THE BORING AND TUNNELLING PROCESS - The Westerschelde Tunnel: Approaching Limits - page 151

deformation of 45 mm to a deformation of 15 mm. However, this favourable
effect was of a temporary nature. Very soon deformation occurred again.

Problems with the tail seal west

As a result of the vertical relocation (110 mm) of the shield during the disaster
with the broken bentonite valve, the space between the inner diameter of the
boring shield and the outer diameter of the segments at the top of the shield
reduced to 0: theTBM's shield lay against the segments. At the bottom on the
other hand, there the air space had indeed increased enormously; the
brushes of the tail seal could barely seal the space created. The fear existed
that this would lead to problems again. A few days after the stabilization of the
broken valve problem, it appeared that this fear was justified and a leakage of
the tail seal came about, which fortunately only involved a small quantity of
6m 3 . In the meantime, this more or less 'expected' leakage had been antici-
pated: injection facilities (packers) were fitted in the tunnel segments.
The moment the leakage happened, a chemical agent could therefore
immediately be injected just behind the tail seal through the segments in
order to stop the inflow of water and sand. In the following period of two
weeks, a solution was sought for the problem. In this, it was of particular
importance that the boring machine had to be steered and the rings had to
be built as such, that once again there was a situation of an equally distrib-
uted air space around the rings.

Diagrammatic
representation of
tail seal

Fig. 11.10

3

1

475 475 475

450

1 - Shield
2 - Lining
3 - Grease chambers
(dimensions in mm)

2

2000

Boring with extra provisions

In order to continue boring without leakages, use was made of an extra pro-
vision for a number of metres, which was also used during the construction
of the Second HeinenoordTunnel (the first traffic tunnel which was bored in
the soil of the Netherlands). In order to create an extra seal, (wooden emer-
gency partition) formwork was applied around the circumference of the tun-
nel ring against which the jacks pushed off. The space between the inner
diameter of the shield and the outer diameter of the tunnel ring was thus
sealed and also filled with grease.
After having bored for a number of metres using this provision - an extremely
time consuming matter - the replacement of the steel brush seal could
commence after having attained a normal distribution of the air space
around the tunnel rings.