A look back at the West Seattle Bridge process; Is a tunnel still a better long term solution?
When the long delayed decision to repair the West Seattle Bridge was made on Nov. 18 it was the culmination of a process that involved dozens of online meetings, and thousands of work hours by the Seattle Department of Transportation (SDOT). It was primarily an engineering driven decision but the practical need to restore the corridor to service was a major consideration.
In the end Mayor Durkan took the repair path to help business and the public as soon as possible with safety as the overarching requirement.
When the bridge damage was first seen as near catastrophic, SDOT took what some saw as a drastic step. They closed the bridge that day. As time has passed, that decision was examined by multiple engineers and was seen as the right move (though some questioned why it did not happen sooner).
Then began the more drawn out task of evaluating the physical status of the bridge, including reinforcement to assure that the structure would remain standing, followed by the development of potential replacement scenarios.
To make that happen SDOT set forth some terms in their initial Request for Qualifications. That RFQ was initially a completely “bridge-centric” document. Only after prodding by City Council member Lisa Herbold was an amendment made to include any tunnel building experience. Still the terms included were restrictive.
Here they are:
Minimum qualifications are required for a Consultant to be eligible to submit a proposal response. Your submittal response must show compliance to these minimum qualifications. Those that are not responsive to these qualifications shall be rejected by the City without further consideration:
1. The prime consultant firm must have a minimum of 10 years of experience in bridge planning, design and construction in a seismic zone 4 urban, marine environment.
2. The proposed team shall include leads for each key discipline who are licensed Professional Engineers in the State of Washington, and the lead structural engineer shall also be a licensed Structural Engineer.
3. The prime firm shall have 3 similar bridge projects ($200M or above in total project cost) in the last 10 years with a government agency, or an equivalent combination of large and/or complex bridge projects in the last 10 years.
4. The team shall have experience in delivering federally funded projects (FHWAUS DOT), projects with complex funding structures and alternative delivery methods.
Reading these you might wonder why, with a project whose effects are not just city-wide but echoing out to the state and region, and costs potentially in excess of $600 million, it was set up this way. Why was it at first limited to “bridge planning, design and construction” with no consideration of any tunnel building experience? Why was it limited to only Professional Engineers in the State of Washington? Why not seek the best in the world as has been done in projects in other states?
This is a common practice but must it be?
Note the language in section 3 referring to “bridge projects” again suggesting that a bridge was the only way. How was the RFQ promoted? Did it go out to a world wide audience of potential applicants? Or was it largely limited to those who had done business with SDOT before? It was published on a City of Seattle procurement website How would firms from outside the are even know about it?
As a side note the United States Geological Survey states: “If you live in seismic zone 4, you have a one in ten chance that an earthquake with an active peak acceleration level of 0.4g (4/10 the acceleration of gravity) will occur within the next fifty years.” More on this below.
During one of late stage meetings with the West Seattle Bridge Task Force a set of alternatives was released. In that presentation SDOT shared a series of potential choices all presented as “in-line” options to keep the options “equal” that is comparing “apples to apples.” except that it wasn’t a serious comparison. They showed potential fixes for the bridge in the repair options and some different choices for the replacement option including one suspension bridge that would have meant making the bridge three times as long. But more interestingly they showed “tunnel option” in the same footprint as the existing bridge whose presentation featured a hastily drawn piece of art showing the path for the tunnel heading deep underground in a completely impractical fashion. Privately, they admitted this was a mistake. No one had suggested such a path and no one took it seriously. But no attempt was made to update or correct it.
It's not new
The tunnel concept for crossing the Duwamish River is not new. In fact it was discussed after the previous West Seattle Bridge was put out of commission by the freighter impact in 1978. But at the time, it was not considered practical. Since then tunnels and tunnel construction have come a long way. The public’s appetite for tunnels was soured understandably by the SR99 tunnel’s problems which centered on driving the world’s largest tunnel boring machine under downtown Seattle to create a corridor with fewer exits and less capacity.
