Large Diameter Elevated Transmission Pipeline Crossing of the Snohomish River Estuary: A Case Study Addressing Unique Design Challenges

Originally constructed in 1929 and 1935, Pipelines No. 2 and 3 convey filtered water from the Sultan River water supply source to the City of Everett. Due to deterioration of the existing pipeline piling support system and pipelines, the City of Everett implemented a plan to replace these pipelines across a relatively inaccessible portion of the Snohomish River Estuary. The Phase 6 project consists of replacing approximately 1,219 meters (4,000 LF) of two parallel 122 cm (48-inch) diameter steel pipes across the western half of Ebey Island just south of and parallel to the State Route 2 highway trestle. The pipeline design needed to address a number of unique design challenges. The project was located within federally regulated wetlands associated with the Snohomish River estuary that fall under the jurisdiction of the United States Army Corps of Engineers. Due to the presence of these Category 1 wetlands, construction access was severely restricted; with access available only from the two ends of the pipe alignment, all construction equipment activities needed to be supported off of a temporary pile supported trestle system or by the new permanent pile bent system that would support the pipeline. Other unique design challenges included: the presence of very soft soil materials, extending to 27 meters (90-feet) and more below the ground surface, the presence of other pile supported structures resulting in a complex web of potential underground conflicts, horizontal angle points resulting in thrust restraint, thermal expansion/contraction of the above ground piping, the need to cross under an existing pedestrian bridge overpass and a buried crossing through an existing dike structure. This paper describes the alternatives investigated and the design solutions developed to address these complex, challenging issues for a major above ground pipeline system while protecting the surrounding wetland resources.