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

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.

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.

Well thought out and certainly deserving of further consideration.

Finally, something that makes sense!

"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.

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."

Bob Ortblad replies:

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.

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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…

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..........

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.

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.

I think the cable idea seems to me great idea because

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!.