Blue Ridge Reservoir

This project for the Tennessee Valley Authority included a new discharge tunnel that provided the Blue Ridge Reservoir, located on the Toccoa River in north Georgia, with an alternate facility to regulate reservoir elevations. Jacobs Associates was Lead Designer on this design-build project that constructed new intake and outlet structures, and a 1,000-foot-long (305 m) tunnel running under the abutment of the existing dam. The steel-lined tunnel has a 9-foot (2.7 m) finished diameter and was 12 feet (3.7 m) in excavated diameter. It was constructed primarily through moderately weathered metamorphic rock using drill-and-blast methods. Primary support consisted of rock bolts and steel sets with shotcrete, as required.

A number of creative approaches were employed to deal with environmental issues and to keep the project within budget and on schedule. The design was developed to minimize turbidity within the reservoir during construction by limiting the excavation to the area within a small cofferdam. Finite difference seepage models were used both in design and during construction as ground conditions were revealed by inlet excavation. These models were used to estimate the cut-off provided by the rock and to verify the uplift forces on the 35 x 46-foot (10.6 x 14.0 m) steel cofferdam tremie slab. In addition, four pressure monitoring wells were installed through the concrete slab to continuously monitor pressures during the changing shaft excavation sequences and increasing lake water level. Relief pressure holes were drilled to relieve pressures below the tremie slab acceptable levels. These measures verified design assumptions and allowed the project to remain on schedule. Construction was completed in 2004.