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Fixing, sealing and strengthening Highway 520 bridge pontoons has cost at least $81.1 million, with more expenses ahead, the state has revealed.

That work, the result of a state design team’s engineering error, has also delayed opening the bridge by at least a year.

Four of the giant pontoons showed severe cracking last year, after they were cast in a basin at Grays Harbor.

A second batch of pontoons was delayed several months, before they could be floated to Seattle in April. The walls were redesigned, and some reinforcing steel was moved partway through construction, to add strength.

But the first batch still needs work. So this summer, contractors are squeezing the boxlike pontoons from either side with steel cables, a common technique known as post-tensioning.

The cracks didn’t pose a risk of bridge failure, but they would have let water seep in, allowing corrosion or erosion to reduce the 75-year design life of the bridge.

The eastern pontoon is currently perched on a dry dock at Vigor Industrial
at the north end of Seattle’s Harbor Island, after being floated from Lake Washington back through the Ballard Locks.

The enormous piece sits on blocks, while epoxy dries within a half-dozen cracks that stretch from the ends down to the underside.

Next comes the cable squeeze, in which wires will be pulled through four conduits on each end, at 1 million pounds pressure. Finally, carbon fiber will be wrapped around the lower 20 feet of the end walls.

“It’s like building your fiberglass boat,” said John Clark, member of an expert oversight panel, and a former engineer for the west landings of the 1963 Highway 520 bridge.

Last winter, workers at Lake Washington planted pillars into the top of that pontoon to support the future highway deck that will slope down from the east shore to the lake level. That led to the odd sight of a tall, spiky structure passing Seattle’s four north-end drawbridges this month.

A second pontoon, 360 feet long, is at dry dock in Portland. A dry dock allows easy access for a faster repair than doing the job in water, but spaces are limited.

So two more pontoons will be fixed in the lake using a coffer dam, a partial enclosure that allows water to be pumped away from the walls before repairs.

The state’s costs are $9.9 million to fix damage to the first batch, when an awkwardly shaped interior wall broke apart at Grays Harbor; an additional $48.8 million to seal cracks in the first batch; and $22.4 million to strengthen the second batch. The state issued change orders in the $367 million contract to reimburse the builders, Kiewit-General-Manson.

Kiewit made mistakes in concrete curing, but those caused mainly superficial cracks of less than 0.006 inches, that have been patched at Kiewit’s expense.

“We are continuing to make progress on what will be the new world’s-longest floating bridge,” said Julie Meredith, 520 program director. About 400 workers are on the project, half of them at Grays Harbor, where casting is under way for the third of six batches.

The DOT’s contractors have built 32 of the 77 pontoons for the 1½-mile floating highway segment. Eight pontoons, of a smaller shape to add buoyancy on the flanks of the bridge, are being floated out of a Tacoma basin this week.

The six-lane floating span, once aimed at late 2014, will now be open to traffic by late 2015 or early 2016, state Transportation Secretary Lynn Peterson said Tuesday. The date hinges on negotiations with Kiewit and partners, she said.

There will be more change orders, worth tens of millions of dollars, to cover design changes to the remaining pontoons, and overall construction delay.

Peterson said $100 million remains in the project’s contingency fund, to cover costs of changes or errors. The state has funded and approved $2.7 billion in construction to date for a $4.1 billion corridor, which will require new taxes or tolls for the Seattle segment.

Earlier this year, the DOT fired state bridge engineer Jugesh Kapur and demoted Patrick Clarke, design manager for floating bridges and special structures.

Expert reviews of the cracking episode say DOT’s bridge division erred by taking the pontoon design and risks under its wing late in the bidding preparations — rushing to meet former Gov. Chris Gregoire’s 2014 goal, and attract lower bids — rather than push the risk of cost overruns to the contractors.

And then, the DOT team designed the end pontoon with 72 post-tensioning bands in the long direction, but did not compress the critical end walls from the sides. The Interstate 90 Lacey V. Murrow Bridge was built successfully in 1993 without such crosswise compression, and DOT was satisfied to do likewise for the 520. Engineers failed to model the forces, according to former Secretary Paula Hammond.

But the 520 pontoons are almost twice as large, so the stresses differed, to the point where walls cracked where they intersected with heavy interior beams. “It’s easy to scale things up from what you’ve done before, (but) some of these things are effects that don’t happen until you get to a certain scale factor,” said John Clark, the expert reviewer.

The 2009 Hood Canal Bridge, buffeted by tremendous winds and currents, was designed to include extra tensioning.

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