I was living in the San Francisco Bay Area when the 1989 6.9 Loma Prieta earthquake broke the Bay Bridge. The hardest hit areas were the bridge, the marina area of San Francisco, and in West Oakland where the double decker Cypress elevated freeway collapsed, killing 42 people. Many people wondered why the hardest hit areas were 40 miles from the epicenter of the earthquake with so little damage closer. It was explained that the areas were built on bay fill subject to liquefaction.
“Liquefaction is the scientific process that happens to certain soils when an earthquake takes place. Science has been able to identify in past years areas of weakness where liquefaction could occur should an earthquake happen. Soil liquefaction happens when certain types of loose and clay soils are violently shaken by an earthquake and soil and water come together like small internal floods creating ground failure.” (www.es.ucsc.edu/~es10/fieldtripEarthQ/Damage1.html)
Hayden Island, anchoring the South end of the I-5 Bridge, consists of rich but vulnerable alluvial soil that has been deposited by the Columbia River, soil that is subject to liquefaction. That is, it could act like a liquid with an earthquake. Other indications that Hayden Island soil is unstable are the closing of the hotel southwest of the current bridge and Zupan’s supermarket. Before Zupan’s closed, I was told by in store management that Zupan’s is falling into a slough, the hotel into the river. The current bridge spans have 70 to 90 foot piers built on top of wooden pilings. That is, they do not go to the bed rock and can fall into the river if liquefaction occurs. The replacement bridge piers will go 250 feet deep to the Troutdale rock formation. These piers add significant cost to the new bridge.
The current bridge is claimed by some to have a 50 more years of life. Seismic fixes were examined by a panel consisting of: two each from Washington State and Oregon departments of transportation and various other agencies and engineering firms. The panel considered a retrofit to bring the current bridge to modern seismic standards. A retrofit would cost $260 million to $280 million but would do nothing about Hayden Island. When an earthquake driven ground failure occurred, a retro fit would likely mean the strengthened bridge would simply collapse in fewer pieces.
“Too many unknowns about the foundations of the existing structures and injection grouting for this application suggests that soil remediation may not be a viable option” (CRC Seismic Analysis). Additionally, a retrofit would not correct the numerous other problems with the current bridge.
While we do not have a lot of seismic activity in this area, we are part of the Cascadia Subduction Zone. We had a 6.8 magnitude earthquake at Tacoma in 2001 and a 5.7 at Mt. St. Helens in 1980. A span-collapsing quake may or may not happen soon but when it does, it will make the 2007 collapse of the Twin Cities’ bridge across the Mississippi seem a mere inconvenience, for a fallen I-5 will block shipping on the Columbia for months, cripple inter-state bridge commerce and render commuting nearly impossible. The Glenn Jackson Bridge will not be able to handle the traffic by itself. The impact on our economy will be tremendous. Most importantly there will be likely loss of life. Even building a new, smaller bridge and retaining the old one will not work, as the old can still fall into the river.
One of our recent election candidates told me vehemently that he would support tearing down the 1-5 bridge “only when they tear down the Golden Gate!” For his credibility, it is hoped that the gentleman was unaware of the difference: The Golden Gate Bridge is anchored on rock with piers solidly on bed rock versus our I-5 crossing with piers resting atop wood pilings and anchored on an island subject to liquefaction.
Any suggested alternative that will keep the current bridge in place puts us and our economy at risk. We must build a replacement bridge as soon as possible, for we are on borrowed time. Those who actively oppose the replacement are not seismic or structural engineers. Neither am I, so it would be arrogantly irresponsible to ignore those who are. And the expert consensus is clear: that the only responsible option is to remove and replace the current bridge, much as we would wish for a cheaper fix.
A good friend, a civil engineer with a master’s degree from UC Berkeley, a specialty in soil mechanics and who worked in Caltrans soils research, commented: “Whenever you have a structure built on soil subject to liquefaction and in an area prone to seismic bounces from Mother Nature, you have potential disaster proportional to the importance of the structure and involvement of human life.”
You should check the information supplied here for yourself, and not listen to a politician who either has not read available materials or, worse, knows the truth and still risks our future for political gain.
Ralph E. Schmidt is a Camas resident.