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What’s happening now?

We are preparing for major construction activities, expected to begin in early 2022. In the coming months, we will finalize our contract documents and job specifications, hire contractors to do the work, and get all the paperwork in place to get started.

construction signs lined up against a wall 

As we near construction, here are some things to keep in mind:

  • King County is upgrading four miles of sewer pipeline across north Mercer Island and southwest Bellevue. In addition to the new sewer pipeline, King County will upgrade the County’s North Mercer Pump Station, as well as the City of Mercer Island’s Lift Station 11 at Fruitland Landing. The pipes and associated facilities, installed in the 1970s, are aging and reaching capacity.
  • Construction overall will take four years, but most locations will see much shorter work durations. There will be times when construction is occurring in several locations simultaneously.
  • King County is committed to being a good neighbor during construction. We provide advance notification of construction activities as well as a 24-hour construction hotline.
  • This is a large and complex project and most people won’t be impacted by all of it. That’s why we’re planning targeted communications for different locations and different concerns.

phases_graphic_construction

Contact us

Contact Kristine Cramer, community services lead at:
 NMEsewer@kingcounty.gov
 206-477-5415

 Feedback form

24-hour construction hotline: 206-263-1915

Get to know the work area

We expect construction to get underway toward the end of this year. You can take a virtual tour of the work area using the map below.

storymap of construction

Construction activity by location

This project is using a number of different construction techniques. Each construction method is the best for that area.

map of the pipeline alignment from North Mercer Island to Enatai. Map displays construction methods by location.

Exterior (street view) of North Mercer Pump Station. Conceptual design showing a driveway leading to the pump station building surrounded by landscaping

We expect construction at the North Mercer Pump Station to begin in early 2022 and last approximately three years. Upgrading the North Mercer Pump Station ensures its compatibility with new conveyance system, addresses the condition of aging equipment, and allows the system to handle the expected increase in flows through 2060.

Construction activities in this area include:

  • Installation of temporary pump station during construction
  • North Mercer Pump station upgrade
  • Odor control system upgrade
  • New generator building
  • Restoration and landscaping

If you are a North Mercer Pump Station neighbor, look for:

  • Flyers on your door with advance notice of construction activity
  • Regular web updates
  • Listserv announcements

From the North Mercer Pump Station to the Mercer Island Boat Launch along the East Channel of Lake Washington, we’ll be installing about two miles of new pipeline across as part of this sewer upgrade project. Approximately half of the new pipe will be in public streets, with the rest under the I-90 Trail. The work could have an impact on everyone who lives or travels on the affected streets or trail.

map displaying open cut construction along the project work area

Crews will use “open-cut” trenching to install the pipe along the streets highlighted in this map.

In open-cut construction, crews dig from the surface to the pipe depth, install the pipe, then cover the pipe and restore the surface. Frequently, crews will complete a temporary surface restoration until the permanent restoration can be completed. Contractors often use machines to compact the soil that can create vibrations in the surrounding area. Crossing Mercer Island, the trenches will vary in size from approximately 8 to 30 feet wide depending on the pipes being installed and the available space at each location.

Learn more about open cut construction


Learn more about open cut construction on Mercer Island residential streets .

If you are a resident on a street that will see construction, look for:

  • Flyers on your door with advance notice of construction activity
  • Regular web updates
  • Listserv announcements

If you are a commuter, look for:

  • Signs at the Mercer Island Park and Ride
  • Alerts through partner agencies
  • Listserv announcements

If you are a biker or pedestrian, look for:

  • Detour routing
  • Listserv announcements

Conveyance work that affects use of the I-90 Trail on Mercer Island is expected to begin in 2022 and will last approximately one year. As the work progresses over that timeframe, portions of the trail will be closed for construction, with the rest remaining open for use. Crews will close sections up to about 1,000 feet long—possibly multiple sections at any one time. Sections where construction is underway will be closed for up to five months at a time.

2108_bike-trail-construction

Learn more about open cut construction


Learn more about construction and restoration on the I-90 Trail .

If you are a biker or pedestrian, look for:

  • Signs along the I-90 trail
  • Detour routing
  • Listserv announcements

The existing Mercer Interceptor pipe is buried under Lake Washington. The new pipe will be buried under streets and trails. As a result, King County must connect parts of the City of Mercer Island sewer system to the new conveyance line, which will pass 97th Avenue SE at the top of the hill near the I-90 trail.

King County will install bigger sewage pumps at Lift Station 11 in Fruitland Landing, upgrade the electrical system, and upgrade an existing pipe in the street to connect the two systems together.

