Ballard Siphon Replacement

(Seattle, WA)  Landau Associates provided geotechnical and environmental services for King County’s Ballard Siphon Replacement project. The previous siphon had served the Ballard community since the 1930s, and consisted of two 36-inch diameter wood stave pipes. The new pipe, which will meet north Seattle’s wastewater conveyance needs through the end of the century, consists of about 1,970 ft of 84-inch diameter pipeline under Salmon Bay between the Ballard and Interbay areas of Seattle. Trenchless construction techniques were used to install the new pipeline. The launch and retrieval shafts each have an inside diameter of about 30 ft and depths of about 148 ft and 89 ft, respectively. After completion of the installation of the new pipeline, the existing dual siphons were taken offline, cleaned, and slip-lined with HDPE pipe.

Landau Associates’ scope of services included conducting upland and over-water borings along the project alignment, conducting geotechnical laboratory tests, preparing a Geotechnical Data Report, and preparing a Geotechnical Technical Memorandum with geotechnical recommendations related to construction dewatering, temporary shoring design, and handling and disposal of contaminated soil and groundwater. We also conducted Phase I Environmental Site Assessments (ESA) for three properties and Phase II ESAs for two properties that were acquired to facilitate construction of the project. Landau Associates provided construction monitoring services for the project, which included ongoing plan and specification review and construction field monitoring of deep excavations at the launch and retrieval shafts. These excavations were constructed to facilitate connection of the new siphon with the existing sewer infrastructure and include internally-braced excavation trenches up to 33 ft deep. Special soldier pile and lagging techniques were used to allow complete removal of the shoring system at the end of construction. 

Photo 1: John Wulff