An immersed tube tunnel could replace the damaged West Seattle bridge. it would be 500 feet long and be installed by digging approach trenches on either side, then a trench in the river bed of the Duwamish into which two 250 precast tunnel sections could be sunk and the tunnel formed.

Civil Engineer (ret), MBA, CPA

All the king's horses and all the king's men

Couldn't put Humpty together again.

The West Seattle-High Level Bridge is a shallow arch bridge supported by two box girders. These box girders have a thin concrete shell. Restoring the strength of these cracked girders can be compared to super-gluing a cracked egg. Cracks have crossed the box girder floors and climbed up the walls at 45 degrees to within inches of their roofs. 

These cracks are forming a 200-foot long, 4,000-ton upside-down keystone.

Even if the bridge is somehow repaired, it will be forever crippled. Nevertheless, the city plans to spend a stopgap $33 million to shore it up and prevent a complete collapse.

Let’s not repeat past mistakes. The current West Seattle Bridge was built on fill and loose alluvial soil. To support the bridge, 200-foot pilings were driven into firm glacial deposits. Pile caps and 150-foot bridge piers were built on these pilings. Then a 15,000-ton, 590-foot concrete arch was built between the center bridge piers. An earthquake would liquefy much of the fill and loose alluvial soil, leaving the heavy center arch swaying 350 feet from solid glacial ground. The current bridge was also built with a 6% grade making it dangerously slippery during wet and icy conditions. This grade also increases traffic danger by slowing heavy trucks and transit vehicles.

Let’s consider a safer alternative that can be far less expensive and faster to build.

In 1956, three decades before the West Seattle Bridge was built, British Columbia built the George Massey Immersed Tube Tunnel (ITT) under the Fraser River. Before SDOT spends millions on bridge consultants, they should travel through this tunnel under the Fraser River to Vancouver.  Every day 80,000 vehicles use this 4-lane tunnel. The tunnel is now beyond capacity and in a unanimous vote, Metro Vancouver’s board of directors has backed a task force’s recommendation to parallel the aging George Massey tunnel with a new eight-lane immersed tube tunnel, with six for vehicles, and two for light rail.

Metro Vancouver considered a high-level bridge but rejected it.

After British Columbia, SDOT should fly to Japan and travel through Japan’s 25 earthquake resistant ITT’s. Stop in Hong Kong and travel by car or train through the five ITT’s that connect to the mainland, then drive to Macao through the world’s longest ITT (4.2 miles).  Fly to the Netherlands and tour their 30 ITT’s.  The Dutch are the most prolific builders of ITT’s and world leaders in their design. 

Rotterdam’s Maastunnel was completed in 1942 was Europe’s first ITT and is the model for 150 others worldwide.

Washington State has a long history of casting concrete boxes and using them to create floating bridges across Lake Washington and Hood Canal. The Duwamish River is only 30 feet deep and about 500 feet wide. Two 250-foot long concrete boxes could be cast in Tacoma or Aberdeen and floated to the Duwamish, then sunk in a dredged trench. East and West down ramp trenches of about 1,000 feet long would connect to the tunnel.

An ITT can be built much faster and cost less than a new bridge. The George Massey Immersed Tube Tunnel built in1959 is four-times longer and almost twice as deep, and was built in two years and two months.  An ITT needs no costly piles, bridge piers, or complex, heavy, long span center arch. An ITT is an ideal soft ground solution. Its natural buoyancy, makes it almost immune to earthquake liquefaction.

A tunnel offers a simple and economical design for light rail. One of Sound Transit’s proposed routes to West Seattle is north of the swing bridge.  It would be an ideal location for an ITT because construction could start even while the current bridge is being shored or later demolished.

Submitted by Rhondda James (not verified) on Fri, 04/24/2020 - 4:03pm

I so agree. I'm from Australia and we did the same think under the Bridge to cross the Sydney Harbour.

Submitted by Randolph Urmston (not verified) on Fri, 04/24/2020 - 4:59pm

This proposal makes sense. I hope SDOT responds. Why is this not the best approach compared to repair or replacement of the defective bridge from cost, schedule, earthquake, mass transit, minimum disruption and other considerations.

