The Brightwater Marine Outfall at Point Wells near Edmonds is the terminus of a 13-mile pipeline that carries cleaned wastewater from Woodinville to Puget Sound. 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’s geotechnical analyses showed that much of the granular slope deposits along the outfall alignment were susceptible to liquefaction flow failures. More than half of the slope could fail during a 7.5 magnitude earthquake.

The outfall, with diffuser ports more than 600 feet deep, is one of the deepest in the world. Since treatment and stabilization of the slope soils was impractical, the outfall pipeline required a seismic anchor/restraint system to maintain its integrity.

HWA developed a simple and cost-effective seismic restraint using sheet piling that would resist extreme seismic loads and ensure operational integrity of the outfall during the design-level earthquake. The simplicity and constructability of the restraint helped the project finish two years ahead of schedule and more than $8 million under the original $38 million estimate.