The Immersed Tube Tunnel: Is the West Seattle Bridge Task Force Ready?

Built in 1959, the George Massey Tunnel carries 80,000 vehicles a day under the Fraser River into Vancouver, B.C.

Retired Civil Engineer Bob Ortblad makes a convincing case for a 500-foot tunnel to replace the failed West Seattle Bridge, currently at risk of a 150-foot collapse. 

https://www.westsideseattle.com/robinson-papers/2020/04/24/op-ed-bridge-history-its-time-immersed-tube-tunnel

Some dredging will be needed to accommodate the two 250-foot segments to retain the 30-foot depth for ship traffic.  The tunnel segments can be built at adequate sites in Aberdeen (520 Bridge pontoons) and Tacoma. 

Earthquake Threat Resolved.  Generally unknown to Seattlites, the West Seattle Bridge sits on a fault, and is subject to seismic liquefaction in an earthquake event.  Bridge support columns can move, crack and fail, as witnessed with the late Alaskan Way Viaduct.  By contrast, immersed tube tunnels rest on the ground.  San Francisco’s BART travels successfully through immersed tube tunnels over several faults.  Japan runs traffic through 25 earthquake-resistant immersed tube tunnels.  The Netherlands operates 30 ITTs under its many bodies of water.  

Funding.  41% of Seattle is water, yet the City provides only $7 million annually on maintenance for all the bridges that span our bays, lakes and rivers.  (Perspective:  the cost to finish Seattle’s stalled streetcar project along First Avenue has soared to $252 million.) The City clearly needs to prioritize the task of moving the 100,000 cars a day across the Duwamish as our paramount transportation project, with strong federal and regional intervention and funding.   

“Skeptic”.  A new way of looking at the West Seattle Bridge replacement is at hand.  The issue is whether the Mayor’s Task Force is up for the challenge of thinking outside the elevated bridge box.  Seattle Times Transportation Reporter Mike Lindblom quotes former Mayor Greg Nickels, Task Force Co-chair, as calling himself  “‘a bit of a skeptic’ about trying heavy construction under the river” (Seattle Times, May 28, 2020, p.A9).  The construction of the tunnel tubes occurs on land (as did the 520 Bridge cement pontoons), not under the river.   

One can hope that the Task Force reaches out for a tried and true engineering solution such as that advanced by Mr. Ortblad.

For an overview of Stritmatter Kessler Koehler Moore coverage of other Highway Design issues, go to www.keithkesslerlaw.com and www.stritmatter.com.