People might be surprised how quickly they'll see work begin on the Highway 99 tunnel, after its supporters fended off a political challenge in Tuesday's election. The Alaskan Way Viaduct will close for nine days in October and be permanently severed at Royal Brougham Way South.

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People might be surprised how quickly they’ll see work begin on the Highway 99 tunnel, after its supporters fended off a political challenge in Tuesday’s election.

In a few days, the Federal Highway Administration is expected to sign the environmental-impact statement, freeing the state Department of Transportation (DOT) to tell its tunnel contractors by Sept. 1 to move into final design and construction, starting with Sodo utility work.

The $2 billion tunnel, part of the $3.1 billion replacement for the Alaskan Way Viaduct, is supposed to open in January 2016.

The waterfront portion of the viaduct will remain open while the tunnel is built, but drivers will experience a slower commute and see a lot of activity.

A noticeable jolt will come in October, when the viaduct will close for nine days and be permanently severed at Royal Brougham Way South. After that, drivers will take an S-shape detour onto the viaduct until the tunnel is done.

That’s the start of a four-year effort at the Sodo end of the huge bored tube that motorists, bus passengers and bicyclists will witness as they meander through the congestion. The detour includes new aerial spans that are being finished in Sodo and later will become the permanent southbound highway.

The detour likely will add a few minutes to the average nine-minute delay that West Seattle drivers heading into downtown faced starting in May, when part of the highway was narrowed from three lanes to two each way. There will be no stoplights, but an advised 25 mph speed limit because of curves and merging lanes near CenturyLink Field.

Once traffic is shifted, the viaduct south of Royal Brougham will be demolished. Because of specialized machines that hammer and chomp the old concrete, the pieces will be only a few feet wide and not at risk of tumbling onto the detour lanes, said Matt Preedy, DOT’s Sodo construction director. Similar work to break a First Avenue onramp moved rapidly last winter.

In January, contractors will drive huge vertical piles into the soil, to reinforce what will become the south tunnel entrance. These columns are 5 feet across and will rest against each other, said Chris Dixon, a tunnel-team executive.

As machines dig the tunnel portals, there will be certain layers of soil where they slow to a few inches per scoop, to avoid breaking Native American and settler artifacts. Work at the north portal, near Seattle Center, will require a 30-day pause to examine soil beneath the Denny Regrade, and there could be some digs using hand shovels. Archaeologists will monitor work at both ends.

Meanwhile, legislators in Olympia will be debating tolls, both for Highway 99 and other busy roadways.

The voter mandate — votes counted as of Wednesday showed 59.4 percent approving the tunnel — ought to help Seattle argue for lower toll rates, said Councilman Nick Licata.

“It shows the Legislature that we have a workable majority in the city, and the infighting, if not dropped, has been drastically reduced,” he said. Even the DOT acknowledges that peak tolls of $4 or more, published in the environmental statement, would prompt thousands of drivers to clog city streets.

The project’s giant tunnel-boring machine, 58 feet across and manufactured in Japan, is to be launched near King Street in June 2013, and travel under the old viaduct, Pioneer Square and downtown until late 2014. As the machine grinds forward, concrete rings will be assembled immediately behind it, to fashion the four-lane highway tube.

Views at the surface will be hindered by noise walls, but travelers will be able to peek from the elevated roadway just to the north and south of the portal.

More than 1 million cubic yards of soil will be mined. Trucks will remove it from the portals, but soil from the deep-drilling operation will exit on conveyor belts to barges on the waterfront, Dixon said. The soil will be dumped in a Port Ludlow landfill, across Puget Sound.

Once the tube is drilled, it will take workers another year to install the highway decks, lights, ceilings and exhaust fans. That’s a less exotic phase, but it became notorious in Boston after failures in the so-called Big Dig, where a falling ceiling panel killed a car passenger.

Back in Sodo, workers will erect a new stadium-area interchange and truck overpass. But the northbound stadium exit won’t open until the tunnel opens, said the DOT’s Preedy. Until then, the viaduct’s midtown ramps at Columbia and Seneca streets will be available.

After the tunnel opens, the waterfront portion of the viaduct will be torn down, some 15 years after the Nisqually earthquake launched Seattle’s epic debate over a replacement, and four columns began to sink a few inches.

A related safety project, replacing the central piece of Seattle’s rotting Elliott Bay Seawall, would begin in mid-2013. The city is trying to get that done before 2016, so shoreline traffic can be detoured under the viaduct, said Jennifer Wieland, a seawall-planning manager. The timeline hinges on a prospective property-tax measure passing in 2012, at a cost of $310 million to $390 million. Another phase, to the north, would be financed and built afterward.

The state agreed to finance a new waterfront Alaskan Way with bike lanes, a short bridge connecting Belltown to this new surface route, and a widened six-lane stretch from the ferry terminal to Sodo.

One purpose of putting Highway 99 underground, rather than on a new or fixed viaduct, is to make the waterfront more pleasant. An ambitious ring of waterfront parks, perhaps with lawns, footbridges, small beaches and a new Pier 62, is envisioned in 2018.

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