The Tunnel: Seattle’s deep dig

Picture this: Washington’s jumbo ferry, the MV Tacoma, runs aground in downtown Seattle between Safeco and Qwest fields, hangs a left onto First Avenue and then starts burrowing underground and disappears.

Got that?

If you do, then you have a pretty good picture of the size and operation of the machine that will be used to drill a deep-bore tunnel to replace the Alaskan Way Viaduct, said Ron Paananen of the state Department of Transportation.

Paananen is in charge of the overall $4.24 billion project to replace the viaduct with a 1.7-mile deep-bore tunnel, rebuild parts of the seawall and improve the Alaskan Way surface street with a promenade, as well as build a new surface street in the footprint of the viaduct after the structure is removed.

His main focus these days is the $1.9 billion tunnel and the machine that will dig it.

The tunnel boring machine will weigh 5,000 tons, about the same as a jumbo ferry, Paananen said. Its power plant will be 400 to 500 feet long, about the same length as a jumbo ferry. The diameter of the “drill bit” will be almost 55 feet.

Industry experts say it will take 18 months to two years to build that machine, at a cost well over $50 million.

“It would be one of the biggest machines ever made,” said Craig Bournes, product manager for Lovat Inc., a Toronto-based firm that manufactures tunnel boring machines. Lovat built two of the machines for King County’s Brightwater, a project that eventually will carry sewage in underground pipes from an inland treatment plant to a discharge point in north Puget Sound. Those machines are much smaller – less than 19 feet in diameter.

Lovat, which is now owned by Caterpillar, is one of several international firms interested in building the tunnel boring machine for whichever design-build team wins the contract to build the tunnel. Herrenknecht AG of Germany, The Robbins Co. of Ohio, Mitsubishi Heavy Industries, Shield Tunneling Association of Japan, Kawasaki Heavy Industries and WIRTH in Germany are others.

GOING DEEP

When the project is finished, the tunnel is expected to carry 85,000 vehicles a day. An additional 25,000 vehicles will be using the Alaskan Way surface street. The viaduct currently carries about 110,000 vehicles a day.

The tunnel boring machine for the Alaskan Way project will have to dig a single tunnel that is large enough for four lanes of traffic – two lanes stacked on top of two other lanes, each pair carrying traffic in opposite directions.

That single tunnel idea is relatively new. Originally, the DOT was going to build two tunnels – one for each direction of travel. But the technology has advanced to allow four lanes in a single tunnel, and that chops about $600 million off the cost, Paananen said.

This scale of tunnel is not unprecedented. For example, Interstate 80 in the San Francisco Bay area goes through a 1,700-foot long, double-deck tunnel through Yerba Buena Island.

Over the past few weeks, work crews in Seattle have been drilling holes every 100 to 400 feet along the eventual route of the tunnel to see what kind of soil the tunnel boring machine will encounter. The tunnel will be 1.7 miles long and will be as deep as 200 feet in some places, which means most of it will be below sea level. The tunnel has to be deep enough to get underneath a 60-foot-deep railroad tunnel that carries freight trains under downtown Seattle. There also are sewer lines closer to the surface.

Paananen said so far the soil samples show native glacial deposits of sand and gravel along most of the route, but there is fill material at the southern end of the tunnel.

Unlike an earlier proposal for a cut-and-cover tunnel that would have been built under Alaskan Way, the single deep-bore tunnel will be about 500 to 600 feet farther inland, farther away from the shoreline. The southern tunnel entrance will be at King Street and will follow under First Avenue until it gets about one block past the Pike Place Market. At Stewart Street it will veer further to the east and resurface at Denny Way onto Aurora Avenue North, also known as Highway 99.

That means the Battery Street Tunnel probably will be closed and filled in, Paananen said. Its fate lies with Seattle city officials, he said.

SLOW GOING

Tunnel boring machines move at a pace of about 2 meters (6 feet) an hour and are guided by laser. The face of the machine chews up the soil and rock to that 6-foot distance and seals off a chamber at the front of the machine.

Rails will be laid behind the machine. The ground-up dirt, rock and other materials are turned into a slurry and are either piped out behind the machine or are carried out on conveyor belts or rail cars for disposal, Bournes said.

Thereafter, prefabricated concrete sections are bolted into place to form a ring around the circumference of the tunnel. That takes about two hours, although once the work gets under way it could take less time, he said. Then, the machine moves forward to grind up the next 6-foot segment.

It takes about a dozen workers to operate the boring machine, but there are many more workers providing support.

The machine will inch forward underground for all 9,000 feet, emerging at the north portal near Seattle Center like a giant sandworm.

After the tunnel is built, the double-decked traffic lanes will be built.

Paananen said it will take four or five months just to assemble the tunnel boring machine on the lots that have been cleared at the south end of the project to serve as a staging area for construction work.

The state will be buying property and getting easements from each of the property owners so the state can tunnel under their properties.

WORK STARTED

Work on the Alaskan Way project already is well under way. Two Seattle City Light electrical transmission lines that are attached to the viaduct are being relocated. Most people don’t realize that an earthquake that is severe enough to bring down the elevated structure would disrupt much more than traffic. Two-thirds of downtown Seattle would lose power if the viaduct were to collapse.

“It’s not just transportation that’s vulnerable,” Paananen said.

New power lines are being buried in a trench to the east of the viaduct and the switch-over will take place this fall. Other utility lines also are being moved.

A tunnel is better able to withstand an earthquake than an elevated structure, but regardless of which alternative was built, it will have to meet current seismic resistant standards.

The DOT also plans to seek contractors, probably in October, to tear down the southern end of the viaduct, the portion between Holgate and King streets. That is an estimated $300 million job.

It will be nearly two years – mid-2011 – until actual construction work begins on the deep-bore tunnel, but the legislative green light has set in motion more preparatory work.

In September, the DOT will send out a request for qualifications, which is basically a call for engineering firms and contractors to show that they can team up and have the wherewithal to design and build the tunnel.

“We know there are teams of engineers and contractors forming already,” Paananen said.

By the end of October, the DOT expects to announce which three or four teams have been chosen to proceed to the next phase of the project, and compete for a contract that could pay them between $800 million and $1.3 billion, he said.

Each of the finalists will be paid to further refine their proposals, and that will take the better part of a year, Paananen said. The winner of the contract probably won’t be chosen until September 2010.

During the legislative session that ended in late April, DOT officials told lawmakers they needed approval for the deep-bore tunnel concept because they needed to order a tunnel boring machine as soon as possible so they could get on a yearlong waiting list.

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