It is the existence of discontinuities in a rock mass that makes rock mechanics a unique subject. The word discontinuity denotes any separation 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...