After a 2001 earthquake damaged a double-deck section of State Route 99
running through Seattle, city planners decided to just bury it. Rather
than rebuild the so-called Alaska Viaduct, they are replacing a
two-mile stretch with a tunnel. So how do you dig a
nearly-60-foot-diameter tunnel under downtown Seattle? With the world’s
largest tunnel-boring machine (TBM). “Bertha,”
as Seattle has named the machine, is currently being assembled in a pit
at the head of its path and starts chewing through buried flannel
shirts and Starbucks cups this summer.
Cutterhead
With
teeth optimized for digging the softer soil found under Seattle,
Bertha’s cutterhead grinds through the earth, collecting displaced dirt
and rock in the excavation chamber directly behind her head.
Conditioners are added to the soil in the chamber to create a goop.
Articulation and
Shove Jacks
The
task of steering such massive equipment beneath a populated downtown
area falls to articulation jacks between the cutterhead and the rest of
Bertha’s outer shield. Shove jacks pushing against completed tunnel
segments maintain face pressure and move the TBM forward.
Concrete-Panel Erector Arms
Giant
arms fix the concrete-wall sections in place, building the tunnel one
ring at a time. Each ring is completed inside Bertha’s shield and sealed
with grout as it is pushed outside. Most TBMs have just one erector
arm, and it’s only Bertha’s sheer size that allows for the use of two.
Soil Transportation Screw and Belt Conveyor
The
soil transportation screw essentially acts as a giant rotating scoop
that moves the paste-like soil mixture from the cutterhead to the belt
conveyor. The conveyor in turn transfers the muck out of the tunnel,
where it is placed on barges headed to a disposal site.
Trailing Gear and Supply Train
Located
behind Bertha’s head are three “cars” that make up the TBM’s trailing
gear. The first car includes the control room, lunchroom, and workshop
for Bertha’s 25-member crew. The boring process is automated; the
operator and two engineers provide technical oversight. Other tasks
include maintenance, surveying, and operation of the tunnel-wall-segment
delivery and installation. The second car contains the hydraulic power
packs, electrical control panels, and backfill grout equipment. The
third car carries high-voltage transformers, a water-regulating tank,
and a temporary power generator.
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