Civil Engineering Reference
In-Depth Information
Disaster with the tail seal east
A disaster occurred twice during the boring of the Westerschelde Tunnel
which formed a direct threat to the safety of the people present in the tun-
nel, the TBM and the tunnel itself. One was the uncontrollable situation
described before as a result of the broken bentonite valve. The second
calamity occurred by the sudden collapse of the tail seal of the easterly
boring machine.
4,000 m
3
of slurry flow into the tunnel
Although it was not expected that a steel brush sealing with grease chambers
would collapse suddenly, this nevertheless happened. On 30th November
2001 in the easterly machine which at that moment was positioned under
Everingen in the tunnel, an enormous amount of soil and water entered.
About 350-400 m
3
of material per hour squirted in through the leak and only
after 13 hours the situation was under control again. Later it appeared that
the second row of brushes at the position of the leak, across a distance of 0.8
metres in the circumferential direction, had disappeared completely, whereas
in the third row a number of brushes were missing as well. Attempts to con-
tinue boring normally - using extra grout and grease injection - did not have
the desired effect for stabilizing the situation: the material continued to flow
in. As such it was a lucky circumstance that the inflowing material mainly
consisted of bentonite which came from the excavation front. The problem
would have increased substantially if it were large quantities of sand coming
in which would be difficult to pump away.
Fig. 11.14
Leakage as a result of
the collapse of the tail
seal of the easterly
tunnel boring machine
In order to replenish the bentonite at the excavation front a permanent sup-
ply of new bentonite was necessary. In order to have sufficient bentonite at
hand, the boring by the other machine was therefore also halted: it was more
favourable to maintain the bentonite flow than to stop the supply which
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