Solution to great 520 Bridge debate could be ... a tunnel

You are heading for Seattle on the Evergreen Point Bridge. Just as you eye the tailgating yachts at a University of Washington football game, you sink below Union Bay. You enter an underwater tunnel, which leads you beneath Montlake and Portage Bay, zipping along by car, bus or light rail.

Then you resurface onto Interstate 5.

A team of engineers has been asked to determine in the next month whether that kind of tunnel technology could be used to replace the Seattle segment of the state Route 520 corridor -- to the delight of many.

Tunnel supporters in Montlake, Laurelhurst, Madison Park, North Capitol Hill, Roanoke and Portage Bay neighborhoods say the technology could break a bridge-design stalemate and end years of wrangling over how to replace the aging span -- the goal of state-appointed mediators.

Engineers from COWI Inc., a Danish consulting engineering firm, told the 520 mediation team last week that while each locality poses different challenges, immersed tube, bored and cut-and-cover tunnels are built worldwide.

COWI engineers expect to present a draft report at a Nov. 20 mediation meeting and a final report to mediators by Dec. 1.

"In Seattle, we think there's a better way to do 520, and one way may be undergrounding it or putting it under water," said Ted Lane, a Roanoke-Portage Bay neighborhood representative on the mediation team.

A 520 bridge design will not be completed when citizens vote Nov. 6 on Proposition 1. The regional $47 billion transit measure includes $1.1 billion toward the estimated $3.4 billion to $4.4 billion cost of a new six-lane bridge. Costs of a tunnel system are uncertain.

But a floating bridge across Lake Washington, which would then drop into a tunnel system beneath Union Bay, is being studied. Supporters say a tunnel option could overcome size, environmental, transit, boating and other concerns in Seattle neighborhoods.

There's a lot to like about a tunnel, said Lane, who was among those who met with engineers last week to discuss tunnel options. Residents are concerned about traffic congestion, air and water pollution, noise and other issues, he said.

"It solves all the environmental problems; there is no environmental pollution," Lane said of tube-and-tunnel technology. "There is no noise. It gets rid of the visual problem."

Another benefit, tunnel supporters say, is the possibility of reclaiming Washington Park Arboretum land now in the 520 corridor and returning it to open space.

Tunnels were not among the options featured in a draft environmental impact statement released last fall by the state Department of Transportation. They are being studied as part of a mediation process mandated by the Legislature. Senate Bill 6099 seeks a six-lane replacement -- two general-purpose lanes and one dedicated transit lane in each direction -- but does not specify a configuration.

The bill also sets aside money for mediators to hire an engineering firm to study the feasibility of "a combination of tunnels and submerged tubes" under Lake Washington; a partial tunnel from I-5 to the west end of the 520 Bridge; and a proposal to move 520 "from its current alignment through the Arboretum."

"We are glad tunnels are still a live issue; people are excited," said Colleen McAleer, a Laurelhurst community representative on the mediation team who also met with the engineers.

"We had a lot of questions, and the engineers were very good listeners. We just want it studied. ... I absolutely believe it could break the stalemate over bridge design."

Louis Hoffer, a Broadmoor resident, welcomes what he calls a long-overdue technical study.

"If you go to northern Europe, Asia, Spain, Turkey or Sydney, Australia, you see tube tunnels; they are built all over the world; why not here?" he said.

Tunnels would relieve congestion and feature a big bonus -- accommodating light rail, Hoffer said. "I want to see people look at all the possibilities."

Craig Dalby, a Seattle resident who commutes across 520 to his job in Kirkland, also spoke with engineers about an Arboretum bypass, which calls for a tunnel beneath Montlake Cut and will be analyzed by COWI.

"I think there are a lot of parts that could be mixed and matched to solve this puzzle," Dalby said.

"That's really what the mediation process is all about -- looking at all alternatives to see if there is something workable, that gains a following, and is not too expensive."

Mediation team members also await answers to other questions, such as whether a tunnel could accommodate bicyclists and pedestrians.

Mike Grady, a mediation team member representing the National Oceanic and Atmospheric Administration/National Marine Fisheries Service and the U.S. Fish and Wildlife Service, said the agencies are concerned about wetlands, the effect of shade on fish habitat and migration from a bridge, interchanges and lake pollution from stormwater runoff.

Theoretically, a tube tunnel from I-5 to I-405 "would probably be the best alternative of all of them ... it would avoid those impacts," he said.

"If you bore a tunnel underneath the lake, you avoid a lot of environmental issues. If you dig down deep enough, it is non-disruptive."

But construction is another issue, said Grady, who, like most mediation team members, has vowed to keep an open mind.

Almost any bridge replacement other than a deeply dug tunnel would be problematic, he said, causing noise, air and water pollution, dredging and turbidity. But with a tunnel, some of those effects likely would disappear after construction.

The state Department of Transportation has studied tube tunnels but has environmental as well as other concerns like possible tribal archaeological sites in the corridor, higher costs, geology (the lake is more than 200 feet deep in the middle) and underwater topography.

"We're interested in what COWI comes up with," said Ron Paananen, 520 project manager and Alaskan Way Viaduct project director.

In the meantime, Paananen said, concerns about bridge safety -- particularly in the event of an earthquake or high winds -- are prompting some steps, like designs for new pontoons that could be installed if the bridge went down.

COWI engineers Casper Paludan-Muller and Poul Marinus Nielsen just began studying the issue and have no conclusions, Paludan-Muller said. They told the mediation team that COWI is working on tunnel projects in 14 countries, including Ireland, South Korea and China.

Each bridge or tunnel project is "unique to its surroundings," Paludan-Muller said.