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Wednesday, December 12, 2007 - Page updated at 03:12 PM

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Culvert repair forces I-405 lane closure

Seattle Times Eastside bureau

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Attention, I-405 drivers

Closure information: The state Department of Transportation advises drivers who use Interstate 405 to check these sources for information:

• Drivers information line: 511

• Department of Transportation travel page:

• Web page created for this project:

Transportation crews plan to shut down one lane of northbound Interstate 405 in Renton around the clock for two weeks beginning this weekend after discovering a huge sinkhole caused by the collapse of a steel culvert during last week's storm.

Without repairs, the freeway itself could collapse if another significant storm were to hit the region, said Greg Phipps, spokesman for the state Department of Transportation (DOT).

"This an emergency; the work needs to be done immediately," he said.

The failing culvert, or steel pipe, carries Thunder Hills Creek under the freeway near Benson Road South and Main Avenue South.

Last week, crews discovered a 30-foot-deep sinkhole, 80 by 30 feet wide, just 12 feet from the southbound shoulder of the freeway. More than 700 cubic yards of rock were poured into the hole to prevent more sliding. Upon further examination, crews determined that a 400-foot-long culvert that runs 60 feet below ground had collapsed next to where it passes under the freeway, causing the sinkhole, Phipps said.

"It's like when you put your thumb over a water hose," Phipps said. "It was an enormous volume of water from a historic amount of rainfall. It blew open a section of the culvert."

That stretch of I-405 carries about 139,000 vehicles daily, Phipps said. The DOT will try to open all northbound lanes during the heaviest morning commute but is hoping motorists will change their driving patterns to keep traffic manageable, he said.

"If drivers don't change their normal patterns, we'll see significant congestion," Phipps said. "We need roughly a third of the drivers to find another way around. That may mean working from home, taking another route or talking with their work about adjusting their schedules."

The first stage of the work will involve setting up an extensive pipeline system that will pump the creek water away from the damaged culvert to a nearby wetlands area that can handle the flow, Phipps said. That work will take about two weeks, during which one of the three northbound lanes will be shut down to accommodate equipment and personnel.

After that, crews will begin work to replace the 48-inch culvert, which will take about six weeks, Phipps said. All lanes of the freeway will remain open during this work. However, the construction will likely be noisy and may affect some local residents and businesses, Phipps said.

The repair is estimated to cost at least $5 million.

Because the work is being done on an emergency basis, the DOT doesn't have time to go through the normal environmental permitting for replacing a culvert, Phipps said. To compensate, the DOT will later come up with a plan to improve the environment in the surrounding area, he said.

Rachel Tuinstra: 206-515-5637 or

Copyright © 2007 The Seattle Times Company

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