Engineering Rock Mechanics

Tailoring testing to engineering requirements 1 75

engineering objective. In Fig. 11.2, there is a histogram of the rock
parameters that have been studied in association with the design of pressure
tunnels for hydroelectric schemes worldwide. The parameters have been
plotted in order of frequency of occurrence along the horizontal axis and
indicate the perceived first 10 (for example) most important parameters
involved in the design of pressure tunnels (this histogram has been
constructed from a study of the literature, but could equally well have been
obtained from existing design practice or numerical modelling require-
ments). It follows that we should design our testing programme accordingly
in line with the design requirements. In this example, in situ stress is regarded
as being of prime importance and should, therefore, definitely be
determined. Conversely, it is assumed that in most cases the in situ stress is
not an important parameter for surface blasting and slope stability, and
therefore would not be determined to support these objectives.
Standardized procedures are advantageous for measuring rock pro-
perties and site conditions, but we must decide whether we are going to
make particular measurements on the basis of the overall objective. If a
slope is being designed, there is a great deal of experience for guidance; if
a new structure is being designed, such as the tunnels housing a super-
conducting energy storage magnet, the most relevant parameters have to
be established. In the latter case, since the magnet expands and contracts
on charging and discharging, it could well be the fatigue properties of the
excavation-peripheral discontinuities that are of paramount importance,
and there are no standardized tests for these.
Finally, while discussing the overall strategy of approach, one has to
consider whether emphasis is to be placed on index tests, fundamental
tests, or a combination of the two. An index test is one that can be

I.R. - Intact rock

D - Discontinuity

R.M. rock mass

Figure 11.2 Importance of rock mechanics parameters for hydroelectric scheme

pressure tunnel design (as established from the literature and compiled by

Arnold, 1993).