Because a deep-bore tunnel as wide as the one approved by legislators this week has literally never been attempted anywhere in the world. (Of course, the Discovery Institute thinks it can be done without cost overruns, and who are we to question them?)
UPDATED: Sorry for the subscriber-only link; here's an excerpt:
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.
A bored tunnel of this size (54 feet in diameter) has never been built anywhere in the world. Most tunnel projects experience cost overruns, even when using conventional engineering techniques. Boston’s Big Dig had cost overruns of $10.6 billion. A cost overrun here of that size would cost Seattle residents approximately $35,000 per household under the new legislation.
there are no specific plans, in place, for the area now occupied by the viaduct.
The middle part is almost entirely tourist/pedestrian.
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