Civil Engineering Reference
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angled and filled with water 'cushions' which were placed longitudinally
between the last tunnel ring built and the shield, had to exert a force onto
the shield to counteract further deformations.The water jacks would be kept
in a fixed position on the inside of the shield, so that they would slide along
with the TBM during the boring, across the outside of the tunnel ring. The
forces exerted onto the outside of the shield of theTBM could then be trans-
ferred via the water jacks onto the tunnel ring.
This alternative was elaborated upon, in which a lot of attention was paid to
the calculation of the extra load that the tunnel rings would have to endure.
Aside from that, naturally implementation aspects also had to be dealt with:
how would the water jacks be built in, how much time would that take, and
what would be the resultant production rate? Eventually the choice between
this alternative and the 'Colossus of Terneuzen' went in favour of the water
jacks. With this type of 'shield support' the boring process in the easterly tube
was - after a stagnancy period of 3 months - resumed slowly and highly
controlled in which the deformations were continually monitored.
Water 'cushions'
Fig. 11.7
Smaller rings
From the moment that theTBMs were boring upwards again and the ground
and water pressures reduced, the deformations of the shield eventually
appeared to lessen. Although, anticipating a possible worsening of the
problem, additional measures were taken prior to the restart of the boring
machines. Thus segments were produced for the construction of rings that
have a diameter which is 20 mm smaller. If the shield would deform further,
by applying the smaller rings, this would not directly lead to a clasping of
the tunnel rings. The segments were not used, for this purpose, because
application also brought on a vital objection: the greater air space which
would be the result, had to be bridged by the tail seal and the possible col-
lapse of the seal caused too great a risk. The smaller rings produced were
later built in at a safer location.
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