Page 152 - Rock Mechanics For Underground Mining
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ROCK STRENGTH AND DEFORMABILITY

































              Figure  4.49  Composite  peak
              strength characteristics for a hypo-
              thetical rock mass containing four
              sets of discontinuities each with the
              properties of the cleavage in the slate
              for which the data shown in Figure
              4.33b were obtained.



                                        be assumed to be isotropic. This would not be the case if one of the discontinuity sets
                                        had a substantially lower shear strength than the other sets.
                                          Because of the difficulty of determining the overall strength of a rock mass by
                                        measurement, empirical approaches are generally used. As discussed in section 4.5.5,
                                        Brady (1977) found that the power law of equation 4.23 could be applied to the
                                        mineralised shale at the Mount Isa Mine. An attempt to allow for the influence of
                                        rock quality on rock mass strength was made by Bieniawski (1976) who assigned
                                        Coulomb shear strength parameters, c and  , to the various rock mass classes in
                                        his geomechanics classification (Table 3.5). Correlations have also been proposed
                                        between other rock mass classification schemes and rock mass strengths (e.g. Barton,
                                        2002, Laubscher, 1990, Laubscher and Jakubec, 2001).
                                          The most completely developed of these empirical approaches is that introduced
                                        by Hoek and Brown (1980). Because of a lack of suitable alternatives, the Hoek-
                                        Brown empirical rock mass strength criterion was soon adopted by rock mechanics
                                        practitioners,andsometimesusedforpurposesforwhichitwasnotoriginallyintended
                                        and which lay outside the limits of the data and methods used in its derivation. Because
                                        of this, and as experience was acquired in its practical application, a series of changes
                                        were made and new elements introduced into the criterion. Hoek and Brown (1997)
                                        consolidated the changes made to that time and gave a number of worked examples to
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