Civil Engineering Reference
In-Depth Information
so-called roller-crushers ('Walzenbrechers'). These served to reduce the last
lumps into workable dimensions.These 'crushers' were not intended to pre-
vent sticking in the excavation chamber, but they were certainly able to pre-
vent large lumps of clay from getting into the outlet pipe which could also
cause blockages and the associated delay.
Design of the cutting wheel
A cutting wheel is mounted at the front of aTBM.This cutting wheel scrapes
the soil loose and mixes it with the added bentonite suspension into a slurry
which can then be pumped out. Keeping in mind the sticking problem, the
cutting wheels of the boring machines used in the WesterscheldeTunnel did
not escape special treatment, since right from the start of the design
process, the properties of Boom clay played a very important role in the
design. In the preliminary design, for example, it was assumed that opti-
mization could perhaps be achieved by letting the cutting wheel revolve in
one direction. Drawings of the cutting wheel from that time, still provide
written proof of this: the digging teeth only being on one side of the spokes
of the cutting disk. Revolving the cutting wheel in one direction then made
it possible to optimize everything in the excavation chamber with respect to
the design and the method of adding bentonite to prevent sticking.
Revolving in one direction was abandoned when the design of the boring
machines was worked out in greater detail, because it would also entail a
big risk since wear and tear on the cutting teeth is much greater and besides
this, any circumferential rotation of the boring machine is not easily corrected
by reversing the rotation. The basic principle of the design, however, still
stands: as many as possible beautiful round slender shapes and as few as
possible hidden corners where clay could be deposited.
The cutting wheel was constructed from a cast steel core, which was con-
nected to the driving mechanism, and six spokes welded to the core fitted
with a total of 64 digging teeth (cutters). A cast steel rim on the perimeter
linked the spokes with each other. The replaceable teeth on the spokes were
positioned in such a way that it was possible to work the whole excavation
front twice in one revolution of the wheel. The spokes were designed as a
hollow box structure and constructed with a trapezial cross section, the wide
sides of which are at the front where the cutters are mounted.The aim of the
whole design was to make the flow of the soil as optimal as possible while
the discharge of the soil to the suction opening was obstructed as little as
possible. Two coulters - vanes - mounted on the inside of the cutting wheel
had to ensure that no material is deposited at the bottom of the shield.
In order to reduce friction of the shield with the surrounding soil, the
drill hole is excavated with the aid of special digging teeth - over cutters - which
was slightly larger than the outside diameter of the boring shield. Making
curves, however, required an even larger diameter and in order to achieve this,
the whole driving mechanism with cutting wheel could be shifted and tilted
with respect to the boring shield with the aid of steering cylinders.
The extra space created as a result between the boring shield and the sur-
roundings was filled with bentonite suspension. Within the space of this
slot, theTBM was then able to push off against the last tunnel section placed
to create the required bend.
An independent active centre cutter
When a cutting wheel revolves, the 'cutting rate' at the centre of the machine
is extremely low. In view of the tendency of Boom clay to stick, the danger
of clumping and the associated stagnation of the boring process would be
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