Civil Engineering Reference
In-Depth Information
amiss with the sealing.The machine was halted immediately. Further inves-
tigation showed that it involved a leakage under one of the neoprene lip
seals which separate the 'chambers' of the main seal from each other.
The position where the boring machine was situated at that moment was
unsuitable for on the spot repairs. At the same time it was clear that, by
means of emergency facilities - thicker grease in one of the chambers (the
labyrinth) of the main seal and the filling of the so-called leakage chamber
with compressed oil and air - further boring was possible. That then hap-
pened: the machine followed its course further into the glauconitic sand,
under the Pas van Terneuzen (the deepest point) and then continued
upwards into the Boom clay. There were no further problems with the main
bearing seal worth mentioning.
Necessary preventative repairs
The question that remained however was, what the consequences would be
of a worsening of the problem at the time that the boring machine would be
positioned in the sand under Everingen. In the sand and besides being
under a fairway, the problem would almost be insoluble; it was a risk which
nobody wanted to take.That is why it was decided to repair the sealing before
the boring machine left the Boom clay. There the chance of success was
greater: the pressures there were a lot lower and the Boom clay was rela-
tively stable. Besides, at the time that the decision was taken to repair, the
TBM was positioned under the Middelplaat and that also meant, that in the-
ory there was also an option to work on a solution from above.
Atmospheric circumstances required
For the reparation of the main bearing seal it was clear that no divers could
be employed; specialists were required who could not undertake the work -
the vulcanization of rubber - with increased air pressure. This meant that
atmospheric circumstances were a requirement - also at the front end of the
tunnel boring machine.
The first idea consisted of boring forwards with the boring machine for 0.4
metres and then withdrawing the cutting wheel. From the working space
thus created a part of the cutting wheel would then be dismantled, the main
bearing would be exposed and the repairs could take place. The experience
that had been gained with the clay in the meantime, had made it clear that
although the clay is stable, there was indeed a problem. Certainly if the task
would take a couple of days: in the clay that would stand in front of the
TBM like a great big wall during the repairs, there are cracks (fissures).
Along those cracks, clay chips (some of which weighing more than 100 kg)
could fall down because the over-consolidated clay relieves. Such working
circumstances provided no perspective.
Solutions from the top: no option
In principle it was possible to work on a solution from a working island to be
constructed on Middelplaat: from this island an underground 'station' would
have to be built to then be able to repair theTBM. For the construction of such
a station, in theory various construction methods were eligible, like making
use of diaphragm walls, boring piles or the freezing technique. The construc-
tion of the required working island would mean that time (and money) would
be lost in the application of the required permits. Besides, the possible imple-
mentation methods would bring along their own problems and risks: the
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