2010 EEA National Gold Award Winner

HWA GeoSciences' Brightwater Marine Outfall

ACEC Washington Impact 1/26/2010
Most people don't know that there is a 13-mile-long underground pipeline that carries cleaned wastewater under neighborhoods and highways, from Woodinville all the way to Puget Sound. The Brightwater Marine Outfall (BMO) at Point Wells near Edmonds is the terminus of this pipeline.

As part of its new $1.8 billion wastewater treatment system, King County's Wastewater Treatment Division needed to ensure seismic stability of the submarine slopes under beach areas that would support the pipeline. HWA GeoSciences Inc. was the geotechnical consultant to the design/build team selected by King County and led by Triton Marine Construction.

HWA's geotechnical analyses, focused on this primary seismic geotechnical challenge, showed that much of the granular slope deposits along the outfall alignment were susceptible to liquefaction flow failures. HWA's analyses showed that more than half of the slope could fail during earthquakes with magnitudes of up to 7.5 on the Richter Scale.

With its diffuser ports more than 600 feet below sea level, the outfall is one of the deepest in the world. Submarine flow failures would introduce instantaneous drag loading along the twin outfall pipes that could cumulatively range upwards of 2.8 million pounds. Since treatment and stabilization of the slope soils was impractical, the outfall pipeline required a seismic anchor/restraint system to maintain its integrity.

After considering numerous alternatives, HWA developed a simple and cost-effective seismic restraint employing sheet piling that would resist these extreme seismic loads and ensure operational integrity of the outfall during the design-level earthquake. The simplicity and constructability of the sheet pile anchor/restraint was influential in Triton's completion of the project two years ahead of the County's schedule and more than $8 million under the original $38 million estimate.