SR 520 Floating Bridge and Landings Project

CLIENT Washington State Department of Transportation

Location Seattle, WA

The SR 520 Floating Bridge and Landings project replaced the original SR 520 bridge as the world's longest floating bridge. 

The natural features of Lake Washington make floating bridges an ideal transportation solution-the lake measures more than 200 feet deep and is underlain by more than 150 feet of soft soils. The new bridge is supported on 77 floating pontoons, which are held in place by 47 fluke anchors, eight gravity anchors, and five shaft anchors. The east and west sides of the bridge's floating portion transition to an elevated structure supported by shafts and large spread footings. In addition, a large maintenance facility was constructed into the existing steep hillside where the east side of the bridge touches the lake shore.

Shannon & Wilson provided geotechnical services to WSDOT from conceptual design through construction for the SR 520 Bridge Replacement and High Occupancy Vehicle Program. Our services included:

  • Subsurface characterization based explorations performed by WSDOT drillers (and observed by Shannon & Wilson) and laboratory tests
  • Probabilistic seismic hazard analyses and dynamic soil structure interaction analyses to evaluate seismic hazards, such as potential submarine landslides near the lake shore
  • Preliminary geotechnical design, including evaluating anchor types and resistances, stability, foundations, and retaining walls
  • Preparation of design-build request for proposal document
  • Supporting WSDOT during the design-build phase of the work

Once bridge construction bridge was complete, WSDOT asked Shannon & Wilson to prepare an anchor load test summary report to archive the unique load testing approach used for the pontoon anchors. Anchor testing included verification testing on two sacrificial anchors, performance testing on six anchor pairs, and proof testing on the remaining anchors. Our report compiles the design-builder's load test information, and synthesizes the data into a format that will inform future floating bridge construction.

transportation Highways / Roads
Laboratory GIS / Database Management Geotechnical Engineering Contamination / Remediation Geologic Hazards Design-Build


Golden Pontoon awards presented to Carole Leigh and Monique Anderson


Monique Anderson
Vice President

David Asunskis

Elizabeth Barnett
Senior Geologist

Stan Boyle
Senior Vice President

Gerard Buechel
Senior Vice President

Jeremy Butkovich
Vice President

Christian Canfield
Environmental Staff

Sarah Corbin
Senior Biologist

Scott Gaulke
Vice President

Mike Harney
Vice President, Seattle Office Manager

Oliver Hoopes
Senior Associate

Alia Hubbard
Environmental Staff

Carole Leigh
Senior Associate

Elizabeth Milodragovich
Engineering Geologist

Martin Page
Vice President

Bill Perkins
Vice President, Earthquake Engineering Services

Red Robinson
Senior Vice President, Director of Underground Services

Ali Shahbazian

Agnes Tirao
Senior Associate

Katie Walter
Vice President

Stephanie Wanderer
Senior Geologist

Kevin Wood
Senior Engineer

Paul Van Horne

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