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The Today File

Your guide to the latest news from around the Northwest

January 17, 2014 at 5:57 PM

Bertha’s inspection under way

Workers inside the Highway 99 tunnel machine began their close-up inspection of the cutter face late this afternoon, spokeswoman Laura Newborn said.

Teams of five to six people will clean and examine the cutter in three-hour shifts, working in air at 1.4 times atmospheric pressure. Then they will spend one hour in a tank aboard the tunnel drill to gradually decompress to normal pressure.

To prepare the work area, groundwater was pumped away from the surrounding soil, using 10 wells installed during December. Then a slurry of bentonite clay was sprayed in, to stick to the soil walls and form a water- and air-resistant crust.  This allows compressed air to be pumped into the 5-foot chamber where tunnel muck enters the conveyor system, as well as a 14-inch gap between the cutters and the soil.  The pressurized air pushes outward against the clay coating — to fend off water and soil leaks, as shown in this diagram:

Bertha_highpressure_checkup

Examinations might take several days, a Department of Transportation (DOT) update said.

The state DOT has said that a buried steel pipe is only partly to blame for blocking the tunnel machine known as Bertha, but managers would not describe their other theories when asked this week. On Thursday, an independent expert told state senators that the hyperbaric inspections ought to quickly solve the puzzle.

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