wunder beta🏗️ How do you build in the middle of a river?
Before a bridge pier or a dam can be built, somebody has to make dry ground where the river is. The temporary wall that does it is designed to the same safety factors as the permanent work — and how high to build it is arithmetic: a three-y
What you’ll learn
- A dry island in the riverDefine a cofferdam, and explain why a temporary enclosure — built to permanent standards — is the first thing built at any site that is under water.A cofferdam is an enclosure inside a river, not a dam across it. It exists to make one patch of riverbed dry enough to build on, it is designed to the same factors of safety as the permanent work, and it is meant to be taken out afterwards down to the natural bed.
- Pressure, seepage, and pumpsExplain how the difference between inside and outside water levels loads the wall, and how bracing, seepage control, dewatering and cell fill answer it.Pumping the inside down is what creates the load, and pressure is greatest at the bottom. Engineers lengthen the seepage path, brace the wall or weight it with soil fill, and run the pumps to a designed rate — and where a hole cannot be pumped dry, they seal it with concrete first.
- Give the river another routeDescribe how a cofferdam works as one half of a diversion plan, and how engineers price the risk that the river takes it away.A cofferdam narrows the river, so the water needs a planned route: a tunnel, a channel, a finished lock, or a staged opening beside the work. How high to build the barrier is an economic calculation, and the losses when a flood beats it — as one did on the Baker River in November 1924 — are counted in advance.
Questions this course answers
A cofferdam is called a temporary structure. What does that actually mean for its design?
The Corps manual on cellular cofferdams states that although cofferdams are often referred to as temporary structures, their importance requires that they be designed for the same factors of safety as those required for permanent structures.
Why is a cutoff wall driven landward where a cofferdam meets the bank?
Water that cannot pass through the wall will travel around its ends through the ground. A cutoff wall run into the bank increases the seepage path and reduces the velocity of the water.
While a concrete foundation seal is being placed underwater, the water inside the cofferdam is deliberately held level with the river outside. Why?
Pressure on a cofferdam comes from the difference between inside and outside water levels. Keeping them equal removes that difference while the seal is placed; the specification forbids dewatering until the seal is strong enough to withstand the hydrostatic pressure.
What gives a cellular cofferdam its ability to resist the river?
Ring tension in the interlocking piles turns the steel into a container, and the soil fill inside supplies the weight that resists sliding — which is why the manual calls the method relatively inexpensive.
Corps guidance treats the height of a cofferdam as an economic decision rather than an absolute. What follows from that?
The manual asks engineers to weigh construction cost against overtopping damage and delay, and its own example notes a roughly 27 percent chance of meeting a ten-year flood at least once during a three-year job.
Grounded in trusted sources
- U.S. Army Corps of Engineers, EM 1110-2-1605: Hydraulic Design of Navigation Dams (12 May 1987), Chapter 6 — flow diversion schemes, cofferdam heights, and preflooding facilities — https://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-2-1605.pdf
- U.S. Army Corps of Engineers, EM 1110-2-2503: Design of Sheet Pile Cellular Structures — Cofferdams and Retaining Structures (29 September 1989) — https://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-2-2503.pdf
- U.S. Army Corps of Engineers, EM 1110-2-1911: Construction Control for Earth and Rock-Fill Dams (30 September 1995), Chapter 6 — river diversion and closure — https://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-2-1911.pdf
- Federal Highway Administration, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (FP-14), Section 208 — cofferdams, foundation seals and dewatering — https://highways.fhwa.dot.gov/sites/fhwa.dot.gov/files/docs/federal-lands/specs/12851/fp14.pdf
- Bureau of Reclamation, The Story of Hoover Dam — Cofferdams — https://www.usbr.gov/lc/hooverdam/history/essays/coffer.html
- Bureau of Reclamation, The Story of Hoover Dam — Tunnels — https://www.usbr.gov/lc/hooverdam/history/essays/tunnels.html
- U.S. Army Corps of Engineers, Savannah District, New Savannah Bluff Lock and Dam: Building the Coffer Dams — https://www.sas.usace.army.mil/Home/NSBLD-Online-Exhibit/Building-Coffer-Dams/
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