If floating 11,000-ton concrete pontoons in Seattle’s Lake Washington wasn’t spectacular enough while the joint venture of Kiewit/General/Manson constructs a new State Route 520 floating bridge, now you can look into the water and see an oversized steel box.
In an effort to repair cracked 360-ft-long pontoons—these are new, never-used-before pontoons, mind you—a 660-ton steel coffer cell arrived in the SR 520 floating bridge construction zone last week.
Measuring 35 ft tall, 45 ft wide and 144 ft long, the coffer cell will attach to the ends of pontoons U and V. Once in place, crews will pump water out of the coffer cell, allowing them a portable drydock to complete epoxy injections, crystalline waterproofing, transverse post-tensioning and carbon-fiber wrapping to repair the damaged pontoons.
By using the coffer cell on Lake Washington for about the next four months, the $49 million fix proves really the only way to repair pontoons that had already been attached to smaller supplemental pontoons. Project managers also say the coffer cell will allow them to keep the new bridge on schedule.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He also writes for TIME, Popular Mechanics, Sports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.