The first giant pontoons for the new Highway 520 floating bridge should reach the Ballard Locks later this month.

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ABERDEEN, GRAYS HARBOR COUNTY — Sometime in mid-August, giant pontoons of up to 11,000 tons will be towed through the Ballard Locks, leaving just over two feet of room to spare on either side.

The state Department of Transportation is preparing for the journey, after six pontoons for a new six-lane Highway 520 bridge were finished here. They floated out Monday night from a casting basin along the muddy banks of Grays Harbor, making room for work to start soon on the next batch.

The pontoons are the first pieces of what will be the world’s longest floating span, 7,710 feet long, crossing most of Lake Washington east of Seattle. The contracting team says that despite a three-month delay this spring, the floating section can still be finished by the end of 2014.

On Tuesday, final inspections began on the finished pontoons. Engineers expect that to take a couple days, but the DOT might use up to two weeks, depending on whether any crack sealing is needed.

Then they will be towed to Seattle via the Pacific. Special oceangoing tugboats will arrive about Aug. 8, said David Ziegler, principal engineer.

Three of the giant 360-foot-long primary pontoons, a cross pontoon for the east end, and two supplemental stability pontoons were in this first set.

After passing inspection, the pontoons must be towed across the turbulent Grays Harbor bar. Safety requires that waves be 7 feet or less (they were only 3 feet Tuesday).

Pontoons will be towed one at a time, a day apart, each by one tugboat.

They will travel around the Olympic Peninsula at an average 4 knots. The 238-mile trip to Ballard should take about 70 hours, says a technical report for DOT. Four are going to Seattle, the other two to Tacoma for additional work. Six small pontoons were finished and floated out in Tacoma last week.

The ocean trips will be repeated over the next couple of years as more pontoons are finished.

The next batch is to be built by December, when there’s only a 10 percent chance to find a calm day on the Grays Harbor bar.

The Locks

The entrance to Seattle is tricky because of a narrow shipping channel and tight turn where Shilshole Bay meets the Lake Washington Ship Canal, says Dave Carpenter, navigation supervisor at the Locks.

Boaters will be unable to use either the small or large lock while pontoons pass, and most will have the sense to stay clear of the shipping lane then, he said.

Carpenter hasn’t heard yet from DOT about schedules, but he anticipates that the first pontoon would arrive in the daytime, and some kind of public event would occur. But he prefers the others arrive at night, avoiding conflict with summer boat traffic. The locks operate around the clock.

Instead of vessels stopping 520 road traffic — drivers are delayed almost daily by drawspan openings on the lake — this time, pieces of highway will interrupt boating traffic.

At 75 feet wide, the biggest pontoons will fill much of the 80-foot wide lock. Wood bumpers will be set on the sides of pontoons, so there will be less bounce than if air-filled or rubber bumpers touched the lock walls, said Carpenter.

He estimates that a pontoon and tugboat would need 30 minutes to maneuver into the lock, 10 to 15 minutes while the water level is raised, and 15 minutes to exit into Salmon Bay.

Visitors will be able to watch from the railings and hillsides, though parking might be crowded. “It’s great access. You’d never get it at any other lock-and-dam,” Carpenter said.

Asked about arrival times, DOT spokeswoman Suanne Pelley said, “None of that has been decided yet.”

Pontoons will continue directly to their final location in the lake, offshore from Medina. The cross pontoon will be anchored first. The first main pontoon will be moored nearby until a pair of stability pontoons can be attached.

The passage through the locks will be repeated, until 77 pontoons are anchored on Lake Washington.

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