Concrete, WA

Lower Baker Dam Unit 4 Powerhouse

Puget Sound Energy


To maximize use of water resources for power generation, Puget Sound Energy (PSE) built an additional powerhouse at the Lower Baker Dam. The project consists of new construction of an approximately 1,000-foot-long (305 m), 12-foot (3.7 m) internal diameter power tunnel, an underground connection to an existing power tunnel, and a new 30-megawatt (MW) powerhouse along the east shoreline of the Baker River, downstream of the Lower Baker Dam — a 285-foot-high (87 m) concrete arch structure. 

The Baker River originates in the North Cascades and flows southward along the east flank of Mount Baker to join the Skagit River at Concrete, Washington. The dam and existing power tunnel were constructed in 1925 to generate hydroelectric power for northwestern Washington and provide flood-control storage for the Skagit Valley. 

Fast Facts
  • Design-Build
  • 30-megawatt single unit powerhouse
  • 1,000-foot-long (305 m), 12-foot (3.7 m) internal diameter new tunnel
  • New tunnel connects to existing power tunnel at base of existing surge shaft
  • Landslide mitigation system to protect the new powerhouse

Scope of Work

Delve Underground performed the preliminary design of the new tunnel and powerhouse excavation. The scope of work also included geologic mapping and drilling explorations to characterize the rock mass for tunnel and powerhouse design, preparation of design-build plans and performance specifications, preparation of a geotechnical baseline report, cost estimating, and review of design-build proposals.

During construction, Delve Underground performed construction management services, including developing the quality control inspection plan, and construction inspection of the powerhouse excavation, tunnel construction, site civil, and augmented PSE construction management team.

Challenges & Innovations

The new powerhouse site is located south of the existing Unit 3 Powerhouse, along the east shoreline of the Baker River at the toe of the canyon wall. Shortly after construction started, slope instruments indicated the initiation of slope movements immediately upslope of the new powerhouse. Slope movements continued and accelerated through the winter and spring as the powerhouse excavation shoring elements were installed. Delve Underground quickly designed a landslide mitigation system to buttress the slope and arrest movement. The mitigation consisted of an anchored soldier pile and lagging wall designed under a very tight timeline, which minimized project delays.