| OTIA III State Bridge Delivery Program |
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| Web Brief (Jan 07) |
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ODOT works to protect bridges and emergency routes during seismic events
As Oregonians, we have become accustomed to the occasional mild earthquake. The infrequent rumblings may send the dog into a barking fury or wake us up in the middle of the night, but they rarely cause serious damage.
But a significant earthquake—so strong as to occur only once every 500 years—might disrupt water, power and transportation, or injure people. And rescue efforts or evacuation could be hampered if a subsequent phenomenon known as soil liquefaction damaged Oregon’s structures—particularly our bridges and the connecting roadways. To help strengthen vulnerable infrastructure, ODOT is currently applying increased seismic standards on its projects, including bridges in the OTIA III State Bridge Delivery Program.
Soil liquefaction occurs most frequently when an earthquake violently shakes the loose, fine-grained, sandy soils located below the groundwater table. The shaking causes the water pressure in the soil to increase to the point that the soil begins to lose its integrity and starts behaving more like a liquid. This liquefied soil can cause bridge approaches to sink or flow away. Soil liquefaction was a major factor in the destruction in San Francisco’s Marina District during the 1989 Loma Prieta earthquake and the 1964 Good Friday earthquake in Alaska’s Prince William Sound.
In the past, ODOT design standards deemed a bridge seismically safe as long as it withstood an earthquake that occurred, on average, only once every 500 years. In 2004 ODOT implemented new design standards requiring that not only the bridges but also the roadways leading up to them be passable for emergency vehicles immediately following a 500-year earthquake. In addition to this new standard, the new bridges themselves have to be able to withstand a 1,000-year earthquake without collapsing.
“During the 500-year earthquake, a bridge resting on its own foundation should remain standing, relatively unaffected,” said Jan Six, who works as a senior geotechnical engineer in the Headquarters Bridge Division for ODOT. “But if the soil flows, the approach roadway may move with it or sink.”
If studies show that a bridge or road would be affected, ODOT has several options for protecting it, according to Bob Stephens, geotechnical lead for the bridge program’s management firm, Oregon Bridge Delivery Partners. Among several types of ground improvement options are “stone columns,” in which holes are drilled into the ground around the bridge or road and filled with compacted, crushed rock; these stone columns improve the soil’s strength, density and drainage. Another option is to modify the foundation by extending piles or shafts below the liquefiable ground, providing support in deeper, more stable soil.
ODOT is studying two bundles in the bridge program that might be affected by soil liquefaction: Bundle 210, on Interstate 84 near Troutdale, and Bundle 409 on U.S. 20 between Corvallis and Newport. The agency is conducting thorough research to determine whether bridge and road damage could occur and how extensive it could be.
“ODOT is especially interested in Bundle 409, because Beaver Creek meanders back and forth underneath all four of the bridges slated for replacement,” Six said. “These small, two-lane, single-span bridges are on the evacuation route for that part of the coast.”
Should action be necessary, ODOT will determine the best solution and then work quickly to meet the state’s requirements. Afterward, a post-construction verification will make sure the bridges are protected from soil liquefaction.
“Earthquake engineering is a constantly evolving field,” Six said. “ODOT is doing its best to keep up with the latest findings. Our main concern is to keep people safe and the bridges open and accessible for emergency vehicles after an earthquake.”
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