The Boundary Dam Complex produces more than 50 percent of Seattle’s electricity. Its transmission lines — which exit through transformer bays at the bottom of a 500-foot cliff — had in recent history received damage from rockfall. To keep the problem from worsening, Seattle City Light put $20 million towards finding a long-lasting solution.
Seattle City Light sought to protect or relocate the complex’s transmission lines to reduce the potential for rockfall and subsequent power outages. Kleinfelder would need to safely evaluate a 500-foot rock slope face and its surrounding area in order to develop feasible mitigation options for Seattle City Light to move forward in design. Because of magnetism from the transmission lines, normal rock mapping couldn’t be completed with compasses.
Kleinfelder completed slope mapping inside the transformer area away from the power lines. Additionally, Kleinfelder employed a subcontractor to complete a photogrammetry scan of the fractures and rock blocks in the 500-foot slope face, then correlated the underground mapping to the photogrammetry. Using this information, the team evaluated rockfall mitigation concepts based on feasibility and constructability, construction and maintenance costs, construction and long-term risks, long-term benefits, design requirements, power outage lengths, environmental impacts, and permitting requirements.
Kleinfelder’s team developed four mitigation options—ranging from rock anchors and netting to rock deflection sheds to a new tunnel—for long-term protection of Seattle City Light’s transmission lines, all of which would reduce the potential for damage and power outages.