Rock masses 17Figure 2.4 Initiation and propagation of a large wedge failure in an open-pit mine,
Western Australia.
two major shear zones and the open-pit mine wall have formed a rock
block which has slid downwards. The traces of dust in Fig. 2.4a and the
clouds of dust in Fig. 2.4b were not caused by blasting: the unstable rock
block slid down under its own weight, and the dust was generated by
the rock surfaces sliding over one another and escaping through rock
fractures.
Natural fractures in the rock mass, especially joints and faults, can
also cause instabilities underground. Some large unsupported caverns
may be stable, as in the cavern shown in Fig. 2.5, but often the rock