Cofferdams are an enclosure built within or across a body of water to allow the enclosed area to be dewatered. This creates a dry working environment so construction can be carried out properly and safely. Typical cofferdams are welded-steel structures consisting of sheet piles, wales or cross braces, and are normally dismantled after construction. However, new portable or temporary geosynthetic cofferdam systems provide an effective alternative. These systems come in a variety of different designs, materials and installation methods and provide a more economical solution for temporary site dewatering versus conventional structural cofferdams. Portable geosynthetic cofferdams are typically lighter-weight modular systems produced from geomembrane and geotextile fabrics. They limit the impact of construction-generated silt and sedimentation when excavating in submerged or dredging areas, which lessens the environmental impact. Most systems are also modular in design, allowing for bends and turns in the project waterway. They can also be installed and removed more quickly than standard structural cofferdams, which makes them more cost-effective. Properly handled, these dams can also be drained and reused, eliminating the need to purchase a new dam for different projects in some instances. Common applications for geosynthetic portable cofferdams include dewatering for bridge foundations, dam repairs, pipeline crossings, irrigation canal repairs, and shoreline construction and restoration. Smaller-diameter geosynthetic dams are also used extensively for flood control barriers. Figures 1, 2 and 3 show the installation of and some applications of geosynthetic cofferdams.
For this article, the cofferdam system produced by the manufacturer consists of two geomembrane tubes contained by a high-strength woven geotextile outer tube. The two inner tubes are filled with water, which creates a stable, nonrolling water-controlled structure. A center baffle curtain is installed for stability, reducing movement or tipping as a result of hydrodynamic loading. The local water source is normally used to fill the inner tubes of the dam. This cofferdam system was first introduced in the early 1990s and is constructed in a range of sizes from 4–16-foot (1.2–4.9-m) diameter in standard 100-foot (30-m) and 200-foot (61-m) lengths. Special connecting collars are used to join the dams during installation. Standard fill and drain ports are installed into each dam. Installation of the dams requires trained technicians and equipment and can be far more difficult in moving water applications than slow or still water projects. Special patch kits and repair techniques are used in the event the dams are punctured during the installation or in application. Figure 4 shows the modular capabilities of the portable cofferdams.
Design and engineering
This section summarizes the system stability and engineering needs of water-filled manufacturer’s dams. System stability is critical to the successful use and installation of portable cofferdams. The basic principle behind the water-filled dam system incorporates a dual inner tube system with a center baffle to stabilize the structure. The safety factor against sliding depends greatly on the coefficient of friction of the material that the dam is sitting on. In this case, where it is sitting on a wet grass surface, the coefficient of friction used is 0.20 (Noon 1994). The factor of safety against sliding is 1.33 when 24 inches (61 cm) of water is being held back.
As the water builds on one side of the dam, the inner tubes prevent rolling, and the dam behaves as a solid barrier.
Engineering designs with properly calculated safety factors are required based on individual manufactured systems and project-specific site conditions. Under hydrodynamic loads, the portable dams need to be designed to resist sliding, tipping and overturning. For the manufacturer’s dam to move as a result of the pressure exerted on one side, it must either be tipped over or slid across the surface on which it rests. In order to tip, the water pressure must lift the first inner tube up and over the second. The following calculation verifies the dam’s resistance to tipping.
The inner tubes are rectangular when filled to facilitate the calculations. The water level on one side will reach the top of the cofferdam to simulate the worst-case scenario.
The force exerted on the side of the water structure is defined as Equation 1.
Once the force on the side of the cofferdam has been determined, the tendency of the cofferdam to tip can be evaluated. Point A is assumed to be the pivot point and movements are calculated about this point. The movement created by each force is a measure of how much the force contributes to rotating the first column of water around point A. The two formulas (Equations 2 and 3) follow.
Simplifying the expression, it is observed that the stability of the dam is dependent on the relationship between its width, D, and the depth of water it must resist. The relationship above indicates the minimum width of the dam to prevent it from tipping when resisting water with a depth, h, equal to the height of the dam itself. The design height for the water structures to prevent tipping would be described as D > (.82) h.
