Page 21 - Rock Mechanics For Underground Mining

GENERAL CONCEPTS

strength and deformation properties of the orebody and adjacent country rock must
be determined in some accurate and reproducible way. The geological structure of the
rock mass, i.e. the location, persistence and mechanical properties of all faults and
other fractures of geologic age, which occur in the zone of influence of mining activ-
ity, is to be defined, by suitable exploration and test procedures. Since the potential
for slip on planes of weakness in the rock mass is related to fissure water pressure, the
groundwater pressure distribution in the mine domain must be established. Finally,
analytical techniques are required to evaluate each of the possible modes of response
of the rock mass, for the given mine site conditions and proposed mining geometry.
The preceding brief discussion indicates that mining rock mechanics practice
invokes quite conventional engineering concepts and logic. It is perhaps surpris-
ing, therefore, that implementation of recognisable and effective geomechanics pro-
grammes in mining operations is limited to the past 40 or so years. Prior to this
period, there were, of course, isolated centres of research activity, and some attempts
at translation of the results of applied research into mining practice. However, design
by precedent appears to have had a predominant rˆole in the design of mine structures.
(A detailed account of the historical development of the discipline of mining rock
mechanics is given by Hood and Brown (1999)). The relatively recent appearance and
recognition of the specialist rock mechanics engineer have resulted from the industrial
demonstration of the value and importance of the discipline in mining practice.
A number of factors have contributed to the relatively recent emergence of rock
mechanics as a mining science. A major cause is the increased dimensions and pro-
duction rates required of underground mining operations. These in turn are associated
with pursuit of the economic goal of improved profitability with increased scale of
production. Since increased capitalisation of a project requires greater assurance of
its satisfactory performance in the long term, more formal and rigorous techniques
are required in mine design, planning and scheduling practices.
The increasing physical scale of underground mining operations has also had a
direct effect on the need for effective mine structural design, since the possibility
of extensive failure can be reckoned as being in some way related to the size of the
active mine domain. The need to exploit mineral resources in unfavourable mining
environments has also provided a significant impetus to geomechanics research. In
particular, the continually increasing depth of underground mining in most parts
of the world, has stimulated research into several aspects of rock mass performance
under high stress. Finally, more recent social concerns with resource conservation and
industrial safety have been reflected in mining as attempts to maximise the recovery
from any mineral reserve, and by closer study of practices and techniques required
to maintain safe and secure work places underground. Both of these concerns have
resulted in greater demands being placed on the engineering skills and capacities of
mining corporations and their service organisations.
In the evolution of rock mechanics as a field of engineering science, there has been
a tendency to regard the field as a derivative of, if not a subordinate discipline to, soil
mechanics. In spite of the commonality of some basic principles, there are key issues
which arise in rock mechanics distinguishing it from soil mechanics. The principal
distinction between the two fields is that failure processes in intact rock involve
fracture mechanisms such as crack generation and growth in a pseudo-continuum. In
soils, failure of an element of the medium typically does not affect the mechanical
integrity of the individual grains. In both diffuse and locally intense deformation
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