For a tunneller, certain Chalk strata have undesirable characteristics, such as the presence of hard flints or sand-like phosphatic chalk. Tunnelling is expensive, but so are delays and damage to tunnel boring machines which hit unexpected geology. So engineers planning to tunnel through the Chalk have two conflicting objectives: minimum cost and maximal information. Professor Rory Mortimore told a recent meeting how he uses selective borehole drilling to achieve a balance between them, and offered some sage advice.
As I might have said before, Professor Rory Mortimore
Is just the guy you need to give a talk
On the varied lithographics and the complex statigraphics
Of that soft, white, porous limestone we call Chalk.
Inspired, perhaps, by moles, he is happy boring holes
But examines every core for tell-tale signs:
In cold and draughty shacks, he looks for fossils, flints and cracks
To check it stratigraphically aligns
With exposures, known to most, on our sunny Sussex coast,
Where the flint and marl bands show in all their glory.
That way he gets a clue about what tunnellers must do.
But visual correlation’s half the story.
Borehole geophys is the latest thing there is –
It can spot phosphatic chalk. And, what is more,
Cameras can record all the layers freshly bored,
As back-up to each frail, extracted core.
Initially, he’ll drill his boreholes far apart, until
A sequence goes against his expectation.
Then, because it’s made him warier, he drills out more cores in that area,
So tunnellers have better information.
“Expect the unexpected; be excited, not dejected
If things don’t always turn out as they should;
For knowledge must advance by serendipity and chance,
It’s much less fun if everything ’s understood!”