The occurrence of discontinuities 1 15that very often major discontinuities delineate blocks within the rock mass,
and within these blocks there is a further suite of discontinuities. Such
hierarchial systems of discontinuities may well be much more complex, but
this will not affect the ideas presented in this chapter. The engineer should,
however, realize the significance of, for example, the likelihood that most
of the smaller discontinuities could either terminate within one of the larger
blocks or at the boundary of a block. Also, as illustrated by the sketched
outlines of the engineered structures shown in Fig. 7.2, in general we might
expect that a relation of the form
(^1) Dc 1
stability =
number of discontinuities engineering dimension
should exist. This idea is intimated in Fig. 7.2, where schematic outlines of
a borehole, tunnel and large excavation have been overlain on a
photograph of a rock face.
Another factor which should be considered, and which has influenced
development of the study of discontinuities, is to what extent they can be
sampled. From Fig. 7.1 it is clear that a great deal of information can be
obtained by field measurement on a rock exposure, but even this only
provides an essentially two-dimensional sample slice through the three-
dimensional rock mass. In practice, we would like to have at least two such
exposures at different orientations to feel confident that some estimate of
the three-dimensional nature of the rock mass structure was being
obtained. Unfortunately, it is often the case that no such exposures are
available before the construction of a particular project. In this situation,
we must rely on borehole core retrieved during the drilling process, scans
using borehole television cameras or indirect methods using geophysical
techniques.
Bv far the most widelv used method is to studv the borehole core, but this
Figure 7.2 Discontinuities in rock and the engineered structure (A, borehole; B,
tunnel; C, large excavation).