Lynnwood Link Extension

Expansive geotechnical exploration, design, permitting and seismic services for an 8.5-mile light rail extension north of Seattle, Washington.

Sound Transit, the public transit authority for the greater Seattle, Washington region, is working hard to expand its light rail network with more than a dozen major projects in various stages of planning or construction. Contractors have already completed several phases of northward rail expansion. In 2016, a new station opened at the University of Washington, and an additional section of track is pushing the line to Northgate—with three additional stations expected to open by 2021. The Lynnwood Link Extension is the next phase of northward expansion into Snohomish County, with four new stations scheduled to open in 2024 and construction beginning in 2019.

GeoEngineers has been helping Sound Transit with this 8.5-mile light rail extension since 2010, having been involved in the project through alternatives evaluation, conceptual, preliminary and final engineering. Through multiple phases of design, GeoEngineers completed an extensive investigation program along the corridor. The team gathered and characterize soil data from locations all along the proposed route and made recommendations for structures like elevated guideway foundations, retaining walls, stations, parking garages and stormwater facilities.

Portions of the planned light rail route cross areas of soft soils and even some sensitive wetlands, complicating permitting and site access. Other portions of the route involve tall cuts into existing slopes along I-5. As with any project in the Seattle region, the team had to model and analyze the project structures to evaluate and mitigate the seismic risk.

Approach

GeoEngineers’ team provided geotechnical services for the Lynnwood Link project beginning from early surveys and continuing through construction. Services for this project included:

Geotechnical Explorations:

  • Before beginning field investigations, the team reviewed published geologic and hazard maps for project sites, along with historical reports from other consultants.
  • Through preliminary and final design, the team planned and oversaw almost 250 geotechnical borings to depths of 40 to 200 feet.
  • Groundwater monitoring wells were installed in some of these borings and groundwater levels were recorded using dataloggers over a multi-year period.
  • GeoEngineers’ AASHTO-certified lab tested and characterized field samples to form the basis of the team’s recommendations for each specific site and structure.

Permitting:

  • GeoEngineers assisted with permitting and right of entry (ROE) requests for the many individual project sites in King and Snohomish Counties, as well as other permits for explorations within public right-of-way.

Seismic Design and Risk Analysis:

  • The Lynnwood Link route is relatively close to three separate fault zones and Sound Transit has strict seismic performance requirements, including design for continued operation after a small earthquake and life safety following a large design earthquake. In addition to these two Sound Transit-defined seismic events, GeoEngineers also evaluated two additional levels of seismic shaking for some structures, depending on the jurisdiction and governing design codes.

Geotechnical Recommendations for:

  • Elevated Guideway Foundations: The team recommended deep foundations using large diameter drilled shafts for the elevated guideway columns with shallow foundations for guideway abutments.
  • Retaining Walls: GeoEngineers designed earth pressure diagrams for a variety of retaining walls along the at-grade sections of track and around station areas including soldier pile walls, soil nail walls, mechanically stabilized earth walls, and cast-in-place concrete walls.
  • Stormwater Facilities: GeoEngineers created soil and groundwater profiles to support design of stormwater facilities.
  • Stations and Ancillary Structures: The team provided geotechnical recommendations for stations along the route, along with associated parking and ancillary structures.
  • Earthwork: The team recommended soil characteristics for structural fill and subgrade preparation at sites throughout the project area.

Results

GeoEngineers’ team successfully investigated a staggering number of individual sites along the Lynnwood Link route, giving Sound Transit and project partners trustworthy data and geotechnical recommendations on which to base the design.

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