Sewer Pipe Replacements and Upsizing
Project Status
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Project Details/Neighborhood
The eastern portion of Mercerdale Park from SE 34th to SE 32nd Streets and 77th to 78th Avenues
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 will be replacing approximately 85 feet of the existing 8-inch sewer main with a new 12-inch main and abandoning about 865 feet of outdated 10-inch asbestos cement sewer main. Additionally, the City will install 1,075 feet of new sewer mains in sizes ranging from 10 inches to 16 inches along new alignments within and near the eastern portion of Mercerdale Park. To ensure continuous service throughout the construction period, a temporary sewer bypass will be set up.
Project Timeline:
Planning/Design 2027
Under Construction 2028
Estimated Budget
$1.72M
Contact
Chris Marks, City of Mercer Island Utilities Engineer: (206)-667-1027 or chris.marks@mercerisland.gov
Status Updates
September 2024
Updates will be paused until 2027
The City has received the 30 percent design for the upgraded sewer mains through Mercerdale Park. After a thorough review by City staff, it has been decided to pause these improvements. This decision allows time for staff to develop a more comprehensive park plan that addresses both the below and above-grade enhancements. We anticipate resuming the project in 2027.
May 22, 2024
This week, the topographical base map and preliminary geotechnical data report containing soils analysis and groundwater monitoring results were delivered. From here, Staheli Trenchless and their subconsultant Reichhardt & Ebe will begin drafting the plan and profile of the proposed sewer lines. Concurrently, the City is working with ecologists from Shannon & Wilson to navigate the permitting requirements of the project. Both preliminary sewer design requirements and the project's relation to environmental critical areas will help determine overall park impacts.
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)