Page 22 - Rock Mechanics For Underground Mining

ROCK MECHANICS AND MINING ENGINEERING

modes, soil failure is associated with processes such as dilatation, particle rotation
and alignment. This distinction between the different media has other consequences.
For example, soils in their operating engineering environments are always subject
to relatively low states of stress. The opposite is frequently true for rock. Further
differences arise from the relatively high elastic moduli, and the relatively low material
permeabilities of rocks compared with soils. The latter distinction is important. In
most rock formations, fluid flow occurs via fissures and channels, while in soils fluid
migration involves movement through the pore space of the particulate assembly. It
appears, therefore, that rock and soil mechanics should be regarded as complementary
rather than mutually inclusive disciplines.
Having suggested that rock mechanics is a distinct engineering discipline, it is clear
that its effective practical application demands an appreciation of its philosophic inte-
gration with other areas of geomechanics. Rock mechanics, soil mechanics, ground-
water hydrology and structural geology are, in the authors’ opinions, the kernels of the
scientific basis of mining engineering. Together, they constitute the conceptual and
factual base from which procedures can be developed for the control and prediction
of rock behaviour during mining activity.

1.2 Inherent complexities in rock mechanics

It has been observed that rock mechanics represents a set of principles, a body of
knowledge and various analytical procedures related to the general field of applied
mechanics. The question that arises is – what constituent problems arise in the me-
chanics of geologic media, sufficient to justify the formulation or recognition of a
coherent, dedicated engineering discipline? The five issues to be discussed briefly
below determine the nature and content of the discipline and illustrate the need for
a dedicated research effort and for specialist functions and methodologies in mining
applications.

1.2.1 Rock fracture
Fracture of conventional engineering material occurs in a tensile stress field, and so-
phisticated theories have been postulated to explain the pre-failure and post-failure
performance of these media. The stress fields operating in rock structures are perva-
sively compressive, so that the established theories are not immediately applicable
to the fracture of rock. A particular complication in rock subject to compression is
associated with friction mobilised between the surfaces of the microcracks which are
the sites for fracture initiation. This causes the strength of rock to be highly sensitive
to confining stress, and introduces doubts concerning the relevance of such notions as
the normality principle, associated flow and plasticity theories generally, in analysing
the strength and post-failure deformation properties of rock. A related problem is the
phenomenon of localisation, in which rupture in a rock medium is expressed as the
generation of bands of intensive shear deformation, separating domains of apparently
unaltered rock material.

1.2.2 Scale effects
The response of rock to imposed load shows a pronounced effect of the size or scale of
the loaded volume. This effect is related in part to the discontinuous nature of a rock
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