inward-pressing walls — a "blowout." The 2017 Wapatki Bridge cofferdam collapse in Arizona killed three workers when a strut buckled and the wall folded.
• Pile interlock separation. Adjacent sheet piles can splay apart at their interlocks if driving forces or differential settlement misalign them, creating a permeable gap. Sealing leaks with bentonite slurry or cement grout is standard, but a large gap that opens after dewatering can flood the cofferdam fast.
• Scour undercutting. Current accelerates around the cofferdam perimeter, especially at the corners, scouring out the bed at the pile toes and threatening pile pull-out. Rock berms or riprap protection are placed before dewatering.
• Overtopping. Storm or flood raising the river level above the cofferdam wall tops it — the cofferdam fills, work stops, and bracing may fail under the unexpected one-sided load. Modern designs include high freeboard, real-time hydrological monitoring, and contingency flooding plans.

Common pitfalls

• Skipping the tremie seal. In permeable soils with significant head differential, pouring the foundation directly on an unsealed bed risks uplift failure. The 1 to 2 m tremie seal is cheap insurance.
• Underestimating seepage pumping demand. Permeability varies orders of magnitude across the bed; one pocket of clean gravel can multiply inflow tenfold. Provide redundant pump capacity and continuous monitoring.
• Driving piles past their structural limit. Hard driving in dense bed material can over-stress sheet piles, crushing the head or warping the interlocks. Predrilling and jetting are remedies.
• Removing bracing too early. Bracing is sized for full hydrostatic load. Removing one level before the foundation provides equivalent restraint can collapse the wall. Sequence of bracing removal is as important as installation.
• Ignoring ice and debris loads in cold climates. Ice floes against a cofferdam wall can impart 200 kN/m of perimeter — comparable to hydrostatic pressure. Account for it in wall design and bracing.
• Failing to monitor seepage continuously. A growing seepage flow rate is the first signal of an impending sand boil or pile-interlock separation. Real-time pump-rate logging and visual inspection of the floor are mandatory.