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Project update

February 22, 2018

King County’s geotechnical contractor has completed soil and groundwater testing in Coal Creek Parkway. The contractor was able to complete this work over a week earlier than expected, reducing traffic impacts on the parkway. This field testing will help confirm the recommended alignment for the Coal Creek Trunk Sewer Upgrade Project.

Beginning Monday, Feb. 26, biologists will be working in the Coal Creek Natural Area surveying areas along the creek. This work is expected to be completed Wednesday, Feb. 28.

Beginning Monday, Feb. 26, biologists will be working in the Coal Creek Natural Area surveying areas along the creek

Watch for project updates this spring as we make progress designing an upgrade to this important sewer pipeline. Once completed, this project and the new section of Coal Creek Trunk Sewer will provide service for decades to come.

View project library for past project updates.


Contact us

Contact Monica Van der Vieren community services lead at:


Project description

King County Wastewater Treatment Division (WTD) is planning to upgrade a regional sewer line that serves Bellevue and Newcastle. The existing pipeline was built between 1966 and 1991 in an area that looked different from today, with a smaller population. The pipeline is nearing its maximum capacity and needs to be upgraded to meet the growing needs of your community. When complete, the new sewer will provide safe and reliable service for another 50 years or more.

Protecting the Coal Creek Natural Area

The existing Coal Creek Trunk is approximately 2.5 miles long and primarily follows the banks of Coal Creek in the Coal Creek Natural Area between Newcastle and I-405. King County is evaluating options to move the pipe away from the Creek to protect this sensitive environmental area. The preferred route for the new sewer runs west of Coal Creek Parkway SE between I-405 and the parking lot at the Upper Coal Creek West Trail Trailhead.

1711_cc-recommend-align_400Recommended alignment .

Next steps

Additional information about the project area is needed before King County can finalize the route for the sewer upgrade. Over the next year, King County will study soils, groundwater, sensitive environmental areas and areas of cultural significance in the project area to confirm that the sewer can be built along the preferred route. We expect to finalize the route by late 2018.


  • Alternatives analysis: 2016 - 2017
  • Design: 2018 - 2020
  • Construction: 2021 – 2024

Where does your wastewater go?

The Coal Creek sewer line serves the City of Bellevue. The water you use in your home or business travels down the drain to the City’s sewer system. King County collects that wastewater from the City of Bellevue and routes it to South Treatment Plant in Renton.

STPaerial_600South Treatment Plant

Our commitment to you

Whether you live, work, recreate or commute in the area, we know you’ll have questions and concerns about this project. King County is committed to keeping you up-to-date. We will ask for your thoughts and input during the project. As this project gets underway, we’d like to get to know you. Our team is in the process of reaching out to neighbors and organizations in the project area. If you are interested in scheduling an individual meeting or briefing for your community organization, please contact Monica Van der Vieren at or 206-477-5502.