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September 22, 2013

Duck River Dam Project skyrockets to $110 million; up $40 million from original estimate

(Continued)

The ‘clay’ problem

Once major excavation work began earlier this year, engineers say they started to notice unexpected clay layers slicing through the bedrock of the proposed dam foundation.

When his team first started to look at the redesign after discovering the site issues, Newton said the problems came close to potentially derailing the entire project.

“At one point, there was no project,” he said. “So, we sat down and came up with a dam that will fit that site.”

CH2M Hill was working off core and soil borings taken by the U.S. Army Corps. of Engineers in the late 1990s, which had not revealed the clay layers at the site. Officials say they could not have known about the issues until the excavation work was complete, as the Corps.’s samples reportedly did not indicate there were problems with the site.

“What we found was very different than what we were expecting,” CH2M Hill engineer Steve Newton said. “It’s not something that has anything to do with who is doing the work, or a reflection of the job done by the Corps. of Engineers. It’s just a case of dealing with what nature has given us.”

By slicing through different areas of the foundation area, Harwell said the clay deposits could compromise the stability of the dam, which is why the design had to be fully reworked.

“Those clay layers go through different layers of the rock, and that’s problematic for the dam because the stability is not there,” he said. “It’s also not practical to just dig those out, because there are so many different spots. So, that’s why you have roller-compacted concrete in the middle, where the better foundation is available.”

The reworked design also accounts for additional grouting work to create additional stability in the foundation area.

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