Page 23 - Rock Mechanics For Underground Mining

INHERENT COMPLEXITIES IN ROCK MECHANICS

mass. Joints and other fractures of geological origin are ubiquitous features in a body
of rock, and thus the strength and deformation properties of the mass are influenced
by both the properties of the rock material (i.e. the continuous units of rock) and
those of the various structural geological features. These effects may be appreciated
by considering various scales of loading to which a rock mass is subjected in mining
practice. The process of rock drilling will generally reflect the strength properties of
the intact rock, since the process operates by inducing rock material fracture under
the drilling tool. Mining a drive in jointed rock may reflect the properties of the joint
system. In this case, the final cross section of the opening will be defined by the joint
attitudes. The behaviour of the rock around the periphery of the drive may reflect the
presence of discrete blocks of rock, whose stability is determined by frictional and
other forces acting on their surfaces. On a larger scale, e.g. that of a mine pillar, the
jointed mass may demonstrate the properties of a pseudo-continuum. Scale effects as
described here are illustrated schematically in Figure 1.2.
These considerations suggest that the specification of the mechanical properties of
a rock mass is not a simple matter. In particular, the unlikely possibility of testing
jointed rock specimens, at scales sufficient to represent the equivalent continuum sat-
isfactorily, indicates that it is necessary to postulate and verify methods of synthesising
rock mass properties from those of the constituent elements.

1.2.3 Tensile strength
Rock is distinguished from all other common engineering materials, except concrete,
by its low tensile strength. Rock material specimens tested in uniaxial tension fail at
stresses an order of magnitude lower than when tested in uniaxial compression. Since
joints and other fractures in rock can offer little or no resistance to tensile stresses, the
tensile strength of a rock mass can be assumed to be non-existent. Rock is therefore
conventionally described as a ‘no-tension’ material, meaning that tensile stresses
cannot be generated or sustained in a rock mass. The implication of this property for
excavation design in rock is that any zone identified by analysis as being subject to
tensile stress will, in practice, be de-stressed, and cause local stress redistribution.
Figure 1.2 The effect of scale on De-stressing may result in local instability in the rock, expressed as either episodic
rock response to imposed loads: (a) or progressive detachment of rock units from the host mass.
rock material failure in drilling; (b)
discontinuities controlling the final
1.2.4 Effect of groundwater
shape of the excavation; (c) a mine pil-
Groundwater may affect the mechanical performance of a rock mass in two ways. The
lar operating as a pseudo-continuum.
most obvious is through the operation of the effective stress law (section 4.2). Water
under pressure in the joints defining rock blocks reduces the normal effective stress
between the rock surfaces, and therefore reduces the potential shear resistance which
can be mobilised by friction. In porous rocks, such as sandstones, the effective stress
law is obeyed as in granular soils. In both cases, the effect of fissure or pore water
under pressure is to reduce the ultimate strength of the mass, when compared with
the drained condition.
A more subtle effect of groundwater on rock mechanical properties may arise
from the deleterious action of water on particular rocks and minerals. For example,
clay seams may soften in the presence of groundwater, reducing the strength and
increasing the deformability of the rock mass. Argillaceous rocks, such as shales
and argillitic sandstones, also demonstrate marked reductions in material strength
following infusion with water.
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