From CLIVE BAKER
I have been directly involved with the construction of three relatively
long tunnels, each of which was driven and guided from a single shaft. Thus
there was no opportunity to check the alignment until breakthrough at the
receiving end. All three tunnels were in Britain and were driven from north
to south. The first, 7 kilometres long, finished about 140 millimetres right
of the centre. The second, 6 kilometres long, finished about 120 millimetres
right.
Tunnel surveying is not an exact science, so that an error of 5 millimetres
per kilometre represents good accuracy. However, the similar errors of about
20 millimetres per kilometre to the right of the target caused me to speculate
that there may be a Coriolis effect at work (Forum, 23 November and Letters,
18 January).
Before the third tunnel started, which was 4.5 kilometres long, I lightheartedly
warned the contractor that, despite his best efforts, he would finish 90
millimetres to the right of his target. He finished about 70 millimetres
to the right, so perhaps 15 millimetres per kilometre represented Coriolis.
The diversion of fluid particles to the right when they are moving southwards
in the northern hemisphere applies equally to particles moving northwards.
Thus a tunnel driven from each end towards the centre should suffer the
same total error on closure. I have just heard on good authority that, on
checking shortly before breakthrough, the two faces of the Channel tunnel’s
first service tunnel were about 320 millimetres to the right of each other.
The north/south component of this tunnel is roughly 20 kilometres, so the
error was again about 15 millimetres per kilometre to the right.
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I will be pleased to hear if other tunnel engineers have encountered
this phenomenon, and the opinion of physicists as to whether Coriolis could
really be at work.
Clive Baker Lahore, Pakistan
