148 Rock massesof the total failure behaviour of a rock mass: this is because of the presence
of the discontinuities and the manifold ways in which the rock mass
structure can break down.
For example, a block might fall out of the rock mass during excavation
and lead to a ravelling-type failure. Alternatively, discontinuities might be
sheared in one direction in the plane but not in another direction.
Individual blocks may fail due to high stresses. The collapse of the rock
mass may occur through many such events, or through failure on one
significant through-going discontinuity such as a fault.
It is impossible to summarize all these possibilities in any simple
quantitative fashion. Also, the failure of the rock mass and its interaction
with the excavation process has links with the objective of rock
engineering. The way in which one characterizes the failure will depend
on the engineering objective. In civil engineering, the objective may be, for
example, that the displacement in the rock mass nowhere exceeds a certain
amount. In mining engineering, we may wish the rock to be in a continual
state of imminent failure as, for example, in the block caving method of
mining, where a large block of ore is undercut and allowed to collapse in
a controlled way via ore passes.
The only generic way of quantitatively characterizing rock mass failure
is through the Hoek-Brown criterion and/or the rock mass classification
systems. We have already described the former; the latter will be
described in Chapter 12. The discussion of rock mass failure, with specific
reference to applications, will be continued in Chapters 17-20.