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Bored By All Those Viaduct Choices? Think Again

By: Glenn R. Pascall
Puget Sound Business Journal
March 16, 2007


This week's voter rejection of both Alaskan Way Viaduct replacement options on the ballot in Seattle reflects the fact that each has serious drawbacks that prevent it from being the clear choice. The surface-street option, the default winner in the election, has its own challenge: gridlock or a massive shift in commuter behavior.

A viaduct rebuild sacrifices the rare opportunity to remove an urban eyesore and reconnect Seattle’s downtown with the waterfront, while a cut-and-cover tunnel involves high costs and/or design compromises. Both options risk severe construction disruption. Moreover, the route is located in highly unstable ground conditions, with poorly placed fill and soft marine deposits.

The question is whether there is a way to liberate the part of the city that has been violated by the viaduct, while maintaining essential transport capacity, holding construction disruption to an absolute minimum, and financing the project’s dollar cost in the very low billions.

There may be an answer: the Bored Tunnel Alternative. Truth in packaging: I’ve been working as an adviser to a group that has been examining alternatives over the past few months and has focused on this one as the most promising.

Members of the dialogue include Bruce Agnew, director of the Cascadia Center; John Wilson, a principal at the Gallatin Group; and Gary Lawrence, a principal at Arup consultants. As of this writing, they are poised to put forth the case for such an approach soon.

Bored tunnels have been built around the world for decades, but the technology of boring machines (called “moles”) has advanced by leaps and bounds in recent years. This has enabled larger diameters (up to 51 feet), increased productivity and greater control of ground movements in a wide variety of conditions.

Recent bored roadway tunnels include the M30 tunnel in Madrid, the SMART tunnel in Malaysia, the 4th Elbe Crossing in Germany, and the A86 West Tunnel in Paris. This type of technology is being used in Seattle by Sound Transit on its Beacon Hill tunnel, and is proposed for the University Link Extension and for King County’s Brightwater project.

Several years ago, state transportation engineers evaluated a 2.5-mile bored tunnel and ruled out this option because of high estimated costs. However, cost data for recent bored tunnel projects around the world indicate huge gains in economy of construction. Projected or actual costs of $270 million or less per mile are a small fraction of the original Washington state Department of Transportation estimate, and far below costs for an elevated structure or a cute-and-cover tunnel.

A combination of factors makes bored tunnels in this country more expensive than elsewhere. Cost estimates for projects on Interstate 710 in California and at the Port of Miami run up to $860 million per mile. However, these projects would accommodate port-related heavy truck traffic that would not be allowed either on a new elevated structure or a tunnel here.

Even so, using this top end conservative baseline for Seattle translates into $1.5 billion for a single-bore, 1.75-mile tunnel, or $3 billion for a double bore that would carry six lanes. The compact length of the bored tunnel is due to more direct routing through downtown.

Another advantage: When the bore is deep enough that the surface area above the tunnel is not disturbed, construction disruption for a bored tunnel is minimal. This means no open excavations, no utilities diversions (other than those associated with demolishing the viaduct) and no trucks through downtown streets. The vast majority of the visible work would be focused at the portals.

Many possibilities exist for locating a single or twin bored tunnel configuration. All options have a common theme: They would be located in firm ground, away from the very poor fill material and soft marine clays along the waterfront. The alignment would likely run from near the stadiums to the south of downtown and follow downtown avenues before connecting to State Route 99 in the vicinity of Denny Way or Mercer Street.

The tunnels would run at least 40 feet below ground and would pass below the existing freight rail tunnel. They would also bypass the existing viaduct, which would be kept open during construction, thus avoiding the large economic costs of disruption related to proposed alternatives.

The completed portals would represent valuable pieces of real estate. The portal structure can be designed to carry high-rise building loads that would allow future development of the site, adding to residential property values near downtown and stimulating new development in the surrounding area.

The options that have been put forth suffer from shortcomings related to cost, capacity, and design impacts. If this were not the case, the protracted debate would have long since ended and agreement reached on a preferred alternative. But the debate continues because all parties find themselves defending deeply flawed approaches.

The bored tunnel alternative is a proven technology that could break the deadlock among advocates. It combines capacity with minimal disruption, at an affordable price, and offers the bonus of reconnecting the waterfront and downtown.

Glenn Pascall’s column appears regularly in the Puget Sound Business Journal. Pascall is an economist who has taught and done research for the Evans School of Public Affairs at the University of Washington. He has directed economic impact studies for the aerospace and wood-product industries, among others, and developed strategies for state economic vitality and affordable housing.






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For More Information: Cascadia Project — Bruce Agnew
208 Columbia St. — Seattle, WA 98104
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