Civil Engineering Reference
In-Depth Information
Fig. 11.3
Crater at the surface
as a result of
the instability of the
excavation front
Excavation front instability Neeltje Suzanna (westerly TBM)
The westerly TBM was also confronted with an instable excavation front
during the boring process. This instability occurred when the boring
machine was at its deepest point in the boring route: 60 metres
NAP, under
the Pas van Terneuzen. This time the collapsing of the excavation front was
the result of a bentonite valve which had broken off the shield, whereby a
direct connection came about between the outside and the inside of the
shield: bentonite that came off the excavation front and soil and water
flowed inside under high pressure.The supportive pressure fell away and the
excavation chamber was filled with sand. This instability was also solved
rather quickly: the material in the excavation chamber was carefully rinsed
away, after which the boring process could be resumed with as low a sup-
portive pressure as possible.
The leakage that occurred due to the bentonite valve breaking off, and its
repair however took quite some doing. This is dealt with elsewhere in this
chapter.
Shield deformations of the westerly and
easterly TBMs
A boring shield is round or at least: it should be. In May 2000 it appeared
that this was not the case anymore for both machines, and deformations of
the tail section were observed. The first indication of this was an strange
division of the air space - the space between the outer diameter of the
tunnel rings and the inner diameter of the shield - around the circumfer-
ence. The measuring of this air space was a normal procedure during the
building-in of a ring.The division can also vary under normal circumstances,
for example, when making a bend. However, the position of the boring
machine did not show any cause for the strange division. The impression
came about that this could be caused by a 'dent'.
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