The Bath County Pumped Storage project, owned jointly by Virginia Power and Allegheny Generating Company, is located 70 miles (112 km) north of Roanoke, virginia, USA. It is a six unit, pump-generating station rated at 2100 MW, the largest pumped storage project in the world. All six units were declared commercial in December 1985, and the plant has operated continuously since that date.
Initial filling of one of the three power tunnels produced unacceptable amounts of leakage from the tunnel. In addition, seepage pressures buckled and ruptured a short Section of one of the steel liners in an unwatered penstock adjacent to one that was filled. The remedial treatment program that followed, included an extensive, high pressure tunnel grouting program, and an expanded drainage and monitoring program.
During the second filling, the three power tunnels were filled simultaneously. Piezometer readings and. penstock drainage tunnel flow rates generally increased with the reservoir level. However combined leakage from the three tunnels was significantly less than the leakage from a single tunnel had been during the initial filling; piezometric pressures,.though higher generally, rose in a predictable manner. On two occasions, however, piezometric increases adjacent to the upstream ends of the steel penstocks and Within the surrounding rock mass prompted additional drain hole and relief well drilling.
The remedial program consisting of grouting and drainage has been effective in reducing tunnel leakage and the high pressures resulting from it. Pressure migration has been controlled. A maintenance program of flushing and cleaning drainholes has been successful in maintaining the low flows and reduced pressures.
The upper and lower reservoirs are Contained by embankment dams (Figure 1). The upper reservoir dam is approximately 480 feet (145 m) high; the lower dam is approximately 150 feet (47 m) high. Water is conveyed through three, 28.5 foot (8.7 m) diameter, concrete lined power Conduits. Each has an upper and lower horizontal segment approximately 3100 to 3600 feet (966–1122 m) long, connected midway by a 990-foot (302 m) vertical shaft. Each lower tunnel bifurcates into two, reinforced concrete and steel-lined Penstocks, 18 feet (5.5 m) in diameter and 900 to 1260 feet (280–392 m) long. The Penstocks lead to the powerstation located On the west bank of the lower reservoir. Maximum static pressure within the tunnels at the upstream ends of the steel lined Penstocks is about 1320 feet (402 m).
The Bath County Project is located at the northwestern margin of the Appalachian Fold Belt of western Virginia. Rock units in the project area are predominately thinly bedded siltstone and shale of the Millboro/Brallier formation, and blocky sandstone and siltstone of the overlying Chemung formation. Both units are Devonian (Figure 2). The structural trend is northeast southwest, paralleling the trend of the Appalachian Mountains. Rock units are buckled in tight chevron folds in the vicinity of the powerstation and penstocks, but strata in the remainder of the tunnel route generally dip at very low angles (5–10 degrees) to the northwest.