Page 130 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

Figure 4.30 Normalised peak str-
ength envelope for sandstones (after
Hoek and Brown, 1980).

Bieniawski found that, for the range of rock types tested, k 0.75 and c 0.90.
The corresponding values of A and B are given in Table 4.1. Note that both A and B
take relatively narrow ranges for the rock types tested.
Brady (1977) studied the development of rock fracture around a bored raise in a
pillar in mineralised shale in a trial stoping block at the Mount Isa Mine, Australia.

Figure 4.31 Hoek-Brown failure
envelope for Lac du Bonnet gran-
ite based on laboratory peak strength
(Lab Peak), long-term strength (Lab
cd ) and in situ crack initiation
stress ( ci ) determined by microseis-
mic monitoring (after Martin, 1997).
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