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SR 520 - Pontoon Construction Project

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.


November 2015


Pontoon construction now complete 

Why is WSDOT building new pontoons?

The Pontoon Construction Project broke ground in February 2011 at a 55-acre site in Aberdeen. 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, in order to stage construction of 33 pontoons.

The largest pontoons built in Aberdeen were the 21 longitudinal pontoons, later joined together end-to-end on Lake Washington to form the backbone of the new floating bridge. The largest pontoons ever built in Washington, these pontoons are 360 feet long, 75 feet wide, nearly 30 feet tall, and weigh 11,000 tons.

The 33 pontoons were built in six cycles. Once complete, each batch of pontoons was floated out of the casting basin, inspected, and then towed to Lake Washington. The final three pontoons that constituted the sixth and final pontoon cycle were floated out of the casting basin on March 9, 2015. They were towed to Lake Washington by tugboat and arrived on April 9, 2015.

The End Result

The new SR 520 floating bridge is supported by 77 concrete pontoons that form the floating foundation of the new bridge. Of these, 33 were built in Aberdeen, Wash., (the other 44 were built in Tacoma, Wash., as part of a separate contract). Bridge construction progress is tracked on the pontoon tracking and bridge assembly page.

Project Benefits

The pontoons for the new SR 520 floating bridge improve the bridge’s safety and reliability. The new pontoons support a wider, six-lane roadway with dedicated transit/HOV lanes, wider and safer shoulders, and a 14-foot-wide bicycle/pedestrian path. The new bridge is designed to withstand windstorms of up to 89 mph and be forward-compatible to accommodate the future addition of light rail to the new floating bridge.

The project also supported local job creation for construction, supplies, and supporting services in and around Aberdeen.

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 was 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.

Visit the SR 520 Budget and Performance page for additional information.

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 contact the project office:

SR 520 Bridge Replacement and HOV Program
999 3rd Avenue, Suite 2200
Seattle, WA 98104

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