When local civil engineer Bob Ortblad brought up the idea again, Westside Seattle was the only media outlet to take it seriously, at first. Then the Seattle Times took note and as usual other Seattle media outlets picked up the story. Ortblad produced dozens of explanatory graphics, held numerous Zoom meetings, did interviews and took part in the task force meetings. He had reasonable, if somewhat incomplete answers to most of the questions and presented a very cogent case for his concept. But the sticking points at least initially were how would a 1500 foot long tunnel work coming up on the west side? How would it connect to the existing ramps? There were other questions about how a ramp down from the Spokane Street Viaduct might conflict with Terminal 18 access, how railroad tracks might be disrupted by construction and more. Ortblad had answers. The west side would attach at 6% to the existing ramps and the tangle of roads there now would be replaced by a (much misunderstood and widely feared) roundabout to get traffic up to Delridge Way. The design of the ramps on the east would be high enough to simply bridge over the Terminal 18 entrance. Railroad tracks would have temporary in-ground trestles built later to be incorporated into or replaced by reinforcements in the cut and cover approach to the tunnel. No matter, the bridge proponents had other plans.
Ortblad offered his thoughts on the process:
"SDOT's analysis of a proposed West Seattle Immersed Tube Tunnel (ITT) has exposed Seattle's lack of innovative and collaborative and infrastructure investment planning. SDOT, the Port of Seattle, and Sound Transit did not work together to design an Immersed Tube Tunnel but individually created a series of objections to an ITT.
Seattle and Gothenburg, Sweden are both major ports with similar populations. Gothenburg, the world’s most sustainable city, has an innovative and collaborative one-hundred-billion euro infrastructure plan that extends to 2035. The just completed $225 million Marieholm Tunnel (ITT) is part of this plan.
Initially SDOT publicly disparaged the ITT. Public pressure forced an ITT evaluation. SDOT then spent $250,000 on a ITT evaluation by a bridge engineering firm. SDOT got what it wanted, an impractical and costly ITT design.
The SDOT design was too big, too deep, too long, and too costly. American engineers lack ITT experience, and european and asian engineers were not likely consulted. SDOT’s estimate of $2 billion and ten construction years is wildly inflated. Gothenburg just completed a six-lane ITT for $225 million that is twice as long.
The ITT design that was proposed in the WestSide Seattle story “Is A West Seattle tunnel the answer?” (May 1, 2020) calls for a 500-foot immersed tube for a channel depth of 30 feet as required by the Army Corps of Enginners. SDOT’s ITT design as proposed by engineering consulttant WSP was designed for an unnecessary channel depth of 45 feet which require a 1,650-foot immersed tube. They made it this depth to presumably prevent damage to the tunnel from passing ships but that depth is not necessary for that purpose. Since it creates a much longer tunnel, that means they need fans but these too are not required since ventilation could be achieved through vertical vents on the shorter tunnel on both sides of the river. Their plan results in a 400% increase in the cost of the immersed tube segments. The SDOT design also estimated that a new costly graving-yard would be require, This is not necessary, the cut and cover excavation ITSELF can be use to cast two 250-foot segments as demonstrated in Gothenberg.
SDOT’s cost benefit analysis is not an equal evaluation of all alternatives. Of the six alternatives evaluated only the ITT included capacity for light rail. The additional cost of light rail is included the ITT alternative, but the benefit is not."
In the cost benefit analysis SDOT consistently noted that a tunnel would last 75+ years, making it roughly equivalent to the replacement bridges they had proposed. Their own consultant, WSP, the company who wrote the CBA stated in their online presentation about the immersed tube tunnel project under the Elizabeth River (once one of the most polluted rivers in the nation) that the tunnel will last 120 years. That tunnel it should be noted is 4224 feet long. The Duwamish crossing ITT would be (even with WSP's greater suggested length) more than 2500 feet shorter.
In the long form Cost Benefit analysis WSP and SDOT in section 3.1 note:
“The site is located within the mapped northern boundary of the Seattle Fault Zone, an east-trending zone of reverse faulting, which is considered capable of producing a M7.0 event.”
The current bridge is literally around 140 feet away from the Seattle Fault, and the soil of Harbor Island is highly subject to liquefaction.
They acknowledge that a Magnitude 7.0 event could occur (a 75 year event) but don’t really look at the 2500 year event standard now in place for new construction. By all accounts from WSDOT and other agencies across the nation, a tunnel is the most seismically resilient structure. SDOT pointed out that the existing bridge is in better shape than the other bridges in their inventory.
