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SR 520 - Pontoon Construction Project

Status: Construction
Thumbnail of Pontoon Construction Project site map

See larger map and view of the new pontoon casting basin.

Project Photos

Photo by Soundview Aerial Photography.

Work at the SR 520 Pontoon Construction Project site in Aberdeen started in February 2011. View the progress.


April 2015


Pontoon construction now complete 

Why is WSDOT building new pontoons?

The aging SR 520 floating bridge on Lake Washington is vulnerable to windstorms and needs to be replaced. Pontoon construction was an essential step for building the new SR 520 bridge. The Pontoon Construction Project broke ground in February 2011 at a 55-acre site in Aberdeen. There, WSDOT and contractor Kiewit-General (K-G) built a casting basin facility featuring a concrete batch plant, on-site water treatment, and a 4-acre casting basin, where pontoon construction began later that year.

In Aberdeen, crews constructed 33 pontoons, 21 of which are 360 feet long, 75 feet wide, and nearly 30 feet tall. These "longitudinal" pontoons are the largest ever built in Washington. Typically built in sets of six pontoons at a time, each cycle of completed pontoons was floated out of the casting basin, inspected, and then towed to Lake Washington, where they will serve as the floating foundation for the new SR 520 bridge. An additional 44 stability pontoons were constructed in Tacoma, Wash., under a separate contract.

The End Result
The 33 pontoons built in Aberdeen, Wash. - together with the 44 pontoons built in Tacoma - will support the new SR 520 floating bridge on Lake Washington, providing a safer connection from Seattle to the Eastside for at least 75 years.

Project Benefits

The pontoons for the SR 520 floating bridge will improve safety and reliability. They are designed to:

  • Withstand a 1,000-year earthquake event.
  • Withstand windstorms of up to 89 mph.

The new pontoons will support a wider, six-lane roadway with dedicated transit/HOV lanes; wider, safer shoulders; and a 14-foot-wide bicycle/pedestrian path.

What is the project timeline?

  • 2010 - Publish draft environmental impact statement.
  • 2010 - Issue final environmental impact statement.
  • 2011 - Receive Record of Decision from FHWA.
  • 2011 - Begin and complete casting basin facility construction.
  • 2011 - Begin pontoon construction.
  • 2015 - Pontoon construction completed.

Financial Information

The project is funded through the following sources:

With the passage of ESHB 2211 in the 2009 legislative session, WSDOT has been authorized to collect tolls on the SR 520 floating bridge since 2011.

This project had a construction change order of more than $500,000. View all change orders.

How can I get more information?

For comments and questions about the project, or to be added to the mailing list or e-mail list, please e-mail

To listen to a recording of the latest project information and leave a verbal comment or question, please call the project hotline at 1-360-591-8012.

To contact the project office by mail:

Pontoon Construction Project
999 3rd Avenue, Suite 2200
Seattle, WA 98104

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