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The Today File

Your guide to the latest news from around the Northwest

August 28, 2013 at 4:54 PM

A look at the fiberglass that made Bertha cough

Readers of today’s Highway 99 tunnel story — about why the giant drill has advanced only 24 feet this month — have been wondering why there is fiberglass in the muck.  The story described how fiberglass strands became caught in a screw-type conveyor system near the cutter head.

Below is an image, forwarded Wednesday afternoon from the state Department of Transportation, of some of the excavated muck with fragments of fiberglass rods sticking out. This pile sits at Terminal 46, after passing through the machine and onto conveyor belts to the waterfront. You can see rods, but not get the full sense of how several fiberglass rods might clump together and jam the works.

fiberglass muck

screw conveyor

The fiberglass rods are there because workers embedded them into one of the four walls of the concrete pit in Sodo where the tunnel machine “Bertha” began her journey to South Lake Union on July 30.

The other three walls were built using concrete poured around steel cages, formed from reinforcing bar. But the front wall, which Bertha penetrated on its escape from the pit, used fiberglass instead — because teeth on the rotary cutter can’t gnaw through steel. This is a common strategy, according to project director Chris Dixon of Seattle Tunnel Partners. But the fiberglass put up more resistance than expected.

At right is the screw conveyor, where dirt, rocks and concrete pass from the face to the rear of the machine.

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