Gates Prepped at 520 Pontoon Construction Site

Aberdeen, WA - There’s a new doorway at the State Route 520 Pontoon Construction Project site – a huge, 30-foot-tall portal designed to keep the waters of Grays Harbor at bay until next spring when the first batch of concrete pontoons is ready to float out.

Crews used a 160-ton crane this week to install the enormous gate sections at the four-acre casting basin, where 33 pontoons are being built for a new SR 520 floating bridge in Seattle.

Tacoma’s Jesse Engineering Co. built the gate in three sections, each weighing 50 tons and measuring 110 feet long, 10 feet tall and 10 feet thick.

“Installing this gate is absolutely essential to the project,” said Dave Ziegler, Washington State Department of Transportation project engineer. “It’s our front door and will allow us to open and close the casting basin to float out the completed pontoons about every six months.”

WSDOT and Kiewit-General crews are carving a launch channel between the casting basin gate and Grays Harbor, which means excavating 82,000 cubic yards of dirt between the two. The gate will keep the water out until each cycle of SR 520 pontoons is completed and ready for float-out. The first of six cycles of pontoons is scheduled to float out in April 2012.

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During these float-outs, smaller side gates will open to slowly fill the basin until the water level equalizes on each side of the large gate and the pontoons begin to float. A crane will then remove the gate sections, and tugboats will tow the pontoons – some of which are the length of a football field – out of the basin to a mooring site in Grays Harbor and later to Lake Washington.

Once the first float-out operation is complete and the tide reaches its lowest level, the gate sections will be put back into place and the remaining water pumped from the basin. The basin is then ready for crews to set exterior wall forms for the next cycle of pontoons.

“This project is on a fast track,” Ziegler said. “Even as crews are building pontoons inside the basin, we’re constructing interior walls for this cycle and future cycles of pontoons in the construction areas outside of the basin. That way we’re ready to install them as soon as they’re needed. This streamlined process will continue through the final cycle and float-out in 2014.”

WSDOT plans to use the casting facility to construct 33 new pontoons for the SR 520 floating bridge. Work on the casting basin started in February and is scheduled to conclude this fall. Construction in Aberdeen is part of a $367 million project that includes the casting facility and pontoons.