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As one can notice, RQD is often used to represent the entire borehole,
despite the fact that it is computed every 1 to 2 m along the core run.
Also, as are many other rock mass parameters, RQD is seen almost uniquely
in two-dimensional views with sections of plans. In fact, the RQD is a
three-dimensional discrete form of data, and this problem resembles those
of tomography or of computer-aided (or axial) tomography (CAT).
On the one hand, in tomography (Dines and Lytle, 1979; Hounsfield,
1980; Jaffe, 1982), all structures above and below the plane of interest
are blurred, and only the precise information of a thin slice in focus remains.
These findings mean that the plane view is a thin section of
three-dimensional data and not the sums of all slices which will be confused
on the same two-dimensional image as for X-rays pictures, and where any
pattern or anomaly will be difficult to understand. On the other hand, besides
its similarity with CAT, stereotomography offers three more variable factors.
For discontinuities, and incidentally for low RQD values which are their
signature, one will deal with the spatial recurring of fracture patterns, their
various scales [ "homothetism" ], and their persistence or continuity.
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Using the lithologic log, RQD is sorted along the core run at intervals to
establish where the mineral and the waste are more caveable. In our
procedure, a systematic sampling has been made: the 10-m interval.
Nevertheless, any other core interval could be chosen; a smaller one would
give more noticeable results for large scales showing the smallest details,
whereas a larger interval will show the regional details present in small
scales.
Between two boreholes there exists a region where the rock can be more
or less fractured. The question in our RQD study has been to find the
degree of fracturation. Firstly, we measured the RQD each 10 m along the
core run and from the surface, and not from another point of reference such
as a horizontal one at a given level. The choice of the borehole collar as the
reference was determined solely for practical purposes, that is to facilitate
the collection of data. Also, it spared the technician the superfluous and
tedious job of transposing, with the least possible risk of errors, all the data
on some conventional horizontal level which was not
mandatory to the technique.
From then on, we calculated for each 10 m of core run, the modified
recovery (recovery for core lengths greater than 100 mm), and then, using
it in a measurement of RQD we computed each 40-m multiple from the
borehole collar. Such a measurement consisted of an RQD value calculated
10 m above and 10 m below the 40-m multiples. Following that, a "virtual"
line was drawn between the 40-m multiples of adjacent holes: in other
words, from center-to-center of the 20-m intervals obtained from paired
boreholes. Finally, we calculated, for such a line, the mean RQD value from
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page-4
