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manufacturing company Herrenknecht in Germany. Here the building-in of
the closing stone was narrowly monitored with a test-mounted video camera.
From the study it appeared that the seal around the keystone already made
contact with the adjacent segments (stones B and C) even under ideal cir-
cumstances during the sliding-in process, at approximately 0.26 metres
before the final position was reached. The video recorded at Herrenknecht
showed that the seal then displayed the tendency of 'rolling up'. When the
keystone was completely in position, openings could come about because
of this rolling up, or the rubber could be damaged. Besides, it appeared in
some cases that the rubber seals did not fit exactly on top of each other as
intended, but lay next to each other. In some cases the rubber seals even
turned over.
In order to solve the problem, it was decided to broaden the profile and also
to extend it. This resulted in the seal of the keystone making contact with the
adjacent profile at a later stage, and the chances were less great that the
sections ended up next to each other or could turn over. With this new sec-
tion, the number of leakages were considerably reduced.
Investigation into
operation of seal and
examination of
keystone
Fig. 11.21
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