Any plan to tear down the Alaskan Way Viaduct to make way for a tunnel or a new viaduct will be a big undertaking, as Washington State's construction projects go.

In terms of costs, the completion of the I-90 floating bridge linking Mercer Island to Seattle was similar in size to a potential viaduct replacement, said Ron Paananen, the project director for the Washington State Department of Transportation.

But Paananen points out that there are other factors to consider besides costs and concrete.

"It isn't the sheer size of the project, it's where its located, Paananen said. "We've built tunnels, we've done seawalls, and we've (relocated) utilities. But it all rolls up into a pretty compact area."

That area, a 3.5-mile chunk of land between the Duwamish industrial center, downtown Seattle, Interbay and Elliot Bay, contains seven city planning areas with commercial, residential, retail and industrial designations. The site is also made up of 13 zoning codes, city-defined special districts like Pioneer Square, environmentally critical areas and various shoreline designations.

Regardless of what form a new structure takes, an estimated 1,100 people would work in the project corridor, with numbers swelling to as many as 1500 during periods of intense construction.

The project will take from 6 and a half years to 10 years, depending on which alternative is chosen and how severely vehicle access is restricted.

Generally, the project phases are described as beginning with relocating utilities, then building a temporary bridge for access to the Colman Dock Ferry Terminal, building the seawall, removing the old viaduct, replacing the main portion of the viaduct from Pine Street to the Battery Street Tunnel and then replacing the Alaskan Way surface street.

Even the opening salvo of viaduct construction, relocation of utilities, poses challenges.

The state estimates that the bulk of the utility relocation would take some 30 months. That will include relocating and rebuilding water mains, dozens of fire hydrants and hundreds of manholes.

Sewer lines must also be relocated and some can be buried 140 feet below the street and can be very large, in contrast with a typical water main, which might be two feet in diameter. One sewer line in the project boundary, the Mercer Street Tunnel, is more than 14 feet in diameter and capable of storing more than seven million gallons of sewage.

Replacing the utility corridor under the viaduct will also mean untangling years of telecommunications infrastructure latched onto and buried under the old structure. More than 100,000 linear feet of fiber optic cable and approximately 20,000 feet of telephone wire used by hundreds of thousands of customers and affecting more than a dozen telephone companies and cable operators will have to be moved.

So will high voltage transmission lines with attendant vaults, transformers and switches operated by Seattle City Light. Private energy utilities under the viaduct include a natural gas line from Puget Sound Energy and the "Seattle Lateral" portion of the Olympic pipeline, owned by BP, which distributes the bulk of Washington State's refined petroleum. The Seattle Lateral delivers gas to Harbor Island, where trucks and barges distribute to end points around the region.

Finally, there are steam pipes to consider. Seattle has an almost 190 miles of steam pipes - some dating back to the 1880s - still in operation and supplying heat to about 200 buildings. The biggest customers of the Seattle Steam Company, which distributes the steam under a contract with the city, include Harborview, Swedish and Virginia Mason medical centers, which depend on the steam for heat.