LEESVILLE REMEDIAL MODIFICATIONS for the PMF
Appalachian Power Company
Roanoke, VA
The Leesville Hydroelectric Project is located in Leesville, VA and consists of a 980 foot
long concrete gravity dam with integral powerhouse. The dam has a structural height of 91feet, to develop the site’s 72 feet
of gross head for two 20 MW turbines.
The Project, constructed in 1960, included foundation drains to reduce uplift pressures beneath the dam and powerhouse, and the construction drawings stated that the uplift pressure beneath the dam was applied over 50% of the base of the structure. The original design was based on a headpond level that was equal to the maximum normal pond (two feet below the top of the dam) but significantly less than the Probable Maximum Flood which would overtop the dam by fifteen feet. In addition, a foundation cohesion of 450 psi was assumed in the design. Based on the findings of the FERC’s dam safety inspection program, the original design criteria did not meet current standards and remediation was required to increase the dam’s factor of safety for the critical loading condition, the PMF.
A site specific PMF was performed by others, resulting in a seven foot reduction of the level of the PMF impoundment. Kleinschmidt recommended the removal of a 3.5-foot high parapet flood wall from along the top of the dam, further reducing the level of the PMF impoundment by over one foot, resulting in a 5% reduction of the hydrostatic loads on the dam. Forty seven tendon type rock anchors were installed to increase the dam’s factor of safety. The anchors had a design load that varied from 493,000 lbs to 1,514,000 lbs (average: 1,135,000 lbs), and varied in length from 84 to 126 feet. The anchors were fully bonded and encased in a seamless corrugated sheath that extended from the tip of the anchor to the anchor head, eliminating the need to include provisions for long term monitoring of the anchor loads. While cost to stabilization the dam was $2.5 M, the site specific PMF and removal of the parapet wall reduced the remediation costs by 67%.
Other work performed to ensure the dam’s stability during the PMF included: installation of flood proof bulkheads on three vertical access shafts and on the single additional entrance into the drainage gallery, and sealing all openings and power conduits into the drainage gallery to prevent flooding of the gallery and allow the drains to remain functional due the PMF event; and to relocate the standby generator for the drainage system dewatering pumps to an elevation that ensures that the power supply would be accessible and would not be flooded during the PMF.
For more information contact Paul Cyr, P.E.