Two of the four cracked pontoons for the new Highway 520 bridge will be towed back out of Lake Washington and hoisted aboard dry docks — where engineers think they can be fixed faster than if they stay in the lake.
One pontoon, 360 feet long, will be towed to Portland next week, followed by the slightly smaller eastern endpiece, which will travel in July to Harbor Island.
But two other long pontoons will stay afloat, because supplemental pieces already were attached to their sides. Those appendages make the structures too wide to backtrack through the locks in Ballard.
A single steel cofferdam will be brought to the lake to be wrapped around portions of these pontoons so water can be pumped away. That way, the area around the pontoons becomes dry enough for workers to seal the cracks.
The strategy, announced Thursday, is intended to fix the pontoons concurrently, limiting the cost of delays for the state Department of Transportation and taxpayers.
Already, the contractors and state abandoned their late-2014 completion goal and say a July 2015 finish would be ambitious.
The cracks resulted primarily from design errors by state engineers, and repair costs, including delays, are being negotiated with Kiewit-General-Manson.
“It’s going to be in the tens of millions of dollars,” said Dave Becher, state floating-bridge construction manager.
The losses would consume much of the contingency on an overall $4.1 billion megaproject. The construction contract for the floating highway and east approach is worth $587 million, and pontoon fabrication is $367 million, two figures likely to increase.
Becher said dry-dock repairs should take only about eight weeks per pontoon. By comparison, the in-lake repairs entail six to eight weeks per end, with the single cofferdam to be moved four times, for a total of up to 32 weeks.
“It’s easier to take a pontoon, and take it to a dry dock facility,” Becher said. “You can work on both ends, you can work inside of it.”
But few dry docks are large enough and have space available this summer, he said.
Dive inspections last winter revealed significant cracks on the undersides of the pontoons; cracks on end walls stretched around a corner.
There was never a risk pontoons would sink or fail to carry traffic. But if water is allowed to seep in, long-term corrosion could reduce the 75-year design life of the new bridge, expert reviewers have said.
The main crack-sealing task will require cinching steel cabling from either end to compress the pontoon, known as post-tensioning. Epoxy will be injected into cracks.
Some walls will be wrapped with carbon fiber, which Becher said will provide both watertightness and some structural reinforcement.
Minor cracks of less than 0.006 inch, blamed on errors in concrete-curing temperatures, already have been repaired by surface sealants.
Meanwhile, the tug Alaska Mariner is expected to reach the locks Friday with the pontoon to support the bridge’s west end. It’s part of the second batch of six pontoons built at Grays Harbor, which were supposed to arrive last winter.