Page 109 - Rock Mechanics For Underground Mining

BEHAVIOUR OF ISOTROPIC ROCK MATERIAL IN UNIAXIAL COMPRESSION

Figure 4.5 Influence of height to di-
ameter (H/D) ratio on stress-strain
curves obtained in uniaxial compres-
sion tests carried out on Wombeyan
Marble using (a) brush platens, and
(b) solid steel platens (after Brown and
Gonano, 1974).

authorities (e.g. Hawkes and Mellor, 1970; Jaeger and Cook, 1979) recommend that
treatment of the sample ends, other than by machining, be avoided.

4.3.4 Influence of the standard of end preparation
In Figures 4.3 and 4.5, the axial stress-axial strain curves have initial concave up-
wards sections before they become sensibly linear. This initial portion of the curve
is generally said to be associated with ‘bedding-down’ effects. However, experience
shows that the extent of this portion of the curve can be greatly reduced by paying
careful attention to the flatness and parallelism of the ends of the specimen. Analyses
of the various ways in which a poor standard of end preparation influence the observed
response of the sample have been presented by Hawkes and Mellor (1970).
The ISRM Commission (1979) recommends that in a 50+ mm diameter specimen,
the ends should be flat to within 0.02 mm and should not depart from the perpendicular
to the specimen axis by more than 0.05 mm. The latter figure implies that the ends
could be out of parallel by up to 0.10 mm. Even when spherical seats are provided in
the platens, out-of-parallelism of this order can still have a significant influence on the
shape of the stress–strain curve, the peak strength and the reproducibility of results.
For research investigations, the authors prepare their 50–55 mm diameter specimens
with ends flat and parallel to within 0.01 mm.

4.3.5 Influence of specimen volume
It has often been observed experimentally that, for similar specimen geometry, the
uniaxial compressive strength of rock material, c , varies with specimen volume.
(This is a different phenomenon to that discussed in section 4.1 where the changes
in behaviour considered were those due to the presence of varying numbers of ge-
ological discontinuities within the sample volume.) Generally, it is observed that c
decreases with increasing specimen volume, except at very small specimen sizes
91