Page 110 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

where inaccuracy in specimen preparation and surface flaws or contamination may
dominate behaviour and cause a strength decrease with decreasing specimen volume.
This, coupled with the requirement that the specimen diameter should be at least 10
times the size of the largest grain, provides a reason for using specimen diameters of
approximately 50 mm in laboratory compression tests.
Many explanations have been offered for the existence of size effects, but none
has gained universal acceptance. A popular approach is to interpret size effects in
terms of the distribution of flaws within the material. Much of the data on which
conclusions about size effects are based, were obtained using cubical specimens.
Brown and Gonano (1975) have shown that in these cases, stress gradients and end
effects can greatly influence the results obtained. The most satisfactory explanations
of observed size effects in rock and other brittle materials are those in which surface
energy is used as the fundamental material property (section 4.5.3).

4.3.6 Influence of strain rate
The ISRM Commission (1979) recommends that a loading rate of 0.5–1.0MPa s −1
be used in uniaxial compression tests. This corresponds to a time to the attainment
of peak strength in the order of 5–10 min. As the arguments presented below show,
it is preferable to regard strain or deformation, rather than axial stress or load, as the
controlling variable in the compression testing of rock. For this reason, the following
˙
discussion will be in terms of axial strain rate, ε a , rather than axial stress rate.
The times to peak strength recommended by the ISRM Commission (1979) corre-
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spond to axial strain rates in the order of 10 –10 −4 s . For rocks other than those
such as the evaporites which exhibit markedly time-dependent behaviour, departures
from the prescribed strain rate by one or two orders of magnitude may produce little
discernible effect. For very fast and very slow strain rates, differences in the observed
stress–strain behaviour and peak strengths can become quite marked. However, a
2
change in strain rate from 10 −8 s −1 to 10 s −1 may only increase the measured uniax-
ial compressive strength by a factor of about two. Generally, the observed behaviour
of rock is not significantly influenced by varying the strain rate within the range that
it is convenient to use in quasi-static laboratory compression tests.

4.3.7 Influence of testing machine stiffness
Whether or not the post-peak portion of the stress–strain curve can be followed and
the associated progressive disintegration of the rock studied, depends on the relative
stiffnesses of the specimen and the testing machine. The standard test procedure and
interpretation discussed in section 4.3.2 do not consider this post-peak behaviour.
However, the subject is important in assessing the likely stability of rock fracture in
mining applications including pillar stability and rockburst potential.
Figure 4.6 illustrates the interaction between a specimen and a conventional testing
machine. The specimen and machine are regarded as springs loaded in parallel. The
machine is represented by a linear elastic spring of constant longitudinal stiffness, k m ,
and the specimen by a non-linear spring of varying stiffness, k s . Compressive forces
and displacements of the specimen are taken as positive. Thus as the specimen is
compressed, the machine spring extends. (This extension is analogous to that which
occurs in the columns of a testing machine during a compression test.) When the
peak strength has been reached in a strain-softening specimen such as that shown
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