Conference Agenda

King County commissioned an alternatives analysis to increase wastewater conveyance capacity between North Mercer Pump Station, on Mercer Island, and Sweyolocken Pump Station, in Bellevue. Alternatives identified a range of alignments and construction techniques for over 4 miles of pipeline and the preferred alternative was selected based on scoring of the alternatives using a triple bottom line approach. One key section of the preferred alternative is the Enatai Siphon, an inverted siphon from Enatai Beach Park, on the Lake Washington shoreline, to the Sweyolocken Pump Station. This 3,000 foot, 32-inch diameter High Density Polyethylene pipeline required installation by horizontal directional drilling (HDD) under the hillside of the Enatai neighborhood at depths of up to 160 feet. The Enatai Siphon will accommodate high flows, with lower flows continuing to use an existing sewer lakeline that is routed along the shoreline of Lake Washington and Mercer Slough.

Design of the HDD provided solutions for physical obstacles, geotechnical challenges and identified methods for pullback of the pipe from Lake Washington. As well as HDD elements, the design also determined optimal system hydraulics, essential for sizing and minimizing build-up of sediment. Methods of maintenance were also evaluated to enable periodic cleaning with suitable access for cleaning incorporated in the site and structure design. And the design incorporated methods to minimize the impacts of working in and adjacent to critical areas.

With critical areas including shoreline and wetlands adjacent to the entry and exit locations, significant permitting coordination was required to successfully obtain the federal, state and local permits. In addition, with the Enatai Siphon entry structure in the Enatai Beach Park, on WSDOT property and immediately adjacent to the Interstate 90 bridge extensive coordination with required with WSDOT for utility franchise and air lease.

Construction of the work was completed in early 2023 and the presentation will include images of the casing installation, HDD and pullback, and structure installation.

Seattle Public Utilities (SPU) is responsible for 1,900 miles of pipelines providing wastewater and drainage conveyance to 1.5 million customers. As part of their proactive assessment and prioritization process, SPU identified a critical drainage pipe that served a large upstream basin that crosses Interstate 5 (I-5) through heart of downtown Seattle. Inspection of this 58”x36” arched shaped flat bottom corrugated metal drainage pipe indicated that the pipe was deteriorating. The pipe has numerous vertical alignment changes and a horizontal curve, which limited the feasible rehabilitation technologies.

SPU performed an Options Analysis and determined that a spray-on fully structural liner that minimized loss of hydraulic capacity was the most feasible solution. Being the first time utilizing this rehabilitation technology, SPU’s team conducted a thorough evaluation of the use of geopolymers, wrote new specifications for the product, evaluated the hydraulic capacity limitations, and developed a bid package for the work. In addition, the velocities in the drainage pipe necessitated an evaluation of abrasion-resistance of the product, leading to a requirement for the contractor to apply a sacrificial wearing course. And because of the large drainage basin, even a trace amount of rainfall resulted in flows that would be problematic to bypass.

This paper will present how SPU determined that a geopolymer spray applied liner was the best solution, the site-specific design challenges of the project, the complex bypassing and evaluation of risk management versus expected costs, the presentation of the construction activities, and lessons learned. Construction was successfully completed ahead of schedule.