Can the West Seattle Bridge be replaced by an underwater tunnel? »Publications»Washington Policy Center

In late March of this year, due to the crack widening by two feet in two weeks, officials from the Seattle Department of Transportation (SDOT) closed traffic on the West Seattle Bridge.
While SDOT officials tried to stabilize the bridge and determine if the bridge could be saved or if the bridge must be completely replaced, they asked the designer for advice on bridge replacement. , In case we are now able to perform short-term repairs to reopen the bridge as soon as possible, but in the next few years, design support is still needed to replace the bridge. "The contract value ranges from US$50 to US$150 million.
Initially, the New York City Qualification Requirements (RFQ) for engineering companies appeared to be limited to bridge alternatives. However, as community support increased, retired civil engineer Bob Ortblad also enabled New York City to include tunnel alternatives in the RFQ. The City of New York has created an appendix to the inquiry sheet, which states: "Other alternatives will be evaluated as part of the contract, including but not limited to tunnel and sound conversion coordination options."
Interestingly, before finally deciding to become the current West Seattle Bridge, Seattle officials considered nearly 20 alternatives in 1979, of which two tunnel alternatives were eliminated. They can be found in Alternative Methods 12 and 13 in the Final Environmental Impact Statement (EIS) of the Spokane Street Corridor. "Due to high costs, long construction time and high destructiveness, they were removed from consideration."
This is not without objection, because a member of the public who participated in Harbour Island Machine Works commented on EIS: “They dug the tunnel out of the ground at too high a price, and no one provided any figures. Now, what is the figure I am asking, Or have they ever tried it?"
The immersed tube tunnel (ITT) is very different from the SR 99 tunnel. When using the "Bertha" (tunnel boring machine) to create the 99 tunnel, the immersed tube tunnel was cast on-site on a dry dock, then transported and submerged under water installed in the water.
Japan has 25 submerged tunnels. A more local example of ITT is the George Massey Tunnel under the Fraser River in Vancouver, British Columbia. The tunnel took a little more than two years to build, including six concrete sections, and was installed in five months. Ortblad believes that the tunnel through Duwamish will also be a fast and affordable way to build. For example, he provided the 77 SR 520 pontoon needed to cross Lake Washington – just two sunken pontoons can cross Duwamish.
Ortblad believes that the advantages of tunnels on bridges include not only reducing costs and accelerating construction speed, but also long service life and strong earthquake resistance. Although the replacement of bridges in the event of an earthquake is still susceptible to soil liquefaction, the tunnel has neutral buoyancy and is therefore largely unaffected by large earthquake events. Ortblad also believes that the tunnel has the advantages of eliminating noise, visual and environmental pollution. Not affected by bad weather conditions such as fog, rain, black ice and wind.
There are some inferences about the steep slopes entering and exiting the tunnel and how it affects the passage of the light rail. Ortblad believes that the 6% reduction in overall results is because descending 60 feet is a shorter method than rising 157 feet. He added that a light rail passing through a tunnel is much safer than running a light rail over a 150-foot bridge over the water. (I think light rail should be completely excluded from the discussion of alternative options for the West Seattle Bridge.)
While the public is waiting to hear whether Seattle DOT will seek alternative products, it is good to see that the public is participating in viable alternatives. I’m not an engineer and I don’t know if this will work, but the suggestion is interesting and worthy of serious consideration.


Post time: Nov-02-2020
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