Project: Center Hill Dam Safety Rehabilitation Project
Customer: U.S. Army Corps of Engineers
Contractor: Bauer Foundation Corporation - Odessa, FL
Project obstacles and solutions: In 2008, The Nashville District of the Army Corps of Engineers began a safety rehabilitation project for the Center Hill Dam. The 260’ high dam, originally constructed in 1948, had been seeping water, especially when the lake was at high levels. The Corps of Engineers found that the old dam was highly unstable, with a high percentage of loose and fractured rock making up the foundation. They came up with a four-step procedure to rebuild the dam.
In 2012, Bauer Foundation began the $106 million remediation work on the dam walls. This involved the construction of a new barrier wall and reconstructing the foundation of the columnar wall with secant piles. Bauer Foundation is known for their deep foundation drilling. The secant piles use a two step process. The primary piles were drilled at a depth of 186’ and filled with concrete. The secondary piles are placed inside the primary piles, and again filled with concrete.
The concrete for the piles was poured below the water level with a tremie pipe. The tremie method delivers the concrete about three feet below the surface of the concrete. This keeps the concrete from segregating like it would in a normal pour. Thus the water is pushed upward, away from the plastic concrete and will not wash away the fine aggregates and cement in the concrete. Special consideration is needed when placing tremie concrete. First, the slump has to be consistently in a 7.5-9.75 inch range to keep the concrete flowing down the pipe and spreading to the width of the forms. IMI had to deliver a continuous pour at a rate of 150 yards per hour so that there was no gap in the flow of concrete to the bottom of the tremie pipe. Any gap can cause a break in the seal and the tremie pipe would have to be raised to the surface and reset. IMI designed a mix that used fly ash as a pozzolan to increase the flowabililty of the concrete. Set times and concrete temperature were also a challenge. The concrete had to reach a strength of 1,000 psi in three days and 1,500 psi in seven days before the drills could move on to the next pile. The concrete had to remain plastic for 12 hours and it needed to be delivered at 80 degrees F due to the size of each pour. IMI used chilled water to mix in the concrete during the hottest days of summer. The mix was designed to achieve early strength without compromising the temperature.
IMI also aided in the drilling process in a most unusual way. In a construction project of this magnitude, there are several factors that are challenging on the job site. The drilling process removed massive amounts of drilled out waste materials which could not be simply discarded at the job site. In order to move such large amounts of discarded materials, Bauer mixed these with water to create a slurry that was pumped to a series of holding tanks. These were then processed with cement before being removed from the site. IMI was instrumental in removing the old materials. As each load of concrete was delivered for the piles, IMI mixer trucks were then loaded with the slurry, which was driven to a landfill for disposal.
This is the third of the four phases of the Center Hill Dam project. Beyond the secant piles, IMI supplied concrete for the processing plant foundations and walls, the tracks the drills rolled upon to move across the dam, and other retaining walls.