Sewer Pipe Replacements and Upsizing
Project Status
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Project Details/Neighborhood
Various locations around Mercer Island
Background
Most of the City’s wastewater system was installed in the 1950s and 1960s. As this collection system ages and degrades the pipes are susceptible to increased Inflow and infiltration, which is when excess groundwater and stormwater enters. Through computer modeling and analysis, some existing pipes may not be able to provide sufficient capacity during large rainfall events. To improve capacity and ensure the pipe’s longevity, segments of the sewer system are replaced with ones that are larger in diameter.
Description
The City has identified three segments of pipe to be replaced and upsized. These sections are 1) in front of West Mercer Elementary School; 2) through Mercerdale Park and north on SE 77th Ave. SE; and 3) on SE 32nd Street in front of the park. These pipes are 8- and 10-inches in diameter. The project will primarily be increasing the pipe size to 12-inches, though one segment will be increased to 16-inches.
Project Timeline:
Complete design Q4 2022
Estimated Budget
$150,000 (design)
Contact
Chris Marks, City of Mercer Island Utilities Engineer: (206)-667-1027 or chris.marks@mercerisland.gov
Status Updates
April 3, 2024
Beginning next week, crews from Shannon & Wilson and KPG Psomas will be conducting geotechnical investigations and a topographical survey, respectively, at Mercerdale Park. This field work is necessary to map existing site conditions and to further engineering design.
Crews are expected to be on site during working hours the week of April 8 and April 15. No impacts to the park or park users are expected.
March 2024
The results of the options analysis by the consultant determined that abandoning the existing sewer pipe in place and installing a new sewer pipe via open cut trenching is the most cost-effective approach to meet the City’s increased capacity requirements. Preliminary designs upsize the SE 34th St line from 8-inches to 12-inches, place a new 12-inch line running north-south through Mercerdale Park near the 77th Avenue SE alignment, and place a new 16-inch line along the Park’s northern frontage adjacent to SE 32nd St. The project will also include restoration of surface areas directly impacted by the sewer line construction.
October 2023
Current regulations regarding the disturbance and handling of asbestos cement (AC) pipe have led project staff to eliminate pipe bursting as a construction option and reconsider other methods of upsizing sewer pipe segments included in the project. The City’s engineering design consultant (Staheli Trenchless Consultants) is currently performing an options analysis to determine the best path forward from both a constructability and cost standpoint. The results of the options analysis are expected to be delivered next month. The consultant has completed a topographic survey and the design for the non-AC pipe segments is mostly complete. Bid ready design documents are expected Q1 2024, and construction is scheduled for later in 2024.
February 2023
The project scope has been refined to focus on the higher-priority Mercerdale Park and SE 32nd Street locations due to budget constraints. The design is based on using a trenchless construction method called “pipe bursting” to replace and upsize the existing sewer in place with minimal excavation. The pipe bursting will minimize the impact to the ground surface across the park. Following the completion of the design in Q2 2023, the project will be advertised for construction bids. Construction is planned for winter 2023 to minimize impacts on park users.
January 2023
This project budget code was listed in the prior biennium as SU0170. For the 2023-2024 biennium, the budget code has changed to SU0115.
Planning/Design
The City recently had a kick-off meeting with the design consultant, Staheli Trenchless. This month the consultant will be reviewing previous CCTV videos of the pipe and as-builts (the original construction drawings). A survey of the area is also expected to take place within the next month.
SU0115 Project Location (421 KB)