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Millions of dollars must be spent to fix pontoons being built for a new Highway 520 bridge, the state said Tuesday, after inspections on Lake Washington revealed that the pontoons’ worst cracks grew over the winter.

Transportation Secretary Paula Hammond also said the problems likely will force officials to miss their December 2014 goal for opening the floating section of the new six-lane bridge. “I’m hopeful the project will be done within 2015,” she said.

The new costs, yet to be determined, will reach the tens of millions of dollars, and the bill will go mostly to the public rather than contractors — because the most severe cracking was triggered by what Hammond described as the state’s own design errors.

She doesn’t know if the price of the fixes and delays will reach $100 million but says she’s confident it will be less than the $200 million in a contingency budget for the multibillion-dollar 520 project.

Much depends on whether the state and contractor Kiewit-General-Manson can accelerate the job, through double shifts or overlapping tasks, she said.

Disciplinary action will be sought against state Department of Transportation (DOT) bridge-division engineers who Hammond says broke with protocol. Among the problems, she said, was their failure to run models that would have shown the likelihood of cracking at the pontoon ends.

A total of 21 lengthwise pontoons and two end pontoons will be grouted together on the lake for nearly 1½ miles, as a single barge to support the road deck of the world’s longest floating bridge. Some pontoon walls have displayed slow leaks or condensation, raising concerns about long-term durability.

The state chose to design the pontoons itself on a fast track (rather than delegate that responsibility to contractors) as a strategy to attract lower bids and to get the floating section built by 2014, a timeline set by former Gov. Chris Gregoire.

Initially the strategy seemed to work, as the winning bid to build 33 pontoons at Grays Harbor, for $367 million, was $180 million less than the state expected to pay otherwise.

But now, taxpayers and toll payers will surrender much of those savings.

“Everybody wants you to take risks, until something goes wrong,” Hammond said in an interview last week before releasing three investigative reports Tuesday.

Hammond says the errors won’t require higher taxes, because of the contingency money.

But losing the bulk of a $200 million cushion at this juncture puts the state in a vulnerable position, because more cost risks lurk to the west. Future construction phases will churn through Foster Island, shimmy around ongoing traffic at Montlake, and culminate in a yet-undesigned Portage Bay bridge.

Even if the contingency holds up, there’s a much larger problem in paying for the 520 bridge. The $4.1 billion project is $1.4 billion short. The House Democrats’ proposed 2013 transportation ballot measure assumes tolls on nearby Interstate 90 will cover most or all that shortfall.

This winter, divers examined the first four large pontoons in the lake. On the undersides, they found cracks that had been concealed while the pontoons were sitting on the floor of the Grays Harbor casting basin. Similar cracks on top have grown 4 to 6 inches in length since pontoons were towed to Seattle in August, said John Reilly, head of an expert review panel.

These cracks appeared after workers in Grays Harbor compressed the pontoons to strengthen the concrete in a common technique called post-tensioning. Steel bands were cinched at extreme pressures from either end, lengthwise through each massive concrete box. But the geometry of interior walls and beams triggered cracks that eventually reached around the edges to the top and bottom, technical reports say. An untreated underwater crack can let in water at 1 cubic foot per hour.

So now the game plan is to add post-tensioning from side to side to squeeze those cracks shut, or at least stop their spread, Reilly said. Epoxy will be injected, and carbon-fiber patches may be applied. That work will happen by summer, said Jeff Carpenter, chief construction engineer for the DOT.

Hundreds of smaller cracks, blamed by a state audit on concrete-curing errors by Kiewit-General, have been relatively simple to fix by filling with epoxy or spreading waterproof sealant over wide surfaces.

There has never been any doubt that the new pontoons will be strong enough to withstand traffic, lake waves or even light-rail trains in the distant future, said Reilly. The challenge is maintenance over a 75-year design life.

“The product that they will end up with is going to be a good product, safe, sound, and lasting the life of this project,” said Hammond.

Hammond is leaving her post March 8. Gov. Jay Inslee last week named Lynn Peterson, a highway engineer and adviser to Oregon Gov. John Kitzhaber, as the new DOT secretary, seeking a greater emphasis on transit and clean fuels.

Mike Lindblom: 206-515-5631 or On Twitter @mikelindblom