Engineering Rock Mechanics

Geometrical properties of discontinuities 1 29

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Figure 7.15 Diagrammatic representation of discontinuity traces intersecting a

scanline set up on a rock face.

waveforms or fractals. From the practical point of view, only one
technique has any degree of universality and that is the Joint Roughness
Coefficient (JRC) developed by Barton and Choubey (1977). This method
involves comparing a profile of a discontinuity surface with standard
roughness profiles and hence assigning a numerical value to the roughness.
The chart of standard profiles is shown in Fig. 7.16.
Despite the obvious limitations of reducing all roughness information to
a single scalar value, the possibly subjective nature of the assessment and
its wholly empirical nature, the JRC profiles have proved to be of significant
value in rock engineering. The geometrical roughness is naturally related
to various mechanical and hydraulic properties of discontinuities. On the
purely geometrical side, it is possible to predict the amplitude of asperities
from the JRC and profile length. On the mechanical side, shear strength
can be predicted via JRC and other properties. Moreover, there are obvious
implications for aperture and variation in aperture as a function of
discontinuity roughness.
The aperture is the distance between adjacent walls of a discontinuity,
i.e. it is the openness of the discontinuity. This parameter has mechanical
and hydraulic importance, and a distribution of apertures for any given
discontinuity and for different discontinuities within the same rock mass
is to be expected. A limited estimate of the JRC is possible from borehole
core, but in general no indication of aperture is possible from the core itself,
except where the fractures are incipient and the core is not separated.
Current research in the hydraulic context indicates that a discontinuity
cannot be approximated as two parallel planes because of the phenomenon
of channel flow, where the fluid mainly flows through certain channels
within the discontinuity created by tracks of larger local apertures.

7.2.6 Statistical analysis and practical examples
The reader will have noted the recurring theme that most of the topics
being discussed are not deterministically tractable due to insufficient three-
dimensional characterization of the rock mass structure. Consequently,
statistical techniques in the data reduction, presentation and analysis are
helpful. We may not be able to specify all the discontinuities in the rock
mass, but we might be able to provide an excellent engineering
approximation via statistical generators which will allow repeated