Water Supply Line Replacement

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The City is installing a new water supply pipeline to improve the overall resiliency of the City’s water system. The new water supply line will replace a section of the aging Seattle Public Utilities transmission line that currently serves Mercer Island by delivering water to the City’s reservoirs. The new water supply line will be fully owned and operated by the City. See the Project Map here.

In addition to the new water supply line, this project includes improvements to other City infrastructure in the same corridor including replacement of an asbestos-cement (AC) watermain, roadway paving and sidewalk enhancements. Combining these improvements into one project improves project efficiency, scheduling, reduces costs and overall construction impacts.

Follow this page for details of the design and construction of the new water supply pipeline, as well as neighborhood and traffic impacts through the project.

The City is installing a new water supply pipeline to improve the overall resiliency of the City’s water system. The new water supply line will replace a section of the aging Seattle Public Utilities transmission line that currently serves Mercer Island by delivering water to the City’s reservoirs. The new water supply line will be fully owned and operated by the City. See the Project Map here.

In addition to the new water supply line, this project includes improvements to other City infrastructure in the same corridor including replacement of an asbestos-cement (AC) watermain, roadway paving and sidewalk enhancements. Combining these improvements into one project improves project efficiency, scheduling, reduces costs and overall construction impacts.

Follow this page for details of the design and construction of the new water supply pipeline, as well as neighborhood and traffic impacts through the project.

  • City Selects Contractor for Supply Line Project

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    At the May 20, 2025 City Council meeting, the City approved the Scarsella Brothers, Inc.as the Heavy Civil General Contractor/Construction Manager (GC/CM) for the new water supply pipeline for the City of Mercer Island. The GC/CM at this stage of the project is needed to aide with design and construction considerations as the project moves thru the design phase which supports a more efficient and cost-effective project outcome. Design and permitting is anticipated to be completed this year, with construction schedule to begin in late 2025 or early 2026.

    Read more here.

  • Alternative Delivery for Design & Construction

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    The City has selected to use an alternative delivery method to design and construct this project. The City presented the project to the state’s Project Review Committee in December 2024, as required for utilization of alternative delivery methods in public works construction. The project received unanimous approval to use the Heavy Civil General Contractor/Construction Manager (GC/CM) project delivery. This alternative delivery was selected for this project to help reduce project costs, schedule and risk.

  • Project Components

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    The water supply line replacement project consists of several components:

    New Water Supply Line: The new 24-inch water supply line will run from the Boat Ramp area, heading north on East Mercer Way, west on SE 36th Street, and southwest along SE Gallagher Hill Road to SE 40th Street, continuing east on SE 40th to 92nd Ave SE. Additionally, the project will involve relocating the SPU master meter and upsizing an existing 12-inch water line section of supply line at the intersection of 89th Avenue SE and SE 43rd Street to 24-inch pipe.

    AC Watermain Replacement: Approximately 1,500 feet of existing 8-inch and 10-inch AC pipeline will be replaced, as well as the replacement of a large Pressure Reducing Valve (PRV) at the bottom of SE Gallagher Hill Road. Additionally, the project will extend the pipeline to the top of SE Gallagher Hill Road and establish a new connection at the intersection of SE 40th Street and SE Gallagher Hill Road.

    Roadway Improvements: Roadway improvements will be constructed, following the water supply line installation, testing, and startup. The scope of these improvements include:

    • SE 36th Street (Gallagher Hill Road to East Mercer Way) – Restore sidewalks removed for pipeline construction, upgrade all sidewalk ramps to meet current ADA standards, mill and repave the roadway with a hot mix asphalt (HMA) overlay, raise utilities to grade, and install new pavement markings.

    • Gallagher Hill Road (SE 40th Street to SE 36th Street) – Construct new curb and sidewalk along the east side (downhill lane), provide a paved shoulder suitable for bicycle use on the west side (uphill lane), install a HMA overlay, raise utilities to grade, and install new pavement markings.

    • SE 40th Street (Gallagher Hill Road to 93rd Avenue) – Construct new curbs and sidewalks on both sides, install bike lanes on both sides, install a HMA overlay, raise utilities to grade, and install new pavement markings. Together, these improvements will repave approximately 0.9 miles of arterial roadways.

    Shorewood Apartment Connection: Two connections and master meters will be installed to provide water service to Shorewood Apartments.

  • New Water Supply Line Added to 2025-2030 Capital Improvement Plan

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    At the October 1, 2024 City Council Meeting, City staff presented recommended actions to improve resiliency of the permanent water transmission line from Seattle Public Utilities (see AB 6530). The 24-inch concrete cylinder water transmission line located in the SE 40th Street right-of-way was constructed in 1956 and has required several leak repairs over the years. The City and Shorewood Apartments are the two wholesale customers served by SPU’s water transmission line. Given the overwhelming cost responsibility held by the City, and the importance of this water transmission line, the City began exploring options to replace this segment of pipe with a stronger, earthquake resistant transmission line in an alternative alignment.

    This City considered several options, details of which can be viewed here. After careful consideration, the City Council approved the design and construction of a new 24-inch earth-quake resistant supply line to replace the existing SPU supply line. The location of the new 24-inch supply line corresponds with several previously planned projects. To reduce construction costs, schedule, and impacts, this project was combined with these previously planned projects.

    City Council voted to direct staff to include the design and construction of a new 24-inch transmission line in the 2025- 2030 Capital Improvement Plan. City staff will negotiate a preferred pipeline alignment and ownership from Seattle Public Utilities and identify long-term solutions to serve the Shorewood Apartments.

  • SPU Water Main Leak and Repair

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    In the spring of 2024, a leak from the Seattle Public Utilities (SPU) 24-inch diameter water supply line on Mercer Island was discovered. The City Manager declared a Local Emergency due to the reduction in the supply of incoming potable water to Mercer Island, and immediate steps were taken to mitigate the impacts of the leak, ensure safety of the site, and repairs were made to the line over the summer of 2024.

    Given the overwhelming cost responsibility held by the City, and the importance of this water supply line, the City began exploring options to replace this segment of the SPU line with a stronger, earthquake resistant supply line in an alternative alignment.

    Additional information on the leak and other past issues with the SPU line can be viewed here.

Page last updated: 26 Jun 2025, 01:56 PM