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A buried steel pipe is mostly to blame for
stopping the giant tunnel-boring machine Bertha, which has been stuck
since Dec. 6 along the Seattle waterfront near South Main Street.
The long pipe was an 8-inch diameter, 115-foot-long “well casing,”
used to measure groundwater during studies in 2002 on the Alaskan Way
Viaduct replacement project, project officials said.
Matt Preedy, the deputy project administrator for the state
Department of Transportation, said he had no estimates about how much
time and money it will take to remove the rest of the pipe, and to
repair damaged cutting tools on the face of the machine.
Nor does the team have a strategy yet for how removal should take
place. One possible method is to send tunnel-trained divers to work near
the cutter face, under extreme pressures that are exerted by
The well site was listed in reference materials provided to bidders
as part of the contract specifications, DOT says. “I don’t want people
to say WSDOT didn’t know where its own pipe was, because it did,” said
DOT spokesman Lars Erickson. However, Chris Dixon, project director for
contractor group Seattle Tunnel Partners, said the builders presumed
there would be no pipe in the way, because casings are customarily
removed after use.
fragment of steel pipe pokes between spokes of Bertha’s cutting face,
in this photo from Thursday’s inspection.
Dixon said the tunnel-machine crew first noticed metal pieces in
Bertha’s conveyor system in early December — when Bertha’s rotation
actually shoved a segment of pipe through the surface, prompting crews
to remove a 55-foot-long piece. However, the machine kept grinding
forward just fine, Dixon said, leading STP to have what he called “a
false sense of security” that things would be OK. But then on Friday
night, Dec. 6, the cutting face rotated without catching soil. The team
later found unusual damage to cutting teeth, and then on Thursday night
an inspection found a pipe fragment jutting through spaces between
spokes of the cutter.
A modern tunnel machine can chew through dirt and concrete, but not
steel. Even fiberglass rods caused a snag that delayed work several days
this summer. Steel could become tangled in the spokes of the rotary
cutting head, and in a conveyor screw that pushes dirt from the cutter
face onto a belt that moves out the rear of the machine. Dixon said
Friday that “we don’t know” yet whether any moving parts are jammed.
Downtown Seattle contains some of the most frequently poked
and studied ground on earth, which makes the blockage all the more
confounding. Five-foot diameter holes were drilled alongside the tunnel
path to install concrete pilings that protect the old viaduct; the
contractors have used ground-penetrating radar; and geotechnical experts
drilled test holes, which didn’t hit this particular object.
For the last four weeks, DOT didn’t mention the pipe during several
news interviews and press conferences. Asked about this, Preedy said
Friday that initially, the project’s expert review team thought the pipe
was a secondary issue, and that a giant boulder seemed more likely. At
60 feet down, the top of Bertha is in glacial soil, beneath the extent
of fill soils and debris that early Seattle settlers dumped near Elliott
The $1.44 billion tunnel construction, from Sodo to South Lake Union,
is about three months behind schedule. But Bertha in November was
advancing as fast as 50 feet a day, prompting Preedy to say it’s
possible to regain time after the steel is removed.