Blue Lake Hydroelectric Expansion
Summary
In a progressive move toward energy independence, the City and Borough of Sitka expanded their hydropower facility. The expansion included raising the existing concrete arch dam; constructing a surge chamber, new intake structure, tunnel conveyance, and powerhouse; and modifying existing components because of higher water head. The project included underground construction, access road, portal and slope stability, dam foundation excavation and grouting, water conveyance tunnel support, and surge shaft excavation.
- Project increased the capacity of the existing hydropower facility by 27%
- The site location within a narrow gorge with near-vertical walls presented construction and logistical challenges
- A new intake structure and intake tunnel, driven on a steep decline, were constructed from behind a cofferdam next to the lake
Scope of Work
Delve Underground was a subconsultant on the construction management team leading the project’s geotechnical and rock engineering aspects. The scope included significant Federal Energy Regulatory Commission (FERC) coordination regarding tunnel and shaft construction, dam foundation excavation, and dam foundation grouting efforts. Delve Underground coordinated the inspection of the dam foundation grouting program.
Challenges & Innovations
Delve Underground evaluated rock kinematics to address the geologic challenges from the highly jointed rock and provided design support methodologies to address the structures encountered. Large cuts were needed, including a 200-foot-deep (61 m) rock cut for the intake tunnel portal. By providing in-field construction engineering, Delve Underground reacted quickly to minimize schedule impacts. Because of the increase in water pressure from the dam raise, portions of the existing tunnel with low rock cover were slip-lined with steel pipe, which was inserted in sections through a smaller opening in a folded state and then expanded in the tunnel.