Cofferdams

What is a cofferdam?

A cofferdam is a temporary dike built across a body of water, and constructed to allow the water to be pumped out of the enclosed area. Cofferdams are used in order to allow for a dry working environment. Once a cofferdam is in place, crews are able to do things like excavation, repairs, and pouring concrete.

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TrapBags used as a cofferdam

What are the different types of cofferdams?

There is a wide variety of cofferdam design that can be used based on the project requirements. These different types of cofferdams come in different sizes, materials, and configurations.

Types of cofferdams include: 

  1. Braced
  2. Cellular
  3. Double-Walled Sheet Pile
  4. Earthen
  5. Rockfill
  6. Single-Walled Sheet Pile

Braced Cofferdams

Pro: Braced cofferdams are good for bridge pier and abutment repairs.

Con: A confined space work conditions.

Braced cofferdams can be used when it’s difficult to install piles into the bed in the water. In braced cofferdams, two piles are inserted and laterally supported by wooden cribs. The wooden cribs are installed alternately in order to create pockets.

These pockets can be filled with rock and dirt. The frame of the cofferdam is made out of wood and built on dry ground, prior to being installed in the water.

Cellular Cofferdams

Pro: Cellular Cofferdams offer a large work area.

Con: They are usually left there permanently.

Cellular cofferdams are used when the water level is too high for other cofferdams to work efficiently. Cellular cofferdams are created by installing straight web steel sheet piles in an interconnected cellular structure. These cells can be custom-configured in order to fit the project. These cells can be filled with things like sand, gravel, or clay.

There are two main types of cellular cofferdam. 

  1. Circular
    1. Circular cellular cofferdams are composed of circular arcs that are connected by straight diaphragm walls. The cofferdam is made out of interconnected steel sheet piles and filled with non-pervious materials such as sand. 
  2. Diaphragm
    1. Diaphragm cellular cofferdams are composed of larger circular cells which are connected by arcs of smaller cells. The walls of the smaller cells are perpendicular to the walls of the larger cells. These cells can be filled independently. 

TrapBag® cofferdams are cellular cofferdams. TrapBag® is a series of pentagon-shaped bags that are sloped on one side, vertical on the opposite side and open at the top for filling. Each of the cells are connected side by side like an accordion, each cell has a common wall with the next cell, and are collapsed during storage and deployment. The cells are made of high-strength textile. Each of the cells are self-contained yet rely on the next cell for added strength. If one of the cells is compromised, it will not affect the rest of the barrier, which will remain standing strong.

TrapBag® Pro: There is no ground penetration required for installation, thereby minimizing subsurface risk (environmental, cost, and schedule) usually associated with sheet piling, also much less expensive. TrapBags® can be individually removed after use without material discharge into the environment.

Double-Walled Cofferdams

Pro: Double-Walled Cofferdams offer increased support compared to single-wall, used for large sites and high depth of water.

Con: These types of cofferdams are more expensive.

Double-walled cofferdams work well for projects with a larger site and deeper water. Single-walled cofferdams are not efficient in this situation. A double wall helps add stability to the cofferdam, allowing it to hold deeper water.

Earthen Cofferdams

Pro: Earthen cofferdams use materials already on site.

Con: They can be installed when water is low and current velocity is low.

Earthen cofferdams are made of natural and local materials like dirt or sand. They are not intended to hold deep water or withstand high velocity.

Rockfill Cofferdams

Pro: Rockfill cofferdams work better than earthen cofferdams and make use of rocks already on site.

Con: These cofferdams are very pervious.

Rockfill cofferdams are similar to earthen cofferdams, but instead of sand or soil, rock is used. These cofferdams are created when there is rock available onsite. Rockfill cofferdams are, however, quite permeable and must be filled with a soil membrane.

Single-Walled Cofferdams

Pro: Single-walled cofferdams can be used for jobs with higher water depths.

Con: These types of cofferdams are more expensive.

Single-walled cofferdams work in areas with moderate water depth (6+ meters) which have smaller sites. An example of this is bridge construction. These cofferdams are built with wood sheets along the perimeter and supported by steel sheets on the interior. Sandbags add additional support to the cofferdam.

Each cofferdam variety has advantages and disadvantages. The project location, budget, and site limitations should be considered before installing a specific type of cofferdam.

How are cofferdams built?

Cofferdams are built by creating a watertight enclosure from which water is pumped to expose the bed of a body of water in order to begin construction work.

Traditional cofferdams are built by driving sheet piling — typically made of steel — into a sea floor or lakebed to create a watertight dam. These vertical piles are kept in their places by horizontal framing constructed by wood, steel, or both. Cofferdams must have strong sides in order to withstand high velocity when the water is pumped out. This process can be lengthy and costly. 

TrapBags®, conversely are very affordable, quick to install, and proven to withstand intense hydraulic pressure.

Where are cofferdams used?

Cofferdams are used in construction sites within water. These sites include bridge construction sites, pier construction sites, subsurface system construction sites, and more. Different types of cofferdams are used based on the type of project, size of the construction site, and the depth of the water.

How do you seal off a cofferdam?

When an  “engineer determines it is impossible or inadvisable to dewater an excavation prior to pouring concrete” (PileBuck.com) seal courses are required.  A seal course works to seal the bottom of a cofferdam in order to prevent subsurface water from seeping in. Seal courses can also help control soil expansion. To seal off a cofferdam, a concrete slab is placed underwater. The slab must be thick enough to resist being uplifted by hydrostatic pressure.

How are TrapBags® used to create cofferdams?

To create a cofferdam using TrapBags® you simply pull the chain of cells apart and place them where you TrapBags’® flexible design allows you to create many different cofferdam configurations ensuring you have enough work space.  

A TrapBag® barrier cofferdam can be installed at a fraction of the cost and time of conventional solutions like a steel sheet pile wall. Up to certain water levels, the TrapBag® barrier can even be installed in the water. 

Whether you need a short barrier or a tall one, we have you covered with bags in multiple sizes that, like traditional small to large sandbags, can be stacked on top of each other to give you endless protection options. TrapBag® barriers are filled with sand, gravel, or concrete.

TrapBag® will save you time & money! TrapBag® is versatile and may be used for other kinds of protection from security barriers to storm-water control. 

You can't always predict,
but you can always be prepared.

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