Page 95 - Rock Mechanics For Underground Mining

ROCK MASS CLASSIFICATION

Figure 3.29 Computer generated
contoured stereographic projection
(after Rocscience, 1999).

3.7 Rock mass classification

3.7.1 The nature and use of rock mass classification schemes
Whenever possible, it is desirable that mining rock mechanics problems be solved
using the analytical tools and engineering mechanics-based approaches discussed in
later chapters of this book. However, the processes and interrelations involved in
determining the behaviour of the rock surrounding a mining excavation or group
of excavations are sometimes so complex that they are not amenable to enginerring
analysis using existing techniques. In these cases, design decisions may have to take
account of previous experience gained in the mine concerned or elsewhere.
In an attempt to quantify this experience so that it may be extrapolated from one site
to another, a number of classification schemes for rock masses have been developed.
These classification schemes seek to assign numerical values to those properties
or features of the rock mass considered likely to influence its behaviour, and to
combine these individual values into one overall classification rating for the rock
mass. Rating values for the rock masses associated with a number of mining or civil
engineering projects are then determined and correlated with observed rock mass
behaviour. Aspects of rock mass behaviour that have been studied in this way include
the stable spans of unsupported excavations, stand-up times of given unsupported
spans, support requirements for various spans, cavability, stable pit slope angles,
hangingwall caving angles and fragmentation. A number of these assessments made
from geotechnical data collected in the exploration or feasibility study stages of a
mining project may provide useful guides to the selection of an appropriate mining
method.
Although the use of this approach is superficially attractive, it has a number of
serious shortcomings and must be used only with extreme care. The classification
scheme approach does not always fully evaluate important aspects of a problem,
so that if blindly applied without any supporting analysis of the mechanics of the
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