Braced Cofferdam

A cofferdam is a rectangular enclosure of earth retention. Most often cofferdams are used in water for setting of bridge piers. The main features of cofferdams are that they are closed and use internal means to provide bracing. For example, in the Soldier Pile Wall post, when the depth became too great for a cantilevered wall to work anchors were installed. You can't use anchors if you are working below water, otherwise the hole in the wall for the anchors floods the workspace.

Another reason to use cofferdams is that sometimes you can put a wale and use it as a ring beam transferring the forces around the beam until they meet the forces from the other side and balance each other out. This project shown here used that approach. Even though they were not below a water table the cost of the wale was cheaper and faster than an equivalent anchor soldier pile excavation.




As the size of the cofferdam gets bigger the ring beam needs to be braced. The picture above shows corner braces, the diagonal braces. This is the first step in bracing the wale. If the cofferdam were bigger an internal strut would be required. The corner braces reduce the span of the ring beams and don't impact the work space too much. Internal struts on the other hand run the width of the cofferdam and are usually placed right in the center, so they tend to have a major impact on the construction operation.

Here is another picture that might help to get a sense of the scale of the depth of the cofferdam shown above:
The other advantage of using Soldier Piles for this job is that you can allow the sewer piping to penetrate the cofferdam by just removing the lagging at the one spot. Had this been done with sheet piles it would have been a lot more troublesome.

So usually the cofferdams are kept small, like this one:



Wenzel Engineering, Inc