By Mark Crawford
Located along the Kentucky River just outside Lexington, Ky., Lock and Dam No. 9 was constructed from 1901 to 1903. Completion of the structure resulted in a 19-mile-long body of water that has provided drinking water to the city of Lexington for more than a century. The dam has been renovated several times over its history, most recently from 1993 to 1995.
Due to steady population growth and the continued deterioration of the existing structures, the Kentucky River Authority decided to renovate the 14 locks and dams along the river. The goal was to increase longevity and storage volume and meet projected long-term water supply needs for the region.
The reconstruction of Lock and Dam No. 9 was performed from 2007 to 2010, and precast concrete was an important part of the work. C.J. Mahan of Columbus, Ohio, served as the general contractor on the $15-million project and Oldcastle Precast of Lexington manufactured the precast concrete.
Precast meets the challenge
Plans called for replacing the existing dam and raising the pool elevation. Stantec Consulting Services of Lexington conducted a flood study of the river and a structural analysis of the lock and dam components. After reviewing the data, the company recommended building a new dam immediately upstream from the existing structure. The new dam design consisted of eight steel sheet pile circular cells with 7-inch steel sheet pile connector cells. The cells were filled with concrete produced by a batch plant set up near the site.
Each cylinder was 52 feet in diameter and built directly in front of the old Lock and Dam No. 9. Officials decided not to build a lock in the new dam because of the cost. However, a cylinder was placed directly in front of the old lock. It can be removed and replaced with a new lock should the need arise.
The increased fall distance in the emergency spillway created concerns about erosion of both the spillway bank and the base of the dam itself.
“Structural solutions included scour-and-slope protections and a conveyance system to transfer water from upper to lower pools in times of drought,” said Daniel Gilbert, the Stantec Consulting Services resident engineer on the project.
Due to the challenges of the river site – which frequently floods – precast concrete elements were selected to reinforce the areas of concern. In-the-wet placement, which consisted of placing precast in the wet river areas, was used where possible to reduce costs, risks, project duration and environmental impacts.
“We used a slope block and a toe block,” Gilbert said. “Precast blocks were used to armor the slope downstream of the dam and provide toe protection at the new dam to armor against scour from the new dam structure.”
The blocks were designed to be interlocking, in part because of equipment restrictions on the river. Smaller modular precast pieces were interlocked together to create the armoring system. Tight tolerances were required for both in-the-wet and in-the-dry placements. The interlocking nature of the blocks, however, provided some extra ability to adjust and accommodate existing conditions.
“The use of these elements provided the mass and protection against scour in an economical manner,” Gilbert added. “The precast toe block elements were installed and then concreted in place to further lock them together, which was a highly effective solution.”
A team effort
According to Jeff McKinley, general manager of the Oldcastle Precast Lexington plant, Oldcastle worked with the Army Corps of Engineers to develop the block design. Kay and Kay Contracting in London, Ky., fabricated special molds to create the custom interlocking shapes designed by the project engineers. The project required 113 precast pieces weighing 26,000 pounds each – a total of 1,500 tons.
“A key consideration in designing the molds was being certain that the pick points were located correctly so that when the blocks were lifted and set into place they were balanced and level,” McKinley said. “Also, because of tight access conditions, the blocks were designed to maximize delivery on flatbed trucks, so overwide permits would not be needed.”
Upon completion of the new dam, portions of the existing lock and dam were demolished. The lock gates were removed and preserved on site for their historical significance.
Mark Crawford is a Madison, Wis.-based freelance writer who specializes in science, technology and manufacturing.