Some concrete inside a pontoon being built for a new Highway 520 floating bridge crumbled during final construction last week, causing a delay of four to six weeks.

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Some concrete inside a pontoon being built for a new Highway 520 floating bridge crumbled during final construction last week, causing a four- to six-week delay.

This means the damaged pontoon and six others cannot be floated out from Grays Harbor to Seattle until July or August, instead of next month as planned, said Julie Meredith, 520 program director for the State Department of Transportation.

Meredith said there are still many ways for the contractor, Kiewit-General, to recoup the lost time, and finish the floating bridge and road deck on Lake Washington by the end of 2014.

The remainder of the crossing, from Foster Island to Interstate 5, is still in design and lacks at least $2 billion in funding.

The damage happened May 11, while workers at Grays Harbor were post-tensioning the first few pontoons, a common method to strengthen bridges and buildings. Steel cables are stretched lengthwise through a conduit in the concrete and then cinched to extreme pressures — to compress and strengthen the concrete from outside.

Conduits are curved near the ends to help workers secure the cables, said Meredith. But when the cables were tightened on the first pontoon, the curve straightened out, and concrete buckled in multiple places, said Mike Cotton, DOT’s design-build director for the bridge.

State officials say workers will scrape away some concrete in all seven pontoons, add more steel reinforcing bar, then recast the concrete there.

Without such repairs, the bridge might wear out before its official 75-year design life, said Meredith.

The primary pontoons weigh 11,000 tons, are 360 feet long, 75 feet wide and more than 28 feet tall. There are 60 post-tensioning sites on each. Smaller pontoons will support the two ends of the floating bridge, and support it from the sides.

It’s too early to say how much the repairs will cost, or whether Kiewit-General, will absorb the expense, state officials said.

Meredith called Kiewit-General “a great design-builder” on Friday, and said the team still intends to finish by late 2014. The contract requires completion by mid-2015, so contractors risk losing incentive bonuses if delays persist. Time can be gained or lost in several other jobs including columns to be poured near Medina, anchor fabrication in Kenmore, or at another pontoon site in Tacoma.

The new six-lane bridge will replace a four-lane, 1963-vintage bridge that carried 105,000 vehicles a day, until new tolls this year led the figure to drop by one-third.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom.