Geometrical properties of discontinuities 1 1^7plane, incorporating factors such as the shape of the bounded plane and
the associated characteristic dimensions (e.g. the discontinuities could be
assumed to be circular discs for the purpose of analysis and sampling).
- Roughness: although discontinuities are assumed to be planar for the
purposes of orientation and persistence analysis, the surface of the
discontinuity itself may be rough. Discontinuity roughness may be
defined either by reference to standard charts or mathematically. - Aperture: the perpendicular distance between the adjacent rock
surfaces of the discontinuity. This will be a constant value for parallel and
planar adjacent surfaces, a linearly varying value for non-parallel but planar
adjacent surfaces, and completely variable for rough adjacent surfaces. - Discontinuity sets: discontinuities do not occur at completely random
orientations: they occur for good mechanical reasons with some degree of
'clustering' around preferred orientations associated with the formation
mechanisms. Hence, it is sometimes convenient to consider the concept of
a discontinuity set (which consists of parallel or sub-parallel discon-
tinuities), and the number of such sets that characterize a particular rock
mass geometry. - Block size: as is illustrated in Fig. 7.3 and depending on the previously
described characteristics, rock blocks can be present. In terms of
excavation and support, it is helpful to have an estimate both of the mean
block size and the block size distribution, which is an in situ analogue of
the particle size distribution used in soil mechanics.
The degree of work that has been devoted to providing techniques for
measurement, data reduction and presentation associated with each of
these seven main aspects of the geometrical properties has been highly
variable. There is no standardized, or indeed 'correct', method of measur-
ing and characterizing rock structure geometry, because the emphasis and
the accuracy with which the separate parameters are specified will depend
on the engineering objective. Therefore, initially we will describe the salient
features of these parameters, taking each individually, and later invoke this
information with respect to specific engineering themes.
7.2.7 Discontinuity spacing and frequency
In Fig. 7.4, we illustrate a sampling line through a rock mass, which
intersects a number of discontinuities. The length of the sampling line is L
metres, the number of discontinuities it intersects is N, and thus,
discontinuity frequency, A = NIL m-'
and
mean spacing, X = L/N m.The discontinuity frequency, being the number of fractures per metre, is
the reciprocal of the mean spacing.
We can also consider the distribution of the individual spacings