Brightwater Marine Outfall

Edmonds, Washington

HWA was the geotechnical engineer of record for the design-build team led by Triton Marine Construction, of Bremerton, Washington; and Dayton & Knight Ltd. Consulting Engineers of Vancouver, Canada.

HWA conducted a geotechnical engineering investigation to support the design and construction of the marine outfall, which extended in excess of 5,000 feet offshore and terminated in a diffuser section approximately 600 feet below sea level. The near-shore section of the outfall pipe was placed in a trench, and the offshore portion was placed directly on the seafloor. The connection to the main Brightwater Conveyance pipe was comprised of a single 84 inch steel pipe, and the outfall branched through a custom-fabricated steel wye section to allow connection of twin 63-inch HDPE pipes, placed side-by-side.

Of particular significance to the performance of the outfall pipeline was the stability of the submarine slope during a design earthquake. HWA performed extensive seismic evaluations and determined that significant areas of the slope could fail as a consequence of dynamic soil loading and/or soil liquefaction effects, which in turn could contribute to major drag loading of the pipe. Based on these analyses, the project team ultimately developed a design for a seismic anchor restraint that would be constructed in the near-shore sheeted section just above the steel wye-connector. The anchorage system comprises a row of sheet piles on each side of the 84-inch steel pipe segment driven to a uniform tip elevation some 25 feet below invert level of the steel pipe, for a minimum length of 60 feet of the line. The steel pipe is provided with a series of precast concrete collar blocks coupled to the pipe at intervals within the anchor restraint and is also equipped with steel shear collars welded to the pipe between blocks. The anchor restraint is also provided with an end shear collar consisting of reinforcing steel affixed to the sheet piling and the entire 60-foot segment of the pipe within the sheet pile rows is encapsulated in tremie placed concrete extending from 1.5 feet above pipe crown to 2.5 feet below invert level about 15 feet below sea bed elevation. The seismic anchor restraint will carry soil drag loads exerted on the pipeline by slope failure during the design earthquake event and transfer them via shear into the sheet piling extending into stable soils below.

HWA's geotechnical investigation included six offshore borings. The purpose of these borings was to further evaluate and characterize the liquefaction susceptibility and shear strength properties of the loose to medium dense beach deposits. The borings were located to supplement previous borings by others in the pre-design process for King County. Laboratory tests were conducted on selected samples retrieved from the borings to characterize engineering and index properties of the beach deposits for use in liquefaction and slope response analyses. Seismic ground response analyses were performed to evaluate the behavior of the slope during an earthquake. Based on the results of these slope response analyses, the potential effects of this response on the outfall pipeline were evaluated. The pipe weighting and anchorage design was updated and finalized based on these more refined analyses. The ultimate deliverables from HWA's investigation and design efforts were three reports acceptable to King County's construction management and technical review team. These include a final Geotechnical Data Report, a Geotechnical Interpretive Report, and a Geotechnical Reconciliation Report; the latter of which established the formal contractual basis for construction conditions for the facilities, between King County and Triton Marine Construction. Geotechnical aspects of the pipe weighting design were reviewed, and geotechnical input to the pipe deployment was provided.

This was the first major design-build project awarded by King County Wastewater Treatment Division, and it was by all measures a major success.

HWA received a Gold Award on the National Track at the American Council of Engineering Companies (ACEC) of Washington's 2010 Engineering Excellence Awards for the Water and Wastewater category.

Dayton & Knight received the 2009 Consulting Engineers of British Colombia's Award of Excellence for this project, and recognition awards have also followed from the local chapters of the APWA and the ASCE.