To quantify the stability of the dam, the dimensions of the standard dam for D and h are substituted into Equation 1. The results are expressed in terms of a safety factor. The safety factor indicates how many times greater the water pressure or water depth must be in order to roll the dam.
Based on the current dam designs, the safety factor against tipping when water levels are to the top of the structure, as per Table 1, is:
Additional site-specific details and risk assessment must also be predetermined for each project.
This is done by completing a detailed site assessment prior to installation. Important criteria required for this assessment includes the water body type (lake, river, stream, ocean), anticipated water depth, historical water depths, water flow rates, freeboard level and ground surface conditions. Site-specific project assessment forms are required by the manufacturer for each project estimate.
Case study: Kilisut Harbor, Wash.
In 2019, the North Olympic Salmon Coalition and the Washington State Department of Transportation needed to restore the historic tidal channels and fish runs between southern Kilisut Harbor and Oak Bay in Jefferson County, Wash. This included removing the outdated culverts and installing a new elevated bridge, which replaced a causeway as part of creating 2,300 acres (931 ha) of productive fish habitat in the Puget Sound region. The first phase of this project included restoration of the channel on the north side of the highway in a sensitive marine environment. Figure 5 shows the cofferdams being deployed during tide out conditions.
As part of the new bridge construction, the project scope required the contractor to excavate and dewater a large channel in an environmentally sensitive region for the protection of salmon stocks and their migration. To address these environmental concerns, silts and sedimentation from construction had to be tightly controlled. Based on the project constraints, the engineer and contractor chose a temporary portable cofferdam. Layfield USA Corp.’s modular cofferdam system was chosen for the project. The project scope included the manufacturing and installation support of 1,400 feet (427 m) of multiple-sized dams of 4, 6, 8 and 12 feet (1.22, 1.83, 2.44 and 3.66 m) in height. The installation started in September 2019 with a crew of seven field technicians.
The project faced several unique challenges. One of the main challenges was working in tidal conditions that required the installation of the dams to only take place during low tide cycles. The contractor was also not allowed to operate equipment on most of the site in order to protect the pickle grass and wetlands area. This required many of the dams to be unrolled and positioned manually. With the multiple-sized dams, special fabricated collars were manufactured to connect the different sizes on-site.
The installation of the modular dams took seven days to complete and were drained and removed in January 2020 (Figure 6). The dams performed successfully, with the contractor being able to complete solids removal from the channel in dry conditions. This was a significantly faster method than if they had had to rely on a vacuum dredge. The use of the temporary cofferdams was a key component in providing an environmentally safe and successful installation. It also provided major cost savings to the contractor and project owner.
Civil engineers and contractors are increasingly using portable geosynthetic cofferdams to help control water in a variety of excavation and dewatering applications. While there are numerous systems available, each system should be individually reviewed and engineered for each application to reduce the risk of failure and ensure safe work conditions. When properly designed and installed, portable geosynthetic cofferdams provide numerous economic and environmental advantages when used to control water.
Brian Fraser, MBA, is vice president of Layfield Geosynthetics in Lakeside, Calif.
Mike Neal is senior project manager at Layfield USA in Eugene, Ore.
All figures courtesy of the authors.
Noon, R. K. (1994). Engineering analysis of vehicular accidents. CRC Press, Boca Raton, Fla.
Sati, R. (2011). “Case study: Performance of rapid respond flood control system during 2011 flood in Manitoba.” Proc., 2011 GeoManitoba, Winnipeg, Man., Canada.
Wikipedia contributors. (2020). “Cofferdams.” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/wiki/Cofferdam. Accessed Aug. 10, 2020.
SIDEBAR: Project Highlights
Kilisut Harbor, Wash., cofferdams
OWNER: Washington State Department of Transportation
LOCATION: Kilisut Harbor, Wash.
CONTRACTOR: Seton Construction
DESIGN ENGINEER: Cardno Inc.
GEOSYNTHETICS PRODUCT: Aqua Dam Cofferdam
GEOSYNTHETICS MANUFACTURER: Layfield