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In
the case of the NBC's tunnel boring machine, the
cutting wheels, commonly called disc cutters or discs, are 17 inches
in diameter. Disc are used for excavation of rock harder than about
10,000 psi (pounds per square inch). The discs roll against the rock
face and break the rock in a manner similar to a pizza cutter breaking
a crisp crust. Hydraulic cylinders thrust the discs into the rock,
like pushing down on the pizza cutter. The harder the rock, the more
thrust that is required to break it. Each cutter can withstand (or
apply) a thrust up to 25 metric tons, including impact loads.
Numerous discs are mounted on the TBM
cutterhead, a cutting wheel roughly the same diameter as the tunnel
excavation. The cutters are positioned to cut concentric tracks in the
face, generally 2 to 4 inches apart, again depending on the hardness
of the rock and the design strength of the discs. As the cutterhead
rotates the discs crush the rock as they roll on the surface, or
excavation face. |
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October
4, 2002:
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October
19, 2002:
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October
26, 2002:
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October
26, 2002:
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Main
Body
The cutterhead is attached to the main body (the
shield) of the TBM by a specially designed bearing. The shield
performs a number of functions. It houses the motors which rotate the
cutterhead (the main drive motors) and the thrust cylinders which
provide the forward thrust on the cutting discs. Because the drive
motors are turning the cutterhead, the shield must also resist the
opposite forces which are trying to roll the shield in the opposite
direction. Another function of the shield is to protect the
workers who are installing the initial tunnel support system behind
(or inside of) the TBM. The main spine tunnel will be supported by
precast concrete rings which are assembled inside of the shield, then
expanded to fit the tunnel excavation as the shield moves forward and
the rings come out of the tail. Hydraulic ring erectors are mounted
inside the shield and are used to position and "build" the
rings. Once a 4-foot long ring is built, the thrust
rams push against the leading edge of the ring, propelling the shield
and cutterhead forward. During this "push" the head is
rotated, rolling the discs as described above and crushing the rock.
The rock chips, called muck, are scooped into the cutterhead through
holes in the head, called buckets, as it rotates . The muck is dumped
onto a conveyor, inside the cutterhead, which carries the muck through
the shield and into the supported tunnel where it is loaded into rail
cars and hauled out of the tunnel.
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