SR 520 Pontoon Casting Facility

CLIENT Washington State Department of Transportation

Location Aberdeen, WA

The SR 520 Pontoon Casting Facility was constructed to create the 33 concrete pontoons needed for the SR 520 Floating Bridge replacement on Lake Washington. 

Built at the Aberdeen Log Yard site in Grays Harbor County, the project's design-build team was challenged with building a casting basin that was larger than three football fields, on soft and liquefiable soil, and ten feet below the adjacent Chehalis River.

Shannon & Wilson provided geotechnical and hydrogeologic services to support the pontoon casting basin during design and construction. We worked closely with the design-build team to provide an integrated design and construction approach.

For the project, we:

  • Characterized subsurface soil conditions using soil borings and cone penetration tests
  • Analyzed slope stability considering static, seismic, rapid drawdown cases
  • Recommended axial and lateral resistance for 130-foot long driven 18- and 24-inch pile foundations
  • Performed a design and production-phase test pile program with dynamic pile tests
  • Developed strong ground motion time histories for the design ground motion
  • Modeled the two-dimensional dynamic soil-structure interaction of the basin slopes and base
  • Evaluated the influence of cutoff wall depth and dewatering well spacing, depth and configuration using a numerical groundwater model
  • Conducted an on-site pumping test and infiltration test to calibrate the numerical groundwater model
industrial Manufacturing
Geotechnical Engineering Geologic Hazards Water Resources Design-Build


ACEC of Washington Silver Engineering Excellence Award for Complexity


Gerard Buechel
Senior Vice President

Jeremy Butkovich
Vice President

Bob Mitchell
Board of Directors - Chair

Paul Van Horne

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