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

Soldier Pile Wall

A Soldier Pile Wall is a type of retaining wall mostly used for construction. These walls are usually only in place until the below grade portion of a building can be backfilled. A typical project is shown here.

This project is on the U of MN campus in Minneapolis. The project is to tear down an existing building and build a new one in its place. In order to place the footings and the basement portions of the new building Wenzel Engineering had to design a soldier pile wall for excavations of up to 40'.

Here is a shot of the demo, just for fun (below).

This is at the shorter end of the wall. The soldier piles are just to the left of that big yellow box. You can see at this site not all of the structure was demoed, the wall on the right was saved to be re-used on the new building.


This is at the deeper section (below). The soldier piles are the steel beams that are driven like piles. The soil on the front side of them is excavated from top down and as the soil is excavated lagging boards are attached to contain the soil.

When the excavation reaches certain depths anchors are needed. The anchors are attached to the piles by wales. The wales are the horizontal channels in this picture.

Bridge Demolition

The Rock Island Bridge was constructed in 1895 to span the Mississippi. It is a draw span bridge where the entire bridge pivots about the center to open and close it for the water traffic.


The upper level was for train traffic and lower level was originally for electric street rail eventually this level was used for car traffic.

Here's a close up:
The bridge need to be removed due to extensive deterioration.



Wenzel Engineering laid out a plan to get it done.




Wenzel Engineering Inc.
www.wenzelengineering.com