Water Supply Line Replacement

The City’s consultant, RH2 Engineering, completed their 60% design plans in December for the new 24-inch water supply pipeline along SE 36th Street, Gallagher Hill Road, and SE 40th Street, as well as roadway improvements following the water main work. RH2 recently completed 90% design plans for a new 10-inch ductile iron water main along Gallagher Hill Road that will replace an existing asbestos cement (AC) water main. This old water main needs to be replaced prior to installation of the new 24-inch Supply Pipeline. Staff are currently reviewing these designs and will present a project update to the Council in April. Construction of the 10-inch water main could start as early as May 2026.

At the October 21, 2025 City Council meeting, the Council received an update on the Water Supply Pipeline project. Design work by RH2 and construction coordination by Scarsella Brothers Inc has continued and has reached the 60% stage milestone.

The new pipe alignments, general footprint of the project, and roadway improvements are identified. Initial cost estimates have also been completed. While there are some remaining scope elements to be defined, the project is moving towards construction in Q1 2026.

This project has two primary phases: water and roadway. The underground and water work will occur first, starting in March 2026 and this is anticipated to last 10 to 12 months. Roadway improvements will follow, beginning in early spring 2027 and are estimated to take approximately 8 months to construct. Substantial completion of the project is expected by the end of 2027.

Read the full report here.

The City’s design consultant is nearing completion of the 60% design plans, which include a new 24-inch water supply pipeline along SE 36^th Street, Gallagher Hill Road, and SE 40^th Street, a new 10-inch ductile iron pipe along Gallagher Hill that will replace an existing asbestos cement (AC) water main, and roadway improvements following the water main work. City staff, in coordination with RH2 (the design consultant) and Scarsella (the contractor), will be reviewing the 60% plans, specifications, and cost estimates in the coming weeks, as well as discussing construction sequencing and duration.

The City of Mercer Island has secured $1.2 million in funding for a new water supply line in the State of Washington’s Capital Budget that was recently adopted during the 2025 legislative session. The pipeline’s replacement became necessary after a leak in a segment was discovered in April of 2024, which required emergency repairs that reduced its overall capacity.

Given the importance of this water supply line, the City explored options for a stronger, earthquake resistant replacement that will improve overall resiliency of the water system. The City Council authorized staff to include design and construction of a new water supply line in the 2025-2030 Capital Improvement Plan (AB 6530).

At its meeting on July 15, the Council formally expressed its appreciation for the efforts of the 41st Legislative District’s elected officials – Senator Lisa Wellman and Representatives Janice Zahn and My-Linh Thai – who applied for this funding on behalf of the City and voted for its inclusion in the Capital Budget.

RH2, the City’s design consultant, has submitted their 30% design plans, which define the proposed alignment for the new 24-inch water supply pipeline along SE 36^th Street, Gallagher Hill Road, and SE 40^th Street, as well as a new 10-inch ductile iron pipe along Gallagher Hill that will replace an existing asbestos cement (AC) water main. RH2, in coordination with Scarsella (the contractor) and City staff, held a meeting to review the 30% plans and initiate discussions on construction sequencing and potential traffic control strategies during construction. To support the evaluation of traffic detour options, RH2 will conduct traffic counts and perform related analysis in the coming weeks.

The City, in collaboration with RH2 and Scarsella, is currently advancing the 30% design phase, with completion anticipated by mid-July. Comprehensive site surveying and mapping have been completed, including field visits to verify existing conditions. A preliminary alignment for the new water main has been established. In addition, initial coordination with WSDOT is underway to begin the permitting process, as the project crosses WSDOT right-of-way at the intersections of SE 36th Street and East Mercer Way.

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. With GC/CM alternative delivery method, the contractor is involved in the design phase of the project to aide with project decisions which supports a more efficient and cost-effective project outcome.

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After a competitive interview process, the City selected RH2 as the design consultant for the project. RH2 will lead the design in coordination with the City and the GC/CM, who will be selected in the next couple months.

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.