U.S. Army Corps of Engineers’ contractors recently began construction of the East Branch Dam Safety Initiative cutoff wall.
The construction started with the installation of the demonstration section to prove that their repair means and methods will result in an acceptable wall.
The cutoff wall will be constructed in 10- to 30-foot segments along the axis of the dam. For each segment, the first step is to excavate through the dam to the designed bottom level of the cutoff wall. Given the great depth (up to 242 feet) to which the cutoff wall will be constructed, conventional excavation equipment cannot be used.
The primary excavation equipment being used during construction of the cutoff wall is the hydromill. This piece of equipment utilizes a series of cutting wheels to drill down, with a return pipe and pump situated directly above for material transport.
As the hydromill begins digging into the dam, the trench excavation is simultaneously filled with a slurry of water and suspended clay. This slurry protects the embankment from collapsing into the excavation by pushing against the trench faces.
The hydromill plays a key role here also, as cutting wheels spin to help direct the slurry mix, which contains the milled soil and rock from the bottom of the excavation, into a pump situated directly above.
The slurry mix is pumped through a pipe to de-sander machinery, which separates most of the soil and rock fragments from the slurry. Finally, the cleaned slurry is pumped back into the excavation.
As material flows through the de-sander, soil and rock fragments drop into an adjacent pit. After the completion, the collected material in the de-sander is loaded on trucks and transported off-site for final disposal.
When a segment is fully excavated, it is filled with concrete from bottom to top before excavation can begin at another segment.
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