Science
(2009-08-23)
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Two men and a drill. Larry Inselman and John White discuss the WSDOT drilling project near Seattle's Alaskan Way viaduct.
(KPLU)
Drilling machines are on the streets of downtown Seattle--testing the soils beneath the city. It's an early step toward construction of a massive tunnel to replace the Alaskan Way Viaduct.
If you're digging a tunnel, deep underground, the last thing you want is any surprises, such as boulders. Even a big patch of sand can cause problems. We're still years away from replacing the Alaskan Way viaduct, with the widest tunnel in the world. But, John White, engineering manager for the state Department of Transportation, says these first steps are critical for the machine that will dig the tunnel.
"The tunnel boring machine [will be] designed precisely to minimize the risks of drilling in the specific soil conditions we expect to encounter along this tunnel alignment," he says.
There are a half-dozen drilling rigs currently at work in downtown Seattle. The drills go straight down, to depths of 100 to 300 feet. (Imagine being on the roof of a 25-story building and looking down a drainpipe.) They bring out soil samples.
White says hitting a sandy patch of earth could lead to sinkholes on the surface. That happened this spring, north of Seattle, when a homeowner's driveway was suddenly swallowed, as a digging machine for a new Metro sewer tunnel passed beneath.
The highway tunnel will pass under First Avenue in downtown. It's not the modern skyscrapers that make White nervous. He says they have secure foundations. "We are more concerned about older buildings, like in Pioneer Square," he says.
Where the soil is loose, they'll have to inject cement grout to stabilize it. Once the test-drilling is done this fall, the transportation department will begin looking for bids on the tunnel boring machine an aircraft-sized cylinder, custom-built for the job.
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