This project included raising the height of an existing concrete arch dam by 83 feet (25m) to an ultimate height of 225 feet (68m) and increasing capacity to 15.9 MW, thereby increasing Sitka’s total hydroelectric energy production by 27%. Completion of this project allowed Sitka to reduce its dependence on imported diesel as a backup fuel source. This area of Alaska receives three times the precipitation of Seattle (one of the rainiest cities in the U.S.). Therefore, increased capacity of the hydropower dam was the answer to maintain their energy independence.
McMillen Jacobs provided construction management services, engineering support during construction, and final design of specific elements to the City of Sitka. We provided the full field construction management team, office engineering team, QA/QC, FERC coordination support, contractor bid evaluation, and regulatory agency coordination support. Our engineering support and coordination for critical construction activities included dam and powerhouse foundation excavation, dam foundation grouting, tunnel and shaft support, and access roads.
The team executed challenging work on the underground and tunnel portions of work. Scope of work included excavation of three new tunnels and two raises using drill and blast methods. Examples include:
- One 140’ long (8-foot-diameter) exploratory tunnel located 200’ from the base of the existing Blue Lake Dam.
- One 470’ long (12-foot-diameter) adit tunnel to facilitate driving a 350’ tall MRC raise, (surge shaft) to a remote breakthrough location accessible only by helicopter or by foot along a separate walking trail.
- One 850’ long (12-foot-diameter) intake tunnel from a sheer cliff 200’ from the existing Blue Lake Dam and 50’ above the city of Sitka’s drinking water supply.
- Two tunnel bulkhead plugs sealing off an abandoned section of the existing power tunnel and isolating a surge shaft adit from the pressurized power tunnel.
- Access hatch in the adit bulkhead through the bottom center of the plug.
- Excavation of over 2,500 cubic yards of rock using a small surface drill and mini excavator to establish the surge shaft breakthrough location.
- Excavation of a 100’ tall MRC raise from the new intake tunnel, breaking through to surface, which serves as a gate shaft for a new intake gate.
- Removal of surge shaft and intake tunnel rock plugs.
Additional design services from McMillen Jacobs included:
- Review of blasting and excavation plans; review of dewatering plans; and overall quality control and assurance.
- Rock support design for a 200-foot-deep (61m) rock cut for the intake tunnel portal.
- Direction for the placement of rock support in the shaft and tunnels
- Designed the concrete plugs that seal the pressurized sections of tunnel, and performed geologic mapping and documentation.
- Rock reinforcement and canopy access structures at the tunnel portals.
- Design details for the underground and powerhouse excavation for site specific rock stability issues
- Coordination and inspection of dam foundation excavation and grouting.
- Updates to the electrical design package for the entire project complete with SCADA.
- Design of new access roads to the powerhouse and the reservoir.
- Design of portal and slope stability.
- Design of concrete tunnel plugs in the adit and intake tunnels.
- Value engineering to reduce construction costs.
- Cost estimating and benefit/cost analysis.
- Pre-bid constructability reviews.
- Coordination and inspection for installation of scour protection wall at the toe of the dam.
- Coordination and layout of drainage relief holes at the toe of the dam.
- In-field construction engineering including ground support during tunnel and shaft construction. Our team performed detailed inspection of the steel installation and directed annulus grouting for the lining.
- Tunnel portal cofferdam.
The McMillen Jacobs team also served as Construction Manager for the remaining installation and construction including the following elements:
- Construction of the new intake structure, intake tunnel, gate shaft, and surge shaft.
- Replacement of two 3 MW generators with three 5.3 MW generators increasing capacity to 15.9 MW.
- Preparation of procurement documents and assisting with bid review and award.
- New steel powerhouse with precast panel walls.
- Factory and field inspection and testing oversight including all electrical equipment including new powerhouse generators, motor control center, and standby generator.
- Remote/Limited Access
The dam site is located on Baranof Island in Southeast Alaska and is accessible only by air or water which created significant logistical challenges. Because all material and equipment had to be delivered by barge, delivery times could often be several weeks or longer. This required advanced planning to stay on schedule. Once the equipment and materials reached the project site, a considerable amount of effort remained to place it in the area needed. Access to the site was extremely limited as it is located within a narrow gorge with near-vertical walls which required continuous crane support. The crane alone required 40 truckloads up a narrow gravel road before it could be assembled at the site. The work area at the base of the dam was accessible only by the crane or by foot along a walking trail. Others were only accessible by foot trail or helicopter. Therefore, positioning employees and delivery of materials was accomplished via the crane—including concrete placement. Employees accessed the work areas via manbasket from the crane.
- Project Challenges
Seasonal constraints and the established milestones for power production required our team to meet or beat the stringent schedule at every phase. It was also a challenge to work around an operating reservoir. In addition, because Blue Lake provides the city’s municipal potable water supply, water quality needed to be maintained for the duration of the project.