7 Discontinuities
It is the existence of discontinuities in a rock mass that makes rock
mechanics a unique subject. The word 'discontinuity' denotes any separa-
tion in the rock continuum having effectively zero tensile strength and is
used without any genetic connotation (cf. the words 'joint' or 'fault' which
describe discontinuities formed in different ways).
The material comprising the intact rock is natural and has been
subjected in most cases to millions of years of mechanical, thermal and
chemical action. During these processes, the discontinuities have been
introduced into the rock by geological events, at different times and as a
result of different stress states. Very often, the process by which a
discontinuity has been formed (e.g. a joint which has been pulled open or
a fault which has been sheared) may have implications for its geometrical
and mechanical properties, and so it is always important to have an
understanding of the formation of discontinuities using structural geology
principles (see Price and Cosgrove 1990).
In the engineering context here, the discontinuities can be the single most
important factor governing the deformability, strength and permeability
of the rock mass. Moreover, a particularly large and persistent discontinuity
could critically affect the stability of any surface or underground
excavation. For these reasons, it is necessary to develop a thorough
understanding of the geometrical, mechanical and hydrological properties
of discontinuities and the way in which these will affect rock mechanics
and hence rock engineering.
In this chapter these subjects will be discussed explicity; in Chapter 8 the
discontinuity properties will be incorporated in a study of rock masses; in
Chapter 9 discontinuity networks and the associated secondary permeabili-
ty will be explained; and in Chapter 10 inhomogeneity and anisotropy are
highlighted, remembering that these are often caused by variations in the
occurrence of discontinuities.