Federal Way Link Light Rail Extension

In 2008, Sound Transit launched an ambitious 15-year plan to expand the light rail system in the greater Puget Sound region. GeoEngineers played a key role in supporting this effort, providing geotechnical and environmental engineering services for the $3 billion Federal Way Link Extension (FWLE) project. The FWLE extended the region’s light rail network 7.8 miles south from SeaTac Airport to the City of Federal Way and included three new light rail stations.

GeoEngineers began work during the early alternative evaluation phase, reviewing existing subsurface data to plan a comprehensive soil and groundwater investigation. This investigation informed geotechnical recommendations for numerous structures along the alignment, including elevated guideway foundations, retaining walls, stations, parking garages, and stormwater facilities.

The project spanned four jurisdictions, hundreds of parcels, and over one hundred properties, many of which required environmental review and potential remediation. GeoEngineers assessed environmental risks, supported cleanup planning, and helped Sound Transit make informed decisions throughout the fast-paced design-build process. Notably, work at Ml—a federal Superfund site—required explorations and planning for how to construct the project through the landfill. GeoEngineers helped guide the project from design through construction, providing on-site monitoring and coordinating closely with the Washington State Department of Ecology (Ecology) to ensure safe and compliant construction through this historically impacted site.

Approach

• Geotechnical Services
  • Reviewed existing geologic maps and prior reports to provide conceptual geotechnical recommendations and identify complex conditions along the FWLE corridor.
  • Developed and executed a geotechnical exploration program, including 84 borings (20-101.5 ft) with standard penetration testing, and collected disturbed and undisturbed samples.
  • Installed 43 groundwater observation wells to monitor water levels at one-hour intervals.
  • Conducted laboratory testing to characterize soils, including moisture content, organic content, percent fines, grain size, and Atterberg limits.
  • Delivered geotechnical recommendations for elevated guideways, stations, parking garages, ancillary structures, earthworks, and roadway structures.
  • Provided foundation designs for elevated guideways (shallow foundations or drilled shafts) and retaining walls (MSE, soil nail, cast-in-place, soldier pile, cylinder pile).
  • Developed recommendations for site clearing, subgrade preparation, structural fill, and noise wall foundations.
• Environmental Services
  • Completed 156 Phase I ESAs, 48 Phase II ESAs, and over 30 remedial cost estimates to support property acquisition, risk assessment, and cleanup planning.
    • Subcontracted SBEs to complete Hazardous Building Material surveys.
    • Represented hazardous materials for the DBPM and assisted with change management as necessary.
  • Developed project requirements for Design-Build Contract.
  • Oversaw remediation and underground storage tank (UST) removal at multiple sites, including a former dry cleaner.
  • Coordinated with regulatory agencies to ensure compliance with environmental regulations and safety standards.
  • Successful regulatory closure (No Further Action) at 20 remediated sites during project timeframe.
  • Supported Sound Transit in obtaining an Independent Remedial Action Grant from Ecology for up to $300,000.

Results

GeoEngineers played critical geotechnical and environmental roles on this fast-paced and complex design-build project. The FWLE significantly expanded Sound Transit’s light rail network, giving Federal Way and nearby communities easy access to SeaTac Airport, downtown Seattle, and points north. Sound Transit estimates that the three new light rail stations will serve approximately 30,000 daily passengers when it opens in December 2025.