Engineering Rock Mechanics

Natural, pre-existing fractures 95

, Approximately linear^6 < d <^16

e

b

g 50.

I I % I

0 10 20 30 40

Frequency, d rn-l

Figure 7.6 Theoretical relation between RQD and fracture frequency.

8

equal, with a value equal to the reciprocal of the fracture spacing. Along
a scanline in the field, the sample mean and sample standard deviation
are often found to have similar values, especially when 50 or more
fractures have been intersected.
We emphasize that the negative exponential distribution is not being
invoked because it characterizes a random or Poisson occurrence of frac-
tures, but because it is the distribution to which a series of superimposed
spatial distributions converges. Because most rock masses contain sets of
fractures from a number of different fracturing episodes, and these sets
can all have different distributions, the negative exponential distribution
is a good description of the overall distribution.
The Rock Quality Designation (RQD) for a fractured borehole core or
along a scanline such as that in Fig. 7.3 is defined as 100CzI Xi/L%,
where Xi = intact core lengths > 0.1 m, N is the number of these in
the core, and L is the total core length. This is the percentage of core
length containing pieces greater than 0.1 m long. For the case of the
usual RQD threshold value of 0.1 m, and with the assumption of a
negative exponential distribution of intact rock lengths, it can be shown
that the Rock Quality Designation (RQD) is related to the mean fracture
frequency, A, as RQD = 100e-o~lA(O.ll + 1).
Fig. 7.6 illustrates the relation between RQD and fracture frequency
and indicates that there is an approximately linear relation for h = 6
to 16 m-'. The equation of this line is RQD = -3.68h + 110.4.
The RQD concept with a threshold value of 0.1 m can be generalized
for an arbitrary threshold, t. Then the relation between RQD and f is
RQD = 100e-fA(tk + 1).
There are many characteristics of rock mass fractures that are import-
ant for engineering rock mechanics. The International Society for Rock
Mechanics (ISRM) has recommended the measurement of the ten frac-
ture characteristics illustrated in Fig. 7.7. These are: orientation, spacing,
persistence, roughness, wall strength, aperture, filling, seepage, number
of sets, and block size (Barton, 1978; Hudson, 1989) 5.

Barton N. 1978. Suggested Methods for the Quantitative Description of Discontinuities