Engineering Rock Mechanics

24 Geological setting

relied upon. I also believed this to be the case and subscribed

strongly to this view for many years. However, I have come

to believe that by and large they are made, not to a man-

made British Standard, but to Nature’s equivalent to a BS

which follows rules of physics, chemistry, biology, mathem-

atics, engineering and so on. Further, I believe that in the

context of site investigation and in the understanding of

the site, application of geological education and experience

should be able to make a moderately close approximation

of the actual geological conditions from the desk study and

that when this is supplemented by ground and laboratory

investigation, there should be, ideally, nothing that has not

been discovered... it should be a realistic goal to be able

to make a very close approximation to the actual site condi-

tions, particularly if you know what you are looking for and

what questions to ask.

Thus, from a knowledge of the geological and engineering rock mech-

anics principles, we ought to be able to predict what we will find

underground and what will happen when the rock engineering takes

place. The authors agree with this: there should be no ‘unexpected’ fail-

ures. Against such engineering rationality are examples like the saying

in the tunnelling industry that ’the history of tunnelling is the history of

the unexpected’. Indeed, all sorts of problems are experienced in tunnels,

as Whittaker and Frith (1990) have illustrated.

We can say for sure, however, that taking the advice of good en-

gineering and structural geologists and implementing the principles

of engineering rock mechanics will certainly reduce the likelihood of

‘unexpected events’ occurring during rock engineering.

42.8 In an article on geological and geotechnical investigations for

tunnelling, Parker (1 996) estimates that “even comprehensive ex-

ploration programs recover a relatively miniscule drill core volume,

less than 0.0005% of the excavated volume of the tunnel“. Do you

think that sampling only this proportion of the rock mass is enough?

A2.8 The 0.0005% sampling level is equivalent to one vertical 100 mm

diameter borehole for every 100 m length along a 5 m diameter tunnel

and, if we did not have other information to guide us, such a sampling

proportion would certainly not be enough.

However, the reason for highlighting the sampling percentage is

that (given we have supplementary information from regional geology,

outcrops, previous engineering, etc., and hence some form of geological

model already established) such spot sampling can often be enough,

but only because the core provides the method for refining an existing

5Whittaker B. N. and Frith R. C. (1990) Tunnelling: Design, Stability and Construction.

Parker H. W. (1996) Geotechnical Investigations, in Tunnel Engineering Handbook (J. 0.

Institution of Mining and Metallurgy, London, 46Opp.

Bickel, T. R. Kuesel and E. H. King, eds). Chapman and Hall, New York, 544pp.