Learn more about sewer work coming to Fruitland Landing and 97th Avenue Southeast .

Learn more about relining sewer pipes


If you are a resident on a street that will see construction, look for:

  • Flyers on your door with advance notice of construction activity
  • Regular web updates
  • Listserv announcements

map of construction near Fruitland LandingMap of Fruitland Landing during construction

map of restoration near Fruitland LandingFruitland Landing site restoration

A key component of the sewer conveyance line crosses the East Channel of Lake Washington between Mercer Island and Bellevue, underneath the I-90 bridge. The timing for this portion of the project depends on what are called “fish windows” – a period of time when spawning or migrating fish are less likely to be in the area. In Lake Washington’s East Channel, the fish window is different on the eastern and western shorelines. On the east side along Enatai Beach Park, the window is in the summer to early fall. On the west side, at the Mercer Island Boat Launch, a much longer fish window allows work in summer, fall, or winter. We are working with natural resources agencies to plan this work to protect fish species that migrate through the area.

The pipe, extending from shore to shore, will be laid on the lake bottom. Learn more about this portion of the project .

The contractor will use Mercer Island Boat Launch and Enatai Beach Park for equipment staging as well as active work zones. Both areas will be restored after construction including shoreline habitat improvements. On the shorelines, we will rebuild areas affected by construction with logs, heavy gravel, and plantings above the water level to improve habitat conditions. At the Mercer Island Boat Launch, we will replace a deteriorating concrete bulkhead with more natural logs and plantings.

map of construction detail at Mercer Island Boat Launch

map of restoration at the Mercer Island boat launch

Learn more about work planned for the East Channel Crossing and Mercer Island Boat Launch .

If you are a boater, look for:

  • Coast Guard Advisory
  • Kiosk at Mercer Island Boat Launch

If you are a park user, look for:

  • Kiosks at Mercer Island Boat Launch
  • Listserv announcements 

Enatai Beach Park is a focal point of the North Mercer/Enatai Sewer Upgrade Project. The new sewer conveyance system from Mercer Island will cross under the East Channel of Lake Washington near the I-90 Bridge, landing at the north end of the park. At that point, the pipeline will split into two paths, both leading to the Sweyolocken Pump Station, about a half-mile to the east.

Construction activity in Enatai Beach Park will happen intermittently over four years, and the park will be closed at times. Park users will see equipment stored on site for up to two years starting in 2022.

Description of construction at Enatai Beach Park

Map of restored Enatai Beach Park following construction

Learn more about work planned at Enatai Beach Park .

If you are a park user, look for:

  • Kiosks at Enatai Beach Park
  • Listserv announcements

Upgrading the existing Enatai Interceptor increases reliability in the system. With a growing population, there is also a need for additional capacity to transport more wastewater to the South Treatment Plant in Renton. The project is building a brand new, high-capacity sewer line called the Enatai Siphon that will be drilled deep underground from Enatai Beach Park to the Sweyolocken Pump Station. The drilling and installation of the underground pipeline is expected to take approximately 6 months.

Learn more about Horizontal Directional Drilling


map of Enatai Siphon (pipeline) from Enatai Beach Park to Sweylocken Pump Station 

Learn more about the Enatai Siphon and Interceptor .

If you are a boater, look for:

  • Coast Guard Advisory
  • Kiosk at Mercer Island Boat Launch

If you are a park user, look for:

  • Kiosk at Enatai Beach Park
  • Listserv announcements

Crews will install the Enatai Siphon in a multi-step process that starts with drilling a hole near Sweyolocken Pump Station. The drilling and installation of the underground pipeline is expected to take approximately 6 months from start to finish.

Learn more about Horizontal Directional Drilling


Crews will also install a pipe from the end of the new siphon to the existing pump station. Equipment will be stored near the pump station. The access road from Bellevue Way SE to the Sweyolocken Boat Launch will be closed for the duration of the work.

aerial photo of boat ramp near the Sweyolocken Pump StationSweyolocken Boat Launch and Pump Station site during construction of the Enatai Siphon connection

map of work area near Sweyolocken Pump Station

Learn more about work coming to the Sweyolocken Pump Station (PS) and Mercer Slough .

If you are a biker or pedestrian, look for: 

  • Signs along bike trails
  • Detour routing
  • Listserv announcements

If you are a park user, look for:

  • Kiosk at Sweyolocken Boat Launch
  • Listserv announcements

Construction photos

King County employees on a dock next to a maintenance access hatchEnatai interceptor maintenance hole.