Civil Engineering Reference
In-Depth Information
Supporting plates
In order to increase the safety of the divers while carrying out their work, five
supporting plates could be placed in between the stationary spokes in front
of the excavation front with the aid of hydraulic cylinders, which created
a protection against a possible unstable excavation front. The supporting
plates had to stay clear of the excavation front so that in the event of a back-
ward motion the 'bentonite cake', which makes a substantial contribution to
the stability of the excavation front, would be prevented from being damaged.
Working space
Repairs could for example consist of replacing the digging teeth.They were
therefore designed in such a way that most of the teeth could be replaced
from behind the cutting wheel; it was not necessary for a diver to get right
in front of the cutting wheel. An exception to this was the teeth of the active
centre cutter. In order to create workspace for the divers, the TBM was able
to bore 0.4 metres forwards with respect to the standard boring position, so as
to then withdraw the cutting disk. Thus creating a workspace of 0.4 metres.
Tail seal
The seal of the space between the inside of the boring shield and the
outside of the lining being installed within the shield is an important aspect
in the tunnel boring process. When making bends, for example variations
arose in the size of this so-called air space and the standard tail seal had to
be able to follow these variations flexibly at all times and be able to absorb
the soil, water and grout pressures prevailing outside. Naturally, the water-
tightness was also the first requirement.
Grout injection
Grout is pumped behind the tail seal under high pressure, through pipes
welded into the housing of the shield, into the space between the outside sur-
face of the shield and the external surface of the tunnel rings. It is extremely
important to fill this so-called tail slot immediately. It is not only important to
prevent the surrounding soil from distorting as much as possible, it is even
more important to provide the completed tunnel section with bedding imme-
diately. If this is not done and the tunnel is not supported by the surrounding
soil, in the worst case the tunnel could collapse.
Steel brush seal with grease chambers
Initially the manufacturer of the TBM proposed using a seal consisting of
three rows of steel brushes. The rows of brushes in this type of seal are
attached to the inside of the shield and press against the completed tunnel
section. The spaces between the brushes, the chambers, are filled with
grease under pressure. The proposal made by the manufacturer of the TBM
was examined critically at the design stage, in which especially the question
was asked as to whether the seal under the given circumstances - the enor-
mously high pressures of soil and water would be safe enough, and whether
temporary solutions could be found in the event of leakages. It was decided
to have a steel brush seal with an extra row of brushes and an extra grease
chamber. The choice of this solution was mainly prompted by the assump-
tion that a steel brush will not suddenly collapse, as can be the case with a
rubber seal. The idea was that by using steel brushes it could be seen
whether the seal was functioning optimally or not based on the grease con-
sumption or the occurrence of small leakages. Having three chambers
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