Page 154 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

Figure 4.50 Hoek-Brown peak
strength envelope for a diorite rock
mass with c = 100 MPa, m i = 25
and GSI 65 and the equivalent
Coulomb shear strength parameters.

Methods of doing this are proposed by Hoek and Brown (1997), Hoek et al. (2002)
and Sofianos (2003).
It is important to recognise that the Hoek-Brown rock mass strength criterion as
presented here is a short-term peak strength criterion and not a crack initiation or
long-term strength criterion. Furthermore, it applies only to sensibly isotropic rock
masses as in the case illustrated in Figure 4.49. In particular, it should not be used when
failure is governed by a single discontinuity or by a small number of discontinuities.
The limitations of the criterion and the conditions under which it should be used have
been discussed by Hoek and Brown (1997) and are illustrated in Figure 4.51.

4.9.2 Deformability
The study of the complete stress–strain behaviour of jointed rock masses involving
post-yield plastic deformation, is beyond the scope of this introductory text. How-
ever, it is of interest to consider the pre-peak behaviour with a view to determining
equivalent overall elastic constants for use in design analyses.
In the simplest case of a rock mass containing a single set of parallel discontinuities,
a set of elastic constants for an equivalent transversely isotropic continuum may be
determined. For a case analogous to that shown in Figure 2.10, let the rock material
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