Above ground structures are much less secure in seismic events than tunnels due to the way seismic waves propagate. It's possible to build an essentially earthquake proof bridge, Los Angeles has built one to replace their 6th Street Viaduct. The result is smaller than any of the proposed West Seattle Bridge replacement bridges and the cost, approaching $500 million.
And they further note:
“A detailed seismic assessment was not conducted for this conceptual stage of the project;”
So how is it possible to make an adequate determination of ANY of these alternatives if this is the case?
On page 18 of the City of Seattle office of Emergency Preparedness it states:
“Surface, marine, and air elements of Seattle’s transportation system are exposed to earthquake hazards. Liquefaction is a common element to this exposure. Most of the Duwamish Valley is a liquefaction zone. Both of Seattle’s major north-south corridors, I-5 and SR99/SR509 run through this zone, as well as key bridges and elevated structures, including the Alaskan Way Viaduct, the West Seattle Bridge, the First Avenue South Bridge, and approaches to the end of I-90. The King County International Airport is completely in the liquefaction zone as are most of the city’s rail and marine terminals. “
A megathrust earthquake, a 2500 year event that exceeds the 7.0 M level would likely render either a repaired bridge or a replacement bridge inoperable. But a tunnel would likely survive.
Since the CBA pointed out that seismic safety was at the top of the list, how is it that it was not given more weight?
What about concerns about permits?
There was a lot of discussion in the eight month long process about avoiding "in-water work" and about the potential delays given the Superfund status of Harbor Island. No documentation about these concerns was provided.
Westside Seattle reached out to both the Washington State Department of Ecology and the Environmental Protection Agency (EPA) and learned that neither had been contacted regarding any potential tunnel construction (only seismic reinforcement of the existing bridge was discussed). The concerns about potential permitting issues are valid and given their ten year projected timeline, much of that suggested time frame would ostensibly be waiting to get permits. This is something SDOT suggested would be possible to "accelerate" in their proposal for a bridge replacement since no in-water work was required.
But the EPA has conducted studies on Harbor Island for more than two decades, doing new follow up studies every five years. Even a cursory look at their latest report shows that the area in question for a cut and cover approach to an ITT shows that there are no contaminants in the soil. This suggests that since the area is so thoroughly and regularly studied the acquisition of permits might even be much faster. The river bed at the site itself also shows no sign of contaminants.
King County it should be noted regularly dredges the lower Duwamish at a rate of hundreds of thousands of cubic yards a year, and Vigor shipyards is embarking on a brand new salmon estuary at the mouth of the Duwamish some 400 yards from a potential tunnel dredge project.
Sound Transit is planning to bring light rail to West Seattle
The alignment for light rail that Sound Transit has proposed runs either south of the bridge (meaning cutting a new tunnel into the hill below Pigeon Point) or north. Right now they are planning on building yet another 150 foot tall bridge either north or south of the High Rise bridge. But an ITT tunnel matches that alignment too and would also carry traffic and freight traffic too in built to accommodate it. Any bridge configuration would need to be built to a 2500 year standard meaning it would need to be big, expensive and take a long time to build. Sound Transit has a seven year horizon to get something built to cross the Duwamish and get much of the work done toward getting light rail in place by 2035. Recently major cost increases were announced that show an increase in costs of over 53%. How likely is it that those are low estimates?
The corridor is essential
At some point the existing West Seattle High Rise Bridge will need to be replaced. SDOT said it will last anywhere from 15 to 40 years. That is unless a major earthquake strikes. Only a tunnel could be built WITHOUT closing the existing bridge until it was time to switch traffic to it. This is not to suggest that there would be no delays or traffic tie ups. But no two to three year delay would be required.
In the notes of the decision to repair the existing bridge SDOT acknowledged that at some point the bridge will need to be replaced. As part of those notes they announced that a Type, Size and Location study would be conducted on two bridge replacement choices as well as an ITT. That study is expected to take a year to complete. So, sometime in 2021 or early 2022 they will have a much better idea of the specifics regarding costs, mechanical requirements, and time required to build a bridge replacement.
Based on the experience of immersed tube tunnel projects around the world it's clear that regardless of the fears expressed by SDOT that a tunnel would be faster, more seismically secure, align with the preferred light rail path, and let the existing bridge stay in service during construction.