Civil Engineering Reference
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machines which were positioned at such great depth to be able to move for-
ward, it appeared that rather massive jack forces were necessary: the
machines used 95% of the maximum force of 112,000 kN installed, whereas
the prior expectation at about 55-60% would have been enough. Although
there was no “hard” relationship with the deformation problem, something
clearly had to be done about this force problem. When the required force
increased even more, then the capacity would fall short and stagnancy
would be the result. Besides, a boring machine that operates at its maxi-
mum capacity is no longer in a position to be steered: if all the force is used
to push the machine straight ahead, then the making of bends - in which it
is necessary that some jacks provide more force than others - is no longer
possible.
Reducing friction
A commonly used method to reduce the required force is the application of
a lubricant (bentonite) between the shield and the surrounding soil, so that
the soil friction reduces during the boring. During the construction of the
boring machines bentonite piping was already fitted for this purpose.
A second possibility to reduce the friction is the boring of a hole which is
greater than the diameter of the shield. Due to this overcut, the shield has
more space to move whereby the friction can further be reduced by filling
the overcut space with bentonite suspension. Reduction of the friction then
results in a reduction of the required jack forces.
The idea was that by realising a more spacious overcut for both tunnel
boring machines, perhaps the loads on the shield and thus the deforma-
tions would also reduce. The two tunnel boring machines were provided
with fixed overcutters with an overcut of 10 mm. It was decided to enlarge
this overcut for both boring machines to 30 mm. At the locations where
the boring machines stood, no air could be used as a support pressure
medium, so the divers replaced the teeth on feeling and under extremely
high pressure (7 bar) in the bentonite slurry.
TBM Neeltje Suzanna continues
Although the deformation problem arose with both boring machines, it
seemed to be less critical at the westerly machine (Neeltje Suzanna). There
was a single large dent of approximately 45 mm at the left in the middle.The
easterly machine on the other hand, showed a freakier pattern and also a
larger maximum deformation (approximately 50 mm). After the enlargement
of the overcut the question was, what had to happen?: wait until the ring-
reformer was ready, or to start boring carefully using the larger overcut
whereby the shield had to be measured off after each ring was built-in. For
the westerly machine the contractor dared the latter. The idea was also that
the experiences gained with this boring machine, could help in the finding
of a solution for the more serious problems of the easterly TBM: the boring
process was resumed again after a stagnancy period of 2 months, and initially
ran its course as it should, whereby only a minimal change in the deformation
of the shield occurred.
Water jacks as an alternative for the ring-reformer
As an alternative to the ring-reformer, the idea had emerged in the mean-
time, to use water jacks to prevent further deformation of the shield: right
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