Seminoe Concrete Repair

Written by: Jay Dallman, WYAO

A concrete repair and stop log replacement project was recently completed at Seminoe Dam with Great Plains Region Construction Services provided contract administration and inspection for the project.

The river outlet works and stilling basin at Seminoe Power Plant was identified in a Comprehensive Review to have deficiencies in the concrete located around the outlet works and the tailrace deck. The stop logs were constructed as part of the original Seminoe Dam and Power Plant in 1939. Over 75 years of wear and tear took its toll. They no longer provided an adequate seal to allow periodic maintenance of the power plant draft tubes and river outlet gates.

The stop logs function as massive steel slide gates, each composed of an upper and lower section that fit into vertical tracks. Each lower section weighs about 6,500 pounds, and the upper sections weigh about 3,000 pounds. They were designed so each could block one of the two river outlet gate openings, or both could be used to block the twin outlets from any one of the three generator draft tubes.

The tailrace deck also had deteriorated concrete and the stairway leading to the tailrace deck did not meet OSHA standards. Engineers from TSC analyzed the problem issues and came up with a design to address the repairs.

The specification was issued by Great Plains Region Acquisition Services on July 17, 2015. The contract was awarded to the low bidder, RSCI (Record Steel and Construction, Inc.), from Boise, Idaho.

The contractor mobilized on February 8, 2016, beginning with construction of a cofferdam to protect the work site from flows passing through Seminoe Power Plant.

A subcontractor, Global Infrastructure, based in Griffith, Indiana, provided diving services for installation of the cofferdam in the stilling basin of the outlet works. Contractor used a 60 ton Link Belt crane to put the components of the cofferdam in place. The dive subcontractor anchored the cofferdam to the training wall and retaining wall.

Upon completion of the cofferdam, the work area between the upstream face of the dam and the cofferdam was dewatered using a 4-inch pump. The contractor cut around the concrete areas requiring repair. The repair areas were excavated using chipping hammers to the required design depth. New steel reinforcement and concrete was placed in the repair zones. Cracks were sealed using a pressurized epoxy grout system. The contractor completed all of the concrete repairs in the tailrace area protected by the cofferdam by March 23. The contractor removed the cofferdam from the river by April 6, to allow for potential water releases through the river outlet works.

The tailrace deck was resurfaced by saw cutting the surface, using jack hammers to remove deteriorated concrete, and then adding a new layer of fresh concrete. Work on the tailrace deck concrete was completed by May 3.

The repairs allow the power plant crew safely perform future maintenance on the jet flow gates and the generating units. The new stop logs minimize leakage and ensure that the maintenance can be accomplished safely and more efficiently. The new tailrace deck, stairway and associated security fence will provide safer access for operation and maintenance of the stop logs and outlet works. The Contractor will test the new stop log sections in the fall following the irrigation water delivery season.

(Left) Injecting epoxy to repair crack. (Center) Concrete removal on tailrace deck. (Right) Installing new stairway.

New reinforcing bar in river outlet works center pier.

Photo at right shows the pier after reconstruction is complete.

Published on May 30, 2016