Engineering Rock Mechanics

374 Design of underground excavations

Figure 20.2 Interaction of high stresses and rock mass fractures has combined to cause
failure of the rock mass and the installed support in an underground gold mine.

stress-induced failure of the intact rock, or a combination of the two?
The consequences of interaction between high stresses and rock mass
structure are illustrated in Fig. 20.2, where we can see that the individual
rock blocks have been displaced by a considerable amount and the
installed support has been significantly distorted.
As with the design of surface structures on rock masses, it is necessary
to establish the project objective before deciding on the type of analysis
that will be conducted. The project objective determines the perform-
ance requirements of the structure, the modelling that should be used
to provide the necessary predictive capability for rock engineering, and
hence the rock mechanics information required. Although knowledge
of, for example, the structural geology of the host rock and its basic
mechanical properties will always be useful, there is no such thing as a
standard modelling procedure and hence no standard site investigation
procedure for rock engineering.
We encourage you to apply the basic principles of rock mechanics
and rock engineering to all potentially complex analyses, as most of the
trends in rock behaviour can be predicted from basic principles. The type
of approach illustrated by the list of questions following in Section 20.2
is a good way to begin. It is only when the overall rock behaviour is
not clear, or when large numbers of numerical values are required, that
more sophisticated analyses should be implemented. Accordingly, we
concentrate in this chapter on the application of the basic principles and
techniques to a variety of rock engineering circumstances.