Submitted by Brian N (not verified) on Fri, 04/24/2020 - 7:49pm

Good suggestion for connecting the east and west sides of the waterway. But this "down-ramps" don't address the connection to the existing roadways or any new roadways. It also doesn't address the demolition of large sections existing elevated roads which would no longer be needed and would be too steep to connect to the suggested down-ramp. While the ITT may be a cost effective replacement for the span over the waterway, the additional costs to connect either end of the tunnel could likely make this design a tough sell.

Submitted by Tamson Fischer (not verified) on Fri, 04/24/2020 - 7:49pm

Well thought out and certainly deserving of further consideration.

Submitted by Marguerite (not verified) on Sat, 04/25/2020 - 8:46am

Finally, something that makes sense!

Submitted by john (not verified) on Sat, 04/25/2020 - 2:53pm

"The current bridge was also built with a 6% grade making it dangerously slippery during wet and icy conditions. This grade also increases traffic danger by slowing heavy trucks and transit vehicles.

Let’s consider a safer alternative that can be far less expensive and faster to build." Bob Ortblad

---But then Mr. Ortblad ignores the stated gradient safety concerns over existing 6% bridge grades by proposing tunnel 6% grades? In terms of gradients, this proposal makes no difference.

Submitted by john (not verified) on Sat, 04/25/2020 - 4:04pm

The proposal requires the same 6% grade as the existing bridge, so that is no improvement in safety?.

"The current bridge was also built with a 6% grade making it dangerously slippery during wet and icy conditions. This grade also increases traffic danger by slowing heavy trucks and transit vehicles."

Submitted by patr on Sat, 04/25/2020 - 5:46pm

Yes, to keep the approach trenches short at 6% grade is needed. However like the Vancouver's George Massey Tunnel 500 to 600 feet of the trench approaches ( More than 50%) will be covered and protected from the weather.

A tunnel has a beginning down grade allowing constant or accelerated speed that can give a heavy vehicle momentum for the exiting up grade.   

A bridge has a beginning up grade that slows a heavy vehicle, and on the bridge down grade a heavy vehicle my have to apply its brakes.

I need not want to make my article too long adding this explanation.

Submitted by Steve Clancy (not verified) on Sat, 04/25/2020 - 6:03pm

Of course there are probably engineering problems that would need study but this seems so sensible and doable it should be given serious consideration.

Submitted by Perry Cole (not verified) on Mon, 04/27/2020 - 9:54am

This sounds like a great idea and clearly worth moving forward with.

Nearly retired Structural Engineer Perry R. Cole, P.E., S.E., F.ASCE

Submitted by Vince Welnick (not verified) on Mon, 04/27/2020 - 12:25pm

So would the east-west traffic that is currently on the elevated roadway be on surface streets? Getting across this road would be problematic for those wishing to go N-S.

Submitted by F.K. Plous (not verified) on Mon, 04/27/2020 - 3:05pm

Ortblad's suggestion has a lot of merit. Immersed tunnels actually go back 60 years earlier than the Massey tunnel in Vancouver. The first one in North America was the double-tracked Michigan Central Railroad tunnel under the Detroit River connecting Detroit with Windsor, Ont. Construction was started in 1906, and the tunnel was opened in 1910. The whole tunnel is 1.6 miles long, but the immersed section measures only about a half-mile. The steel tubes are sunk in a trench about 30 feet below the surface, and the entire tube structure is covered with concrete delivered from a barge through tremie chutes. The tunnel has suffered no structural failures and has never had to be closed.

In 1943 the Chicago Rapid Transit Co. opened its first subway, which included twin immersed tubes laid in a trench under the Chicago River on the alignment of State Street. The underwater immersed section is about 300 feet long. In 1951 the Rapid Transit Co.'s successor, the Chicago Transit Authority, constructed what is now called the Blue Line Subway, which crosses the north-south segment of the Chicago River twice in twin immersed tunnels under Lake Street and Congress Street. Both tunnels have given good service ever since.

The Detroit and Chicago tunnels both rest in trenches cut into blue clay, a tough, stable material sitting atop the Niagara dolomite that is found around most of the Great Lakes basin. It is a much firmer material than the alluvial soil under the Duwamish, but the geologists and civil engineers no doubt have ways of stabilizing that type of soil. An immersed tunnel probably is the best answer to Seattle's problem.

Submitted by David Allen (not verified) on Mon, 04/27/2020 - 7:22pm

The ITT tunnel sounds like the best and simplest solution. Hopefully our City will choose this option, rather than building another gigantic bridge with all the attendant cost, design, safety and other problems.

