Viaduct tunnel idea rises again

This region's procrastination over how to replace the Alaskan Way Viaduct has brought unintended results. It's given tunneling technology a chance to mature.

An idea that was once considered foolish — drilling a tunnel along the western edge of downtown — is all the rage this month among well-connected business and political leaders.

Backers point to a new generation of tunnel-boring machines that can ream a tube more than 50 feet in diameter. That's twice as wide as what was possible nearly three decades ago.

"Even in the last two years, tunneling has improved," said Ron Judd, senior adviser to Gov. Christine Gregoire. "Tunnels are becoming a much more attractive compromise."

On Tuesday, Gregoire announced she would postpone until sometime this month a decision over how to replace the aging viaduct. She had been expected to make the decision by Wednesday.

Judd said the delay will give the state time to further study the feasibility of a four-lane bored tunnel, instead of the surface route or elevated highway recommended by transportation planners last month.

The kind of tunnel being considered would run in relatively stable soil beneath First Avenue. It would bear little resemblance to the tunnel that Gregoire and Seattle voters rejected in March 2007.

That earlier version was a cut-and-cover tunnel, to be dug like a trench along the mucky shoreline. One disadvantage to that option is that the highway lanes would be closed for years during construction.

By drilling a few blocks inland, the state might be able keep the old viaduct open until the new tunnel is finished.

The tunnel, however, would have fewer lanes than the existing six-lane viaduct. And state predictions say a bored tunnel would drive costs far beyond the $2.8 billion the state has budgeted for the project.

"Bypass tunnel"

The so-called "bypass tunnel" being considered is one of seven tunnel proposals conjured by the state Department of Transportation since 2001, when the Nisqually earthquake damaged the old viaduct.

It would descend into the ground near a new Sodo interchange, going mainly beneath First Avenue. The path would cross under a century-old freight-train tunnel and the Battery Street Tunnel, before being connected to Aurora Avenue North near Seattle Center.

One general lane and one high-occupancy lane would travel in each direction, stacked within a 54-foot wide tube. There would be no exits at mid-downtown or Western Avenue.

There's no design yet for the north portal at Aurora Avenue, which would need widening so that exit ramps could reach Seattle Center or Mercer Street.

An obvious question is whether that junction would dash the dreams of city officials and developer Vulcan, Inc. to transform car-dominated South Lake Union into a gracious, walkable neighborhood. A tunnel would put more pressure on Mercer Street to connect Highways 99 and Interstate 5.

The tunnel would have no exit to Interbay or Ballard via Western Avenue.

Freight from those areas that now use the viaduct would have to travel on Alaskan Way along the waterfront, or take other surface streets to and from north end of the tunnel, said Ron Paananen, DOT's viaduct project director.

Cool to idea

The state has been cool to the idea of tunnel in the last couple of years, because of the higher cost and the unpredictable nature of drilling underground.

The state thinks it would cost around $2.1 billion to construct a two-mile long tunnel, not counting all the other improvements proposed along the existing Highway 99 corridor from Sodo to South Lake Union.

Tunnel advocates say costs could be reduced.

All the estimates may be suspect, though, because the engineering study is less than 1 percent complete on the bypass tunnel. Based on a long, worldwide history of megaproject overruns, the industry practice is to distrust cost figures until 30 percent of engineering is complete.

The DOT has experience using smaller boring machines when it built I-90 Mount Baker Tunnel. And Sound Transit is completing a pair of thin bored tunnels through Beacon Hill for a light rail line due to open later this year.

Of the $2.8 billion earmarked for the corridor, $1.1 billion is already reserved for the Sodo interchange, bus-service increases and other early-stage work.

The state estimates the tunnel option would drive the entire corridor cost to $3.9 billion.

But the price tag drops to $3.2 billion if you strip out ancillary projects — a new Elliott Bay sea wall, utility relocations, streetcars and expansion of surface streets along the waterfront.

Negotiations are under way among the state, the city, King County and Port of Seattle to tackle funding plans.

"What we're trying to do is, we are very clearly trying to identify who is responsible for what," Judd said.

His remark brings up an interesting scenario. If the governor commits to funding a tunnel this spring, she has arguably kept the state's promise to maintain the ability to move traffic through downtown.

Streetcars, bus lanes, boulevards, or sea walls could be left to local governments. Judd said local leaders would still have a few years to find money for those projects before a tunnel would open.

Seattle Times reporter Andrew Garber contributed to this report. Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com