Page 71 - Rock Mechanics For Underground Mining

IMPORTANT GEOMECHANICAL PROPERTIES OF DISCONTINUITIES

Figure 3.8 Relation between RQD
and mean discontinuity frequency
(after Priest and Hudson, 1976).

For values of in the range 6 to 16/m, a good approximation to measured RQD
values was found to be given by the linear relation

RQD =−3.68 + 110.4 (3.4)

Figure 3.8 shows the relations obtained by Priest and Hudson (1976) between mea-
sured values of RQD and , and the values calculated using equations 3.3 and 3.4.
It should be noted, however, that RQD measured from drill core can be an unreliable
predictor of discontinuity frequency because:

it relies on the ability of the logger to discriminate between natural fractures and

those caused by blasting or drilling;
it may be influenced by the strength of the rock material being drilled;

good core recovery depends on the drilling practice used (see section 3.4.2);

RQD is not a good measure of the better rock mass conditions. If a rock mass has

one uniformly spaced discontinuity set with a spacing of either 0.1 m or 5 m, the
RQD will be 100 in both cases; and
in an anisotropic rock mass, the measured RQD will be influenced by drilling

orientation.
Discontinuity spacing is a factor used in many rock mass classification schemes.
Table 3.1 gives the terminology used by the ISRM Commission (1978a).
Persistence is the term used to describe the areal extent or size of a discontinuity
within a plane. It can be crudely quantified by observing the trace lengths of discon-
tinuities on exposed surfaces. It is one of the most important rock mass parameters
but one of the most difficult to determine. Figure 3.9 shows a set of simple plane
sketches and block diagrams used to help indicate the persistence of various sets of
discontinuities in a rock mass. Clearly, the persistence of discontinuities will have a
major influence on the shear strength developed in the plane of the discontinuity and
on the fragmentation characteristics, cavability and permeability of the rock mass.
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