1 16 Discontinuitiesis an essentially one-dimensional sample through the three-dimensional rock
mass, with obvious limitations. For example, a borehole core will provide an
excellent means of examining the discontinuity occurrence and hence
frequency in the borehole direction, but will provide little information on the
lateral extent of the intersected discontinuities. Thus, a key aspect of rock
structure interpretation will be the extrapolation to three-dimensional
properties from either one- or two-dimensional measurements.
One of the most fundamental aspects of discontinuity occurrence is the
mean value and distribution of spacings between discontinuities, and the
associated indices of discontinuity frequency and Rock Quality Designa-
tion. These and several other aspects of rock structure geometry will be
discussed in the next section.
7.2 Geometrical properties of discontinuities
In Fig. 7.3, we present a schematic representation of two planes within a
rock mass. There are no assumptions about whether these planes are real
or imaginary surfaces or sections. Also, the borehole or scanline could be
real, or postulated solely for the purpose of analysis. This diagram shows
the main features of rock mass geometry with, in particular, the following
parameters being illustrated:
- Spacing and frequency: spacing is the distance between adjacent
discontinuity intersections with the measuring scanline. Frequency (i.e. the
number per unit distance) is the reciprocal of spacing (i.e. the mean of these
intersection distances). - Orientation, dip directioddip angle: the discontinuity is assumed to be
planar and so the dip direction (the compass bearing of the steepest line
in the plane) and the dip angle (the angle that this steepest line makes to
the horizontal plane) uniquely define the orientation of the discontinuity. - Persistence, size and shape: the extent of the discontinuity in its own
Figure 7.3 Schematic of the primary geometrical properties of discontinuities in
rock (from Hudson, 1989).