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and theories such as the rock mass classifications NGI and CSIR (Hoek
and Brown, 1980), have also been made for the benefit of geologists and
mining engineers.
Essentially, the RQD, in the standard method, is measured for cores of
54 mm diameter drilled with a so-called NX casing core barrel. But some
other way exists to deal with other core barrels as suggested by Heuzé
(1971). Indeed, such an RQD index is the ratio of all core sections longer
than 100 mm taken for a given NX core run over the drilled length.
Nevertheless it would seem fair to say that, despite the great importance
of the procedure, there are still many pertinent questions to be answered,
questions which deal with the validity of such an index. And, to answer
them fully, to the satisfaction of all those interested, is anything but
easy task.
The objections or questions raised by the majority of authors who have
little confidence in or much suspicion of the RQD index, are mainly the
following: (1) RQD cannot discriminate for rocks of different nature;
(2) it depends on the very moment at which you measure it, and little can
be said for clays that dry within minutes, RQD passing from 100 % to 0 %;
(3) the measurements of the rock quality index is dependent on the manner
that drillers handle the cores, and also on the way that geologists can
adequately sort out the natural fractures from the artificial ones; (4) the
core discing due to stress relief is not necessarily present in the rock mass
undisturbed.
The first question is quite a subtle one which we cannot answer one way
or another by our own means. Concerning the second question, we contend
that the RQD index has some claim to validity. Indeed, its validity depends
on the time the measure is made. Moreover, when the clay loses its
humidity, the RQD of the core run may be truly altered. In that case, such
an RQD passes rapidly from "excellent" to "poor", and consequently the
information on this "indirect in situ deformability index" of the rock mass
cannot be assessed. As for the third question, there are critical and valuable
points to clarify in order to obtain any objective classification. This question
can be considered as the human factor, the subjective one. The only way to
deal with it, at this moment, is to design some arbitrary cut-off levels to
decrease and/or extract such a bias in order to obtain the best reliable
representation of the reality. Finally, the fourth and last question manifestly
deals with low RQD, as does the second one. It deals consequently with the
conservative aspect of safety for the deformability of the rock mass
(Dershowitz et al., 1979; U.S. National Committee for Rock Mechanics,
1981). Hence, it will not interfere negatively with the precision of the
statistical method called "stereotomography" which makes use of redundant
and numerous data. To prove the consistency and the conclusiveness of this
statement, we will try to describe in some detail, in the next sections of this
study, most of the points raised above.
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