Tunnel returns to debate over viaduct options

As political momentum grows for a highway-free Seattle shoreline, some would-be visionaries want to help traffic move by digging a deep tunnel from Sodo to north of downtown.

Such a route — reconnecting to Interstate 5 and Highway 99 somewhere north of downtown — would avoid the need for an elevated structure to replace the aging Alaskan Way Viaduct, supporters say. A tunnel would bypass I-5 through Seattle's narrow downtown and the state convention center.

Costs are unknown, but would be in the billions of dollars. Even if a suitable tunnel path exists, Seattle's loose, watery soils present a challenge in places, and there's not much room at the surface for ramp connections at I-5.

That hasn't stopped the Cascadia Center, a branch of the Discovery Institute think tank, from promoting a tunnel. International tunneling executives at a Cascadia forum downtown regaled 100 people Friday with success stories from Europe.

Such a project would have been impossible just a few years ago, but for the first time, tunnel-boring machines can dig a hole 50 feet in diameter, enough for three lanes northbound and three lanes south, on separate decks.

Unlike the tunnel pushed by Mayor Greg Nickels that was rejected by voters in March, this one would not follow the shoreline and would be deeper beneath downtown.

Tunnel advocates need to show political leaders and voters that tunnel benefits outweigh the relatively high up-front price, said John Reilly, former president of the Underground Construction Association and a well-known cost consultant.

"Seattle is probably a world-class example to demonstrate how to do this," he said.

Sound Transit is using a similar, but narrower, machine to grind a one-mile, $299 million tunnel through Beacon Hill with two stations. The machines are long cylinders with rotating teeth at the front.

Seattle officials, hoping to improve the waterfront, oppose a potential $2.8 billion elevated viaduct replacement; Gov. Christine Gregoire said this week she has become open to a "surface-transit" approach, but that other options remain in play. Many drivers enjoy the convenience and waterfront views of today's elevated highway, which carries 110,000 vehicles per weekday.

City Councilwoman Jan Drago said if a workable surface-transit option can't be found, a bypass tunnel merits a closer look. "We need to make better use of the real estate we have ... including our subterranean assets," she said.

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