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City staff outlines plan for new 24-inch Mercer Island supply main to replace SPU sections and boost resilience
Summary
City staff presented plans for about 5,000 feet of new 24-inch transmission main, removal of asbestos-cement pipe on Gallagher Hill, connection of Shorewood Apartments to the city system, an estimated utility cost of roughly $12.5–$13.5 million (with bonds and a state grant available), and an anticipated two-year construction window with major paving targeted for summer 2027.
City staff described a multi-year project to install a new 24-inch transmission main on Mercer Island to reduce reliance on a steep, slide-prone SPU alignment and to consolidate ownership and control of island supply infrastructure.
"This is a generational investment in system resilience," said Clint (city public works staff), who led the presentation. He said the 24-inch main has an expected useful life of 50 to 75 years and would serve multiple generations of Mercer Islanders.
The project will install about 5,000 feet of 24-inch ductile iron restrained-joint pipe running up Southeast 36th, Gallagher Hill Road and turning onto Southeast 40th, according to staff. As part of that work the city will replace roughly 1,500 feet of asbestos-cement (AC) pipe on Gallagher Hill and relocate a pressure-reducing valve station at the bottom of the hill.
Clint said the plan also transfers ownership of certain SPU segments to the city: the city would take over Reach 4 and intends to abandon the Reach 3 segment that runs along the steep Southeast 40th right-of-way after construction. Staff noted a separate 16-inch emergency…
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