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District finds opportunity during drought, completes repair work at Minneapolis lock

Published Feb. 11, 2013
District maintenance and repair employees place a temporary dam, or cofferdam, on the Lower St. Anthony Falls Lock and Dam in Minneapolis during repairs in the fall of 2012.

District maintenance and repair employees place a temporary dam, or cofferdam, on the Lower St. Anthony Falls Lock and Dam in Minneapolis during repairs in the fall of 2012.

Drought isn’t all bad. Low flows on the Upper Mississippi River provided the district an opportunity to make concrete repairs at Lower Saint Anthony Falls Lock and Dam in Minneapolis.

Jacob Fall, engineering and construction, said the district has been waiting years for river flows low enough to safely divert water at Lower Saint Anthony Falls from the dam to the attached hydropower facility.

He said during the 2005 periodic inspection, Corps divers discovered the concrete sill in all three of the dam gate bays had severe erosion at their monolith joints. "We were concerned, because Lower Saint Anthony Falls is built on a highly erodible foundation," he said. "If that were to be compromised, there’s a good chance that piping could occur in the foundation." Piping, he explained, is when water flows underneath the foundation eroding the foundation’s material. It could result in failure. He added that this happened to the Northern States Power, or NSP, power plant at an adjacent site in the 1980s.

In addition to being highly erodible, Fall said the geometry of this location is unique. In order to dewater the three dam gate bays, he said, staff from engineering and maintenance and repair needed to design and build a dewatering box, which they did with materials the district already had on hand. "We were trying to be economical," he said, "and it [the dewatering box] worked really well."

After designing, building and installing the dewatering box, closer inspection of the concrete revealed that although there was severely eroded concrete at all monolith joints, no water stops within the structure were compromised. Their next steps included prepping the eroded surface for placement, replacing reinforcement and then, finally, placing and curing the concrete. An in-house maintenance and repair crew, specifically Mike Gunderson and Josh Rye, both in operations, completed the work, which took around six weeks.

Because the work location was close to the district headquarters, Fall said, it enabled engineering staff the opportunity to go out and view the work and offer technical advice more often than with other projects. "Working together so closely was a great learning experience for both of us," he said.