A half-dozen probes have not hit any huge object in front of Bertha, adding to the mystery of what’s blocking the stalled Highway 99 tunnel machine.
By Mike Lindblom, December 30, 2013
Wells were installed earlier this month to pump groundwater away from
tunnel-boring machine Bertha, so workers could look around the cutting
face for obstructions Thursday.
Workers have poked the soil in six locations in front of the stranded
tunneling machine Bertha, but their probes haven’t bumped into any huge
obstructions yet, a senior engineer said Monday.
Those results increase the mystery over what might be blocking the
world’s biggest tunnel machine. Project leaders initially speculated
that an unusually wide boulder, deposited by ancient glaciers, might be
in the way.
Progress on the Highway 99 tube has stalled since Dec. 6, when the
rotary cutting blades of the 54.3-foot-diameter drill spun without
scouring much sediment between Pioneer Square and Elliott Bay.
The probe holes are being drilled 110 feet deep and spaced five feet
apart, from east to west. A hole-digging rig was out Monday morning to
take another poke, and more test holes will be drilled Tuesday.
“We’re not hitting anything unexpected,” said Dave Sowers,
tunnel-engineering manager for the state Department of Transportation.
If a school-bus-size boulder were there, he said, the probes should
Meanwhile, contractor team Seattle Tunnel Partners intends to send workers into Bertha’s flooded front end Thursday to inspect for the blockage
through gaps in the cutting head, Sowers said. Many crew members were
gone on holiday, so contractors chose to wait for their return before
starting the inspection, he said.
The tunnel team hopes to remove enough groundwater so the inspection
can be done at normal air pressure. Ten wells have been installed around
the buried machine to extract the silty water that permeates this
stretch of the route.
Bertha’s cutting head, and a 5-foot-wide chamber where excavated soil
enters a conveyor system, are flooded with roughly 90,000 gallons of
water and mud.
The goal is to lower the water table enough so the top half of the
cutter is exposed for a clear view, Sowers said. The cutter then can be
rotated so that the other half can be viewed. “You’ll be able to see
whether spokes are clogged,” he said.
The soil contains enough groundwater to create hyperbaric pressure, similar to working deep in the ocean. Tunnel-trained divers eventually may be needed to hammer or blast away a boulder, Sowers said.
Chris Dixon, project manager for Seattle Tunnel Partners, has said
the team notified Ballard Marine Construction to have divers available
The white-colored well tops protrude a few feet above the surface,
clamped to thick hoses alongside the Alaskan Way Viaduct. Silty water is
dumped into a green holding tank, where sediments can settle out. The
remaining water is released into the city stormwater system.
The four-lane tunnel from Sodo to South Lake Union is supposed to
open for traffic by the end of 2015, a timeline Sowers said is still
Officials say it’s too early to know whether the stoppage will lead
to overruns on the $1.44 billion construction budget, or how big those
While Bertha is stalled, the builders are trying to gain time on
other tasks. About 20 blades are being replaced, by workers who reach
them by crawling within the hollow spokes of the cutter. Temporary
concrete rings and steel frames, used by Bertha to make its initial
push, need to be removed from the launch pit in Sodo.