By Mike Lindblom, December 9, 2013
Nearly three days after an unknown object
blocked tunnel-boring machine Bertha, project managers haven’t yet
determined the size or how to remove it, according to the state
Department of Transportation.
“We don’t know what it is. We don’t know whether it is man made or natural,” DOT spokeswoman KaDeena Yerkan said.
Drilling halted Friday night, about 60 feet deep, along the Seattle
waterfront between South Jackson Street and South Main Street. In normal
conditions, the team might try a “hyperbaric intervention,” meaning
that the tunnel machine could retreat 18 inches, and then divers would explore gaps around the cutting face,
at high pressure. (Tunnel projects keep specially-trained divers on
call, to work in air and soil that exceeds atmospheric pressure – Bertha
includes three hatches where drivers can move to the machine face.) But
in this case, there is watery sand and weak fill soil immediately above
the machine. So if high air pressure were exerted in front of the
machine, the air would push or burst through the soil, said Yerkan.
The problem was discovered Friday night, and reported to state DOT Secretary Lynn Peterson, who was updated on Monday.
“STP (Seattle Tunnel Partners) has not made a decision on how they’re
going to move forward yet,” Yerkan said. “They’re talking to their
experts, we have been talking to ours.” Chris Dixon, STP’s manager,
hasn’t yet responded to messages requesting comment.
The cutting face, at 57 feet, 4 inches, is the widest in the world.
It’s equipped with steel cutting discs to scour and crack boulders, but
apparently can’t defeat the large obstruction. Fragments less than three
feet diameter can slip through openings in the cutter head, and be
removed out the back of the conveyor system.
The tunnel route was intensively sampled by
soil engineers from Shannon & Wilson long before the project
started, but apparently their narrow test shafts didn’t strike this
object. The soil at 60 feet down is considered clean, glacial sediment,
but most of the soil above is unstable fill, including wood debris from
industries more than a century ago, and spoils from the Denny Regrade in 1898.
One scenario might be to simply excavate from the surface, and pluck
out what’s in the way — since the soil above is useless anyhow. A blue
crane was being assembled nearby on Terminal 46 Monday, but it’s unknown
whether that’s related to the drilling.
Another response could be to send in crews with pneumatically driven
drills, hammers or other tools to break the large object. But that task
would be hampered by the loose soil, Yerkan said.
Since its start on July 30, the deep-bore tunnel project has advanced
more than 1,000 feet, or close to one-eighth of its total distance
from Sodo to South Lake Union.
“The machine is running well, it’s functioning,” Yerkan said. There
have been no reports of damage to the drill face, where eroded steel
discs were recently replaced during routine maintenance.
By early 2014 the machine is supposed to dive under the Alaskan Way
Viaduct, causing a temporary highway closure and potential risks of
vibration to old brick buildings nearby.
Seattle Tunnel Partners, led by the US branch of Spanish-based
Dragados and by California-based Tutor-Perini, has been paid $730
million as of September, or about half the total $1.44 billion contract
value, according to a state chart, released under a public-document
Asked why officials waited two days to disclose the problem, Yerkan
offered two theories: some project staff were gone during the weekend,
so Monday was the logical time to regroup for an update; and the team
may have wanted to come up with some progress or strategy to offer,
before reporting to the public there was a hitch.
What Bertha leaves behind, Fri, Nov 29, midnight 2013
The SR 99 tunnel in Seattle takes shape in October 2013. Two workers
walk through the first rings of the tunnel toward Bertha, the SR 99
tunneling machine. Later in the tunnel drive, crews will use special
trucks to make the increasingly long trip to the machine. Learn more
about the Alaskan Way Viaduct Replacement Program at
www.alaskanwayviaduct.org or follow Bertha on Twitter @BerthaDigsSR99.