Construction on the Highway 520 bridge is finally getting down to water level this week, as workers will soon build the fixed sections that extend up from Lake Washington to the Eastside.

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Construction on the Highway 520 bridge is finally getting down to water level, as workers will soon build the fixed sections that extend up from Lake Washington to the Eastside.

Barges and cranes should arrive Thursday next to the old bridge, followed soon by noisy pile driving, and some excavation onshore.

This phase of the massive bridge project includes huge foundations and concrete spans near shore, twin sloping transition spans that reach down to the lake, dozens of floating pontoons, and road decks fastened upon them.

Early on, a cofferdam made of vertical steel walls will be inserted just onshore, allowing crews to dig out a wide basin to pour the first concrete footing. Another cofferdam and pour are planned in the lake in August, after summer salmon runs are past.

Construction workers and machines have already spent months excavating Eastside lands to expand the highway, and hoisted girders to build mammoth parklike lids.

This week’s new activity offshore adds proof that nearly 15 years and $288 million spent on planning, design and engineering will beget a bridge. The Kiewit-General-Manson construction team aims to complete the eastern high-rise and floating segments of the new six-lane bridge by the end of 2014, though the $587 million segment contract allows a mid-2015 finish. The partnership recently built a new Hood Canal bridge.

Spires and belvederes

This week, the public is getting its first glimpse of the final design, for the east and floating sections.

Images show that the state Department of Transportation thinks of the new bridge as not just a passageway but an attraction, whose bike and pedestrian features surpass those on any other Washington state highway.

The bike-ped trail, separated from traffic by a concrete barrier, is 14 feet wide, or four feet broader than the Interstate-90 bridge trail.

Five turnouts called belvederes will allow people to step out of the trail traffic, said John White, DOT’s floating-bridge project director. The belvederes protrude like ears over the lake, have wind shelters, and could include interpretive signs about floating-bridge history.

High-rises at the two bridge ends will feature twin towers that begin near water level and rise in straight lines topped by pyramidal beacons. The spirelike, tall shapes resemble an early 20th-century office tower, or an obelisk. They’re inspired by smaller sentinels built for the 1940 Lacey V. Murrow floating bridge down the lake.

Green lights will be trained on the towers, while amber lights shine out from the beacons.

“Sentinel Elements identify regional gateways between land-based and waterborne structures and signify an arrival onto the world’s longest floating bridge,” says a portfolio of design standards by VIA Architecture.

White said the simple concrete sentinels, less elaborate than some landmark bridges have, fit the linear, uncluttered form of the bridge — and serve a function, because the towers will include stairways for bridge maintenance crews.

The bridge’s official design life is 75 years, but managers think it will last longer. Even the 1963 bridge survived 50 years, and the new one will benefit from advances in concrete chemistry, to make pontoons more crack resistant, White said.

The new fixed spans will be supported horizontally by concrete box girders, like the high West Seattle Bridge, but flatter.

White said DOT wanted a modern, slim shape — and to break from the busy-looking, overhead steel truss that defines the 1963 floating bridge. Tolling devices, fastened to the truss now, will move to land near the first Eastside lid.

“We’re hoping people will be satisfied and happy with this bridge for 100 years,” said DOT Secretary Paula Hammond. “I think it’s very elegant.”

Big work ahead

Drivers this summer will see the second cofferdam in the lake, right at the spot where tolls are being subtracted from their Good to Go accounts.

After that cofferdam is built, water will be pumped, and workers will scrape the silty lake bed down to solid soil. A large rectangular slab for the foundation will be cast there, 170 feet crosswise by 40 feet lengthwise, and 12 feet high. It will support two columns, one for each side of what will be a split road deck near shore.

The bridge splits here for two reasons, said White: to allow a cherry-picker machine to lower maintenance workers between and beneath the dual decks, and so a wide median can exist onshore, where big columns must support the heavy parklike lids.

Piles will be driven just offshore so that when pontoons arrive, they can be tethered to the pilings and fastened together as a group, with the primary lengthwise pontoons attached to sideways supplemental pontoons.

Pontoons won’t be floated out of their casting sites in Grays Harbor and Tacoma to the lake until several weeks from now, said spokesman Jeff Switzer — a change from initial plans to float through the Ballard Locks in April. Pontoons and decks are scheduled to be done by late 2014. At that point, drivers could go partway to Seattle across the lake on the new bridge, then detour onto the west part of the narrower 1963 bridge.

The state remains at least $2 billion short of funding the entire $4.65 billion project.

Hammond says the DOT is seeking a federal loan to speed construction of the next piece: fixed spans westward from Foster Island to the Montlake exit. A state amendment forbids that phase until financing is assured for the entire corridor including Montlake and North Capitol Hill lids.

But the federal government might provide literally a “bridge loan” to reach Montlake a few years sooner, while the state begins collecting the remaining money to go all the way, through I-90 tolls or new taxes. In this scenario, one of three westbound lanes on the new bridge peel off at Montlake Boulevard for a few years, until the whole route to Interstate 5 is done, said Hammond.

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