OP-ED- West Seattle - Cable-stayed Bridge vs. Immersed Tube Tunnel
By Bob Ortblad
The economy and beauty of a cable-stayed bridge make them the default choice for spans from 500 to 2,500 feet. A Duwamish span would be 950 feet. SDOT has recently hired HNTB engineers to design a bridge or tunnel to cross the Duwamish. HNTB is the winner of many awards for its cable-stayed bridges. As professional engineers, they will try to be objective in the analysis of a bridge versus a tunnel. However, their experience and the seductive beauty of a cable-stayed bridge may distort an objective view. Beware, political and engineer hubris may tempt West Seattlites with a new cable-stayed Duwamish bridge.
To have a fair comparison, SDOT should hire an equally qualified immersed tube tunnel (ITT) engineering firm. An ITT is a much better option because it can be constructed in half the time, at half the cost of a bridge, accommodate light-rail, and be much safer in an earthquake. An ITT can also restore a bit of scenery and tranquility to the Duwamish River.
Let’s not repeat history.
A new bridge across the Duwamish is a high-risk solution. The West Seattle Bridge is failing under its own weight and the Seattle Department of Transportation has no idea why. Failure could be due to an engineering error, construction mistake, earthquake, or heavy traffic. The Duwamish mudflat is a horrible site for any style bridge. Two hundred feet of mushy alluvial soils sit on top of the Seattle Fault. The United States Geological Survey predicts this soil would liquefy in a moderate earthquake. Washington State Emergency Management warns a Seattle Fault earthquake will result in heavy damage.
Delays and cost overruns
A Duwamish cable-stayed bridge would be almost 50% longer than the current bridge.
- In 2018 Houston started to build a very similar billion-dollar bridge across its ship channel. After two years of construction and $422 million spent, construction was halted Jan. 2020. A score of engineering errors was discovered and the engineer was fired.
- In 2018 an engineering error caused a cable-stayed bridge near Bogota Columbia to collapse, killing 10 construction workers.
- Work was halted November 2019 on Corpus Christi’s New Harbor Bridge, a billion-dollar cable-stayed bridge. There is uncertainty to its future cost and schedule.
- The Port of Long Beach, CA cable-stayed bridge will open this October after seismic reinforcement delayed completion by a year and added $500 million to the original $1 billion budget.
- In 2016 the Nipigon River cable-stayed bridge failed six weeks after its opening, temporarily severing the Trans-Canada Highway.
- In 2009, $50 million was spent to replace 25-year-old cables on the Hale Boggs Bridge, New Orleans.
Washington State has two iconic cable-stayed bridges, one on the Columbia River between Pasco and Kennewick, and a second over Tacoma’s Thea Foss waterway. There is no need for a third. A risky landmark Duwamish cable-stayed bridge is seductive, but the wrong option to reconnect West Seattle.
Some useful links
Modeling a Magnitude 7.2 Earthquake on the Seattle Fault Zone in Central Puget Sound
Seismic Stability of the Duwamish River Delta, Seattle, Washington
Immersed tunneling - underwater is not a boring tunnel solution
Engineers raise ‘significant concerns’ over the design of $1 billion Ship Channel Bridgehttps://www.houstonchronicle.com/news/transportation/article/engineers-ship-channel-bridge-project-houston-area-15118122.php
What is next for $1 billion Houston bridge after county fired bridge designer?https://www.click2houston.com/news/local/2020/08/13/what-is-next-for-1-billion-houston-bridge-after-county-fired-bridge-designer/
Colombian under-construction bridge collapsed, killing ten workershttps://www.thestructuralengineer.info/news/colombian-under-construction-bridge-collapsed-probably-due-to-high-loads
TxDOT: New HarborBridge changes may add delays, higher price tag
The Port of Long Beach, CA cable-stayed bridge
Nipigon River cable-stay bridge failed 6 weeks after opening
Mistakes added $20M, 2 years to state bridge project -Hale Boggs Bridge
Bob pack it in. The tunnel while a feasible option has major problems.
1: The Duwamish is a polluted mess. The environmental risk of disturbing the soil in the area and potentially spreading PCB's and heavy metals that have embedded themselves in the sediment over time is a non starter. Your looking at potential delays and cost overruns galore.
2: The grade differential isn't super appealing to trains. So the combined approach with light rail is DOA.
3: Economically the cable stay is a non starter as well. The span really dictates a pre cast or CIP segmental twin bridge again. Maybe multi cell to add stiffness to the bridge.
4: Guarantee if they can fix it and kick this can down the road 15 years for someone else to deal with that's the path they will choose.