State rejects Portage Bay tunnel

A citizens' suggestion to include a tunnel through Portage Bay in a future Highway 520 replacement has been rejected by the state, which argues the bay tunnel segment alone could cost as much as $8 billion.

It also would be extremely difficult to build interchanges that connect to the main highway under water, said a report released yesterday by the state Department of Transportation.

The tunnel concept is one idea meant to protect the Montlake neighborhood, which has staved off a bigger above-ground bridge replacement for years.

The state's proposed bridge replacement could be twice the size of what's there today, with six lanes instead of the current four, plus several ramps, which many neighbors consider unacceptable.

Two local activists, Bill Mundy and Theodore Lane, advocated the "tubes" idea, which the state admits would significantly reduce noise. It includes tunnels that would be immersed in the water, or just below the lake floor.

Backers say it would re-connect neighborhoods that are severed by today's 43-year-old freeway, and preserve nearby Foster Island.

But the state study says that the job would require separate tunnel segments in soil, water and wetlands, and that construction work would tear up the island. In addition, the highway would require construction of a new "transition island," where the tubes would rise from the lake floor to a fixed bridge section.

The state's preferred surface highway, including a new floating bridge, would cost around $3 billion, or less than half the cost to tunnel Portage Bay alone.

The report was prepared by Parametrix, the same firm that has worked on the state's other 520 plans.

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