February 16, 2001August 2, 2026

Comments on Seattle Tacoma International Airport Project Third Runway – Embankment Fill and West MSE Wall, and Industrial Wastewater System Lagoon #3 Expansion Project on Second Public Notice

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GeoSyntec Consultants, retained by the Airport Communities Coalition, submitted technical comments to the U.S. Army Corps of Engineers and Washington State Department of Ecology regarding the proposed Third Runway Expansion Project at Seattle-Tacoma International Airport. The letter identifies significant deficiencies in the field investigation, laboratory analysis, and seismic and static analyses related to the embankment fill and West Mechanically Stabilized Earth (MSE) Wall. GeoSyntec concludes that the project documents do not provide a sufficient basis for approval, citing concerns about weak peat and liquefiable sands beneath the wall footprint in a seismically sensitive area.

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As the source for federal transportation grants, the Puget Sound Regional Council was charged with developing a system to meet the growing need for commercial aviation. When the search for a second airport failed, they authorized the Port of Seattle to build the Third Runway, with a mandate to develop a noise abatement and mitigation program. In their effort to stop the Third Runway, the ACC argued over every detail of the Port's efforts--including property buyouts and sound insulation. The dispute was meant to be settled by a three member Expert Arbitration Panel. This is their final report. It finds 2-1, that the Port's program was insufficient in several respects. Despite that, funding for the Third Runway was approved by the PSRC, and the 'Port Package' program, proceeded largely unchanged. Expert Arbitration Panel's final decision finding that the Port of Seattle had not shown sufficient reduction in real on-the-ground noise impacts to satisfy noise reduction conditions required for approval of a third runway at Sea-Tac International Airport. The majority decision concluded the Port's noise abatement programs were insufficient despite being impressive in scope.

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