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The state House of Representatives has approved replacing the Alaskan Way Viaduct with a bored tunnel so Senate Bill 5768 is now back in the Senate, which will have to decide whether it accepts amendments to the bill that were added by the House.
The Senate is likely to make a decision today because it is planning to pass a transportation budget, a Senate staffer said.
One amendment, proposed by state Rep. Judy Clibborn, D-Mercer Island, would require downtown Seattle property owners to foot the bill for any cost overruns on the $2.8 billion project.
The other amendment adopted by the state House on Wednesday would take out a provision that makes state funding for the Spokane Street viaduct contingent on the city of Seattle making street improvements. That amendment was proposed by Representative Sharon Nelson, D-West Seattle.
As recently as last December, the bored tunnel was dismissed as too expensive by the viaduct project team. But then the Washington State Department of Transportation realized it could build a tunnel with a single bore instead of a double bore, and the cost estimate fell by almost $900 million.
“It's less labor, less materials, one machine versus two,” said John White, viaduct program director for WSDOT.
Tunnel boring machines are getting bigger, and it is now possible to build one that is 54 feet in diameter, big enough to hold a tunnel with two 12-foot traffic lanes in each direction, with a 4-foot shoulder on the left and an 8-foot shoulder on the right. The lanes would be stacked inside the tunnel.
To construct a twin-bored tunnel, WSDOT would have had to drill two 40-foot-diameter tunnels as well as cross passages to link them.
“And you have to mine those (cross passages) pretty much by hand,” said John Reilly, a Massachusetts-based consultant who is working on the viaduct project.
The single tunnel will be 54 feet in diameter, wider than any other such tunnel in the world.
Last year two 51-foot diameter tunnels were built in Shanghai, China, according to a report by Arup that was commissioned by the Cascadia Center, which is part of the Discovery Institute. A forceful advocate for the bored tunnel, Cascadia paid Arup $35,000 for that report, according to Cascadia's policy director, Bruce Agnew.
In early December, while the viaduct project team was eliminating the bored tunnel from its list of possibilities to replace the viaduct, Cascadia brought together a group of tunneling experts who wrote a letter to WSDOT saying its cost estimates for the bored tunnel were too high.
The group wrote to WSDOT Deputy Secretary David Dye and said a bored tunnel could be “completed in the 60 months period with a price of $2 billion or less.” That letter was authored by Richard Prust of Arup, Vladimir Khazak of HNTB, Dick Robbins of the Robbins Co., independent consultant Kern Jacobson and Gerhard Sauer of the Sauer Corp.
WSDOT now is taking soil samples along the viaduct alignment on First Avenue in Pioneer Square. It is estimating the tunnel will be 100 feet to 200 feet deep, but it is hoping to make it shallower.
“The deeper you go, the greater the water pressure, the greater the danger to workers,” said principal Peter Chamley of Arup's New York office, who spoke at a forum in March that was organized by the Downtown Seattle Association. Chamley is working on a new Second Avenue subway in Manhattan.
Tunneling conference
Early next month, tunneling experts from around the world will speak at a conference here hosted by the Society for Mining, Metallurgy and Exploration.
The conference will be held May 4 at the Seattle Sheraton, 1400 Sixth Ave. For more information, visit www.smenet.org.
Margie Slovan can be reached by email at margie.slovan@djc.com or by phone at (206) 622-8272
