146 Rock masses
Figure 8.6 Strength of a rock mass containing multiple discontinuity sets.We can consider, on the basis of this single plane of weakness theory,
what would happen if there were two or more discontinuities at different
orientations present in the rock sample. Each discontinuity would weaken
the sample as indicated in Fig. 8.4, but the angular position of the strength
minima would not coincide. As a result the rock is weakened in several
different directions simultaneously as shown in Fig. 8.6. The material tends
to become isotropic in strength, like a granular soil. When plotting the
superimposed curves, care should be taken in interpreting the magnitude
of
The main advantage of the single plane of weakness theory is its
simplicity and helpfulness in the interpretation of rock mass failure. We
have presented here the two-dimensional case (applicable to plane stress)
and one can imagine an extension to the general three-dimensional loading
case in which none of the principal stresses is perpendicular to the discon-
tinuity normal. In reality, the situation is rather complicated because the
stresses will not be transmitted directly through the discontinuity.
However, despite these shortcomings the authors feel that the advantages
gained by understanding such idealized models do help in interpreting the
far more complex behaviour of real rock masses.
We have already discussed the Hoek-Brown failure criterion in Chapter
6 in terms of its application to intact rock. The criterion is especially
powerful in its application to rock masses due to the constants rn and s
being able to take on values which permit prediction of the strengths of a
wide range of rock masses. Recent publications, i.e. Hoek and Brown (1988),
Hoek (1990) and Hoek et al. (1992), provide an update of the failure criterion
as it applies to rock masses, together with the relation between the
Hoek-Brown and Mohr-Coulomb criteria.
From the first of these publications, we have included Table 8.1 which
indicates the relation between rock mass quality and the rn and s material
constants. The table also provides a guide to the relation between these
constants and two commonly used rock mass characterization values, i.e.
the CSIR and NGI classification ratings (the latter being described in
Chapter 12)correctly for each of the discontinuities concerned.