SR 520 - Pontoon Construction Project - Completed March 2015


The Pontoon Construction Project broke ground in February 2011 at a 55-acre site in Aberdeen, Washington. WSDOT and contractor Kiewit-General built a casting-basin facility, featuring a concrete batch plant, on-site water treatment and a 4-acre casting basin, to stage construction of 33 pontoons for the new SR 520 floating bridge.

Bridge pontoons are the foundation of a floating bridge. The largest pontoons built in Aberdeen were the 21 longitudinal pontoons, later joined 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 Aberdeen-built pontoons were constructed in six cycles. (The other 44 pontoons for the new bridge were built in Tacoma, Washington.) Once complete, each batch of pontoons was floated out of the casting basin, inspected, and then towed to Lake Washington. The final three pontoons from Aberdeen left the casting basin on March 9, 2015. Tugboats towed them to Lake Washington, arriving on April 9, 2015.

Learn more about the pontoons by reading our online SR 520 floating bridge booklet (pdf 6.8mb) 

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. 

How you benefit

Improves safety
The pontoons for the new SR 520 floating bridge improve the bridge’s safety and reliability. The new bridge is designed to withstand sustained winds of 89 mph – a 100-year storm. In addition, the bridge’s roadway deck is higher above the water than the old bridge’s roadway, which keeps windswept waves from washing over traffic during storms. The raised roadway also gives WSDOT crews ample room to perform important maintenance and safety work from the pontoon deck beneath the roadway.

Relieves congestion
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. Also, the bridge was designed to accommodate the addition of light rail, with a retrofit, if voters choose to fund this cross-lake transit option in the future.

Economic incentive
The pontoon construction project created hundreds of local jobs and supported the local economy in and around Aberdeen through the purchase of supplies and supporting services.

Partnerships & cooperation
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., by contractor K-G. The other 44 were built in Tacoma, Wash., as part of a separate contract. 


WSDOT awarded a $367 million contract to Kiewit-General for the Pontoon Construction Project. The following sources funded the project:

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.

The 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.


Contact us


206-770-3554 (project info; staffed 8 a.m.-5 p.m., Mon-Fri)

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

Thumbnail of Pontoon Construction Project site map
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photo of pontoon casting basin
Photo by Soundview Aerial Photography

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