The dam is a 340-foot-high double curvature concrete dam with the underground powerhouse generating over 1,000 megawatts of electricity and, at certain times of the year, providing as much as 60% of Seattle’s power. Schnabel performed the 2015 FERC Part 12D dam safety inspection of the facility. The work also included a comprehensive reevaluation of the PFMA.

It was noted during the workshop that the project records did not include a comprehensive understanding of the arch dam’s foundation. We subsequently performed a detailed review of published geologic information, construction photographs, and existing borings and performed additional geological field work.

All of this data was used to develop a 3D geologic model of the site that was focused on the right abutment. The model confirmed the presence of a susceptible failure plane in the rock supporting the level six access bridge below the dam. We designed permanent stabilization measures including cable lashing and post-tensioned anchors to address the concern. A focused spillway inspection and PFMA were performed in 2018 for both radial gate spillway chutes.

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