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plane on the net, that means for the visible and "obvious" ones shown from
structural field surveys, or even for the less obvious, "invisible" ones. The
reason for this is self-explanatory. But an objective way to register the
weakest planes (which could be the invisible ones in this occurrence) must be
used. The only way to do so will be to pass all imaginable cyclographic
traces, but, for reasons of economy, some criteria can be chosen in the
selection of a measure at fixed short intervals of azimuths and dips. In our
study, we chose "arbitrarily" the attitudes of three main discontinuity
families. Nevertheless, remembering the above explanation, we will now
describe a practical example, in which such techniques as described above
were used.
For the stereotomogram (Fig. 3) and for its histomogram (Fig. 4), a similar
process was used. We counted along the cyclographic traces of great circles
all the paired data or the composites. For example, in the case of the
NNW-SSE joint family, N166/52, we did the following. On the stereotomo-
gram we counted : CDC-CC-CF-...-EC-CC-DD-...-DDF-CD-DC-..., and on
the histomogram : 423-54-32-...-33-33-95-...-934-61-29-... . After that, we
combined each value found on the stereotomogram with its correspon-
ding value on the histomogram. Finally, we sorted their composites between
10 equal range classes of RQD identified by their class mark : 95, 85, 75, ...,
15, 5. Then, we computed the weighted RQD mean value of the discon-
tinuity attitude :
RQD (N166/52) =
[(19*0.85)+(276*0.75)+(241*0.65)+(25*0.55)+(7*0.45)] / [19+276+241+25
+7]
RQD (N166/52) = 396.70 / 568 = 69.84 % = D +
We found through the same procedure the weighted RQD mean values
for the other two discontinuity attitudes :
RQD (N046/47) = 449.90 / 632 = 71.19 % = C -
RQD (N105/80) = 253.10 / 358 = 70.70 % = C -
Of course, if all possible discontinuity planes showing on the nets are
scrutinized, we will certainly find the real weakest discontinuity attitudes,
and we may also check retrospectively with old joint surveys that they are
indeed the attitudes of the main discontinuity families of the rock mass.
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