Submitted by Jeff Stapleton (not verified) on Tue, 04/28/2020 - 9:20am

Interesting idea; however, SDOT is not going replace the entire bridge with a new tunnel, when it is only the main three span segment (375’, 590’, 375’) segmental box-girder that is in distress. It is my understanding that the approach spans with prestressed concrete girders are fine, and don’t need to be repaired or replaced at this time.

If the existing main segment cannot be repaired, I would suggest replacing it with a new three span (345’, 650’, 345’) extradosed concrete box-girder bridge. Similar to the new St. Croix River Crossing recently constructed in Minnesota/Wisconsin. This type of superstructure with extradosed stay cables would have increased long-term durability, plus better ductility for seismic events.

Submitted by Steve Colony, PE (not verified) on Thu, 04/30/2020 - 3:22pm

I disagree, On the W Seattle side a 1000 foot approach tunnel would take you to the bottom of a very steep slope. If people think the 6% slope on the bridge is bad, think about the new road that would need to be made up to 35th. I do agree that the bridge may be toast, a rebuilt may be needed (hopefully the existing foundations could be re-used.

Submitted by Margi C (not verified) on Wed, 05/20/2020 - 10:38pm

Wow. I love the fact that Mr. Ortblad seems to understand the true urgency of the situation and is innovating an actual solution, unlike SDOT that is lacking on both counts. I wonder what the next step would be to have the city consider it. I imagine a real feasibility study is far more expensive than what we can fund as residents but what if Mr. Ortblad convened a panel of noted engineers from each of the regions that are experts in these tunnels and each can provide their expert opinion on this proposal? Get the media to cover it, and then the City/SDOT can't ignore this. Our small business owners, commuting residents, people with medical conditions that need regular access to Seattle and others don't have 10 years to wait for SDOT to come up with a solution.

Submitted by L Rose (not verified) on Mon, 05/25/2020 - 5:31pm

Interesting ideas. A cable span bridge would be nice, especially if some concrete structures can be retained on either end. The tunnel in the Duwamish River is a good idea too, but there may be connecting streets that would need to be widened to handle the traffic on both ends. Either way, I'd like to see light rail incorporated into any design so that we don't have to build something, then tear down for light rail and build again.

Submitted by Chief English -ret. (not verified) on Mon, 06/01/2020 - 3:57pm

Several tunnel options are available. Hopefully SDot will seek professional assistance on RAPIDLY scoping and selecting best/ quickest viable options. Region is known as a 'tunnel city' with safety as a high priority. First sprinkler road tunnel in world (Battery St), STransit tunnels, WSDot retrofits and AWV which is one of the safest on the planet. Top tunnel experts live in region.

Submitted by patr on Mon, 06/01/2020 - 6:14pm

Please see the follow up story that describes why

1. An immersed tube tunnel is the only real tunnel option

2. The ramps required actually work very well.

3. Why repairing, rebuilding or replacing the high bridge with another bridge is a mistake.

https://www.westsideseattle.com/robinson-papers/2020/05/30/why-not-duwa…

Submitted by Randy FILLINGIM (not verified) on Sat, 06/06/2020 - 10:10am

Logically - it makes the most sense in terms of time money and being able to do the job. of course that’s what worries me. And if the city of Seattle cannot see this- then it is speaking with a ‘forked tongue.’ Sigh..........

Submitted by Steven (not verified) on Tue, 06/09/2020 - 5:22pm

To me it makes much more since to build the tunnel. Taking in facts of cost and time. Why repair a bridge that has already shown flaws in the architecture and has only another 10 of use behind it. Let 's think smart for once and do the right thing.

Submitted by Harry R. Sohl (not verified) on Sun, 06/14/2020 - 11:02pm

I’d suggest dedicated lanes for emergency vehicles and/or buses (similar to what the lower deck provides now):

1) there is no hospital in WS.

2) even with light rail there will likely still be buses. Perhaps many buses.

Submitted by Andy Gidey (not verified) on Sun, 06/21/2020 - 9:44am

I think the cable idea seems to me great idea because

Submitted by David W Kannas (not verified) on Fri, 07/10/2020 - 2:37pm

Yes, tunnels are the wave of the present as well as the future. The cost is less, and the completion time is reduced. Then there is the cost: less than a new bridge. Do it! Just get her done!.

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