A tunnel is the most expensive option for replacing the viaduct, taking the longest to build, is also the least disruptive to traffic.

A tunnel would take nine-and-a-half years to build and complete and is estimated to cost $3.5 billion before adding hundreds of millions in parallel improvements to streets, transit and the freeway.

But, during construction, the existing viaduct could remain in use.

The construction times for building a replacement for the viaduct were presented to the Alaskan Way Viaduct Stakeholders Advisory Committee on Nov. 20, and discussed in their meeting Nov. 24.

Seven of the eight scenarios schedule demolition of the viaduct during the third or fourth year of a project. A bored tunnel scenario would postpone it until the seventh year, long after Gov. Gregoire's ordered demolition in 2012.

Traffic going through downtown could be switched from the viaduct to the tunnels with little interruption.

Still the waterfront would face another two or three years of disruption while the viaduct was demolished and a surface scenario is built there.

Total construction would take nine-and-a-half years, said Gordon Clark, with the Washington State Department of Transportation, reporting to the advisory committee Nov. 24. It was incorrectly stated Nov. 20 as taking 101/2 years.

A tunnel-boring machine would take one-and-a-half years to build off site, during which crews would move utilities and begin construction on tunnel portals. The machine would be transported to Seattle, needing six months to reassemble here. It would start drilling at the south end, taking 14 to 16 months to bore a 40-foot tube under downtown. It would be turned around at the north end to bore a second tube.

Engineers are considering buying two machines, at $60 million each, to bore both tubes at the same time, but they save only six months. Fire, safety and ventilation systems could be installed in the first tunnel while the second is being bored. And engineers doubt trucks, trains or barges could carry away the debris from two boring machines fast enough.

Engineers also consider buying one large machine, boring a single tube 52 to 54 feet in diameter, which could contain two pairs of stacked lanes. But only one other tunnel in the world is as large. A larger tunnel may be easier for fire, light and safety systems, but ventilation would be more challenging, and more expensive.

A tube this large would be at "the bleeding edge of technology," said Clark. "And the cost would be about a wash."

Forty-foot tubes, on the other hand, are common.

For any project, engineers study schedules that are "construction efficient" or "traffic efficient." Construction efficient means the viaduct is replaced quickly. Crews take over city blocks for staging materials and construction. Parking disappears and little traffic gets through the site - or access is closed entirely -for months, even years.

Traffic-efficient construction means crews must also keep lanes open for cars, trucks and transit while they work. Any closures would be kept as short.

"Costs could go up 10 percent, and construction time two years, with the inefficiency of staging construction around the traffic," said Ron Paananen, deputy director of the Urban Corridors Office at Washington State Department of Transportation.

The worst scenarios for traffic are a cut-and-cover tunnel or a lidded trench. During a construction-efficient schedule, any waterfront roadway would be closed - for demolition of the viaduct, rebuilding the seawall, and digging of a tunnel - for three to nearly four years.

In contrast, a traffic-efficient schedule would close northbound traffic for only three months. Northbound traffic would be detoured off the viaduct onto Western Avenue to get to the Battery Street Tunnel. Southbound traffic would use Broad Street to get to a surface Alaskan Way. Detours could last one-and-a-half to three years while building a lidded trench, three to nearly four years for a cut-and-cover tunnel.

Digging these tunnels on the waterfront could take comparatively longer than boring a tunnel. The dirt on the waterfront is contaminated with wood and brick, debris used to fill in behind the seawall as it was originally constructed.

Completion of a waterfront tunnel would take six-and-a-half to eight years.

While an independent elevated or an integrated highway is built, northbound lanes could be closed for three years, and southbound traffic rerouted over Broad Street to Alaskan Way. Construction would take seven years under a construction-efficient schedule

Under a traffic-efficient schedule, the northbound access would be closed only three months, then rerouted on Western Avenue. Detours could last four years, using surface streets, then moving onto partial completed elevated lanes. Completion would take eight years.

None of this includes the time to develop the underside of an integrated elevated highway with residential and commercial buildings.

Similar detours would last two years for any of the three surface street scenarios. Construction would last five to six years.

To ease traffic during any construction, many other projects are on a fast track.

A new loop exit from Spokane Street will direct traffic from the viaduct toward Fourth Avenue, and be completed by 2010. King County Metro is buying new hybrid buses to get drivers from West Seattle and Ballard out of their cars and onto transit during construction. RapidRide from those neighborhoods begins in 2010.

These projects must be in place before replacement of the southern half of the viaduct. Construction of a new northbound overpass between Holgate and King streets begins in 2009.

The department of transportation will present economic impacts - new construction jobs, loss of downtown business, longer commutes and freight travel times - to the Alaskan Way Viaduct Stakeholders Advisory Committee Thursday evening. This is one of the last of the measures evaluating the eight scenarios.

Then the state department of transportation will dissemble the building blocks of the eight scenarios, and recombine the most promising into two or three "hybrid" scenarios. These hybrids will be presented to the advisory committee Dec. 11. The committee will discuss them at that meeting and the next, Dec. 18.

A public meeting, where the public may comment on the hybrid scenarios, will be 5:00 to 7:30 p.m., Tuesday, Dec. 15 at Town Hall downtown.

The city, county and state departments of transportation plan to recommend a single, final scenario to the mayor, county executive and governor, by the end of the year.

Matthew G. Miller may be reached via wseditor@robinsonnews.com