Civil Engineering Reference
In-Depth Information
enough to counter the relocation of the ring in the grout. Aside from that, the
research showed that it was not so much the compressive strength of the
grout that was of importance, but particularly the shear resistance.
Due to the high pressure, the water would press out of the grout into the
soil, resulting in an increase in the shear resistance. By attaining the required
shear resistance, the grout could no longer flow away and the bedding was
also guaranteed. During the further construction process cement was only
added to the grout at the position of the cross connections, so as to create
an extra safety precaution here for the connection of the frozen soil to the
tunnel tubes.
Excavation front instabilities
Instability at cement-bentonite wall
After having bored approximately 60 metres, the easterly machine reached
the cement-bentonite diaphragm wall around the southerly access ramp.
This was the time to switch the alternative teeth which were used for the
boring through of the impermeable block to the original type of cutting
teeth; an operation this time round, which had to take place under increased
pressure - by employing diving personnel.
After the first team of divers had carried out their work on the cutting wheel,
a second team made preparations to enter the pressurized section of the
TBM. Precisely at that moment, an instability occurred at the excavation
front: the supportive pressure fell away and the excavation front collapsed
consequentially followed by the soil and ground water flowing into the
machine. At the surface level a huge crater was the result. Obviously the
tooth-switching activities were stopped and the crater was filled to the sur-
face level. Thereafter the working chamber and excavating chamber within
the shield were slowly filled again with bentonite suspension and the bor-
ing was resumed with the idea of switching the teeth elsewhere. Other than
at excavation front instabilities such as those which occurred at the Second
Heinenoord Tunnel, in this case it appeared that an instability as such,
did not have to lead to lengthy stagnancy. It was possible to get the cutting
wheel going by rinsing it with large quantities of bentonite. With as low a
supportive pressure as possible, the shield could then be pressed past the
bad patch.
Collapse of the excavation front due to 'cracks' in the hard
soil layer
A second excavation front instability occurred directly after the passing of
the surrounding dyke - this happened after boring about 110 metres - which
once again involved the easterly machine, and the instability likewise
occurred during the preparation of a diving operation. The cause of this
instability was presumably the presence of a dense soil layer (clay) between
the TBM and the surface level. Underneath, the air built itself up which pro-
vided the supportive pressure at the excavation front during the diving
operation. The layer was eventually no longer resistant to the pressure
build-up and cracked, or burst: the supportive pressure fell away and the
excavation front collapsed. At surface level this caused a crater with a diam-
eter of approximately 6 metres. The hole was filled in and the boring was
resumed as fast as possible.
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