The proposed Gorge 2nd hydropower tunnel (G2T) was designed through approximately 11,000 feet (3,353 m) of solid rock between Gorge Powerhouse and the intake at Gorge Dam on the Skagit River. Once constructed, it will parallel the existing tunnel, increasing generating efficiency by reducing head loss.
Gorge Powerhouse on the Skagit River began powering Seattle, Washington, in 1924. The original power tunnel, still in service, conveys water to three turbine units. When SCL added a fourth generator and built a higher dam in the 1950s, the new loads increased head losses in the existing tunnel and kept the plant from reaching its generation capacity. Consequently, Seattle’s electric utility began considering a second parallel tunnel to optimize the Gorge Powerhouse facilities.
The G2T will be a largely unlined, 22- to 24-foot-diameter (6.7 to 7.3 m) tunnel bored through hard rock with connections to the existing power tunnel upstream of the Gorge Powerhouse and downstream of the intake at the Gorge Dam.
Delve Underground led the preliminary design, including an exploration program, tunnel diameter optimization and layout studies, hydraulic modeling, and transient studies, which identified recommended alignment, connection configurations, and other important project features incorporated into construction cost estimates for various tunnel sizes. Final design, also led by Delve Underground, resulted in the development of Contract Documents comprising plans and specifications, and an engineer’s cost estimate for the recommended tunnel diameter and alignment.
Other project team tasks included a complete inspection and mapping of the existing tunnel during a planned outage, energy model/revenue evaluation, and assistance with the Federal Energy Regulatory Commission (FERC) License Amendment application.
A unique feature of the exploration program is that two of the boreholes were drilled 1,200 feet (366 m) inside the existing Devil’s Elbow Adit, a midpoint adit that was used to construct the existing power tunnel. These boreholes extended horizontally about 400 feet (122 m) in either direction along the new tunnel alignment. Ventilation, air quality monitoring, special illumination, and on-site safety teams were required for these borings.
During construction of the G2T, the existing power tunnel will be in operation (i.e., it will be pressurized). The separation between the two tunnels must be large enough so that construction of the G2T does not impact the integrity of the existing power tunnel. Stress field interaction between the two tunnels was evaluated, and additional evaluations using numerical modeling were performed during final design to verify that this separation will be adequate and to assess potential impacts of leakage from the existing power tunnel into the G2T excavation.