Engineering Rock Mechanics

Concluding remarks 11 1

rz = 0.87

5.0 -

.

4.0

....

.

..

r/

.

.

II I I I I I I

n 0.2 0.4 0.6 0.8 I .o 1.2

2

cc

Figure 6.20 The Hoek-Brown empirical failure criterion.

intact granite, say), the compressive strength is found to be about 20 times
the tensile strength. Note that these strengths are now the rock mass
strengths, because not only the intact rock fracturing but also the large-scale
rock mass fracturing is incorporated into this criterion via the parameter
m. As we will see later in Chapters 8 and 12, relations can be postulated
between the parameters m and s and other measures of rock mass quality
using classification schemes. Also, inter-criteria relations can be found, in
particular linking the Hoek-Brown criterion with the Mohr-Coulomb
criterion, i.e. linking m and s with c and $.
The Hoek-Brown criterion has recently been recast (Hoek et al., 1992) to
take into account the experience gained over the 10 years since its
development.

6.6 Concluding remarks

We have presented three failure criteria that are extensively used in rock
mechanics and rock engineering. The reader should be aware of the
shortcomings of each of the criteria, and be prepared to make use of
whichever is best suited to a particular application. Further criteria are
available as listed in Fig. 6.21. None of the criteria take into account the
specific structure of the rock mass, in particular its fracturing configuration.
The occurrence of the natural pre-existing fractures in rock masses is the
next subject to discuss because of their profound influence on the
deformation, strength and failure of rock masses.