Page 108 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

Figure 4.4 Influence of end restraint
on stresses and displacements induced
in a uniaxial compression test: (a)
desired uniform deformation of the
specimen; (b) deformation with com-
plete radial restraint at the specimen–
platen contact; (c) non-uniform nor-
mal stress, n and shear stress, in-
duced at the specimen end as a result
of end restraint.

Varying the standard conditions will influence the observed response of the spec-
imen. Some of these effects will be discussed briefly in sections 4.3.3 to 4.3.7.
More extensive discussions of these effects are given by Hawkes and Mellor (1970),
Vutukuri et al. (1974) and Paterson (1978).

4.3.3 End effects and the influence of height to diameter ratio
The objective of the test arrangements should be to subject the specimen to uni-
form boundary conditions with a uniform uniaxial stress and a uniform displacement
field being produced throughout the specimen (Figure 4.4a). Due to friction between
the specimen ends and the platens and differences between the elastic properties of
rock and steel, the specimen will be restrained near its ends and prevented from de-
forming uniformly. Figure 4.4b illustrates a case in which complete radial restraint
occurs at the specimen ends. The result of such restraint is that shear stresses are
set up at the specimen–platen contact (Figure 4.4c). This means that the axial stress
is not a principal stress and that the stresses within the specimen are not always
uniaxial.
As a consequence of these end effects, the stress distribution varies throughout the
specimen as a function of specimen geometry. As the height to diameter (H/D) ratio
increases, a greater proportion of the sample volume is subjected to an approximately
uniform state of uniaxial stress. It is for this essential reason that a H/D ratio of at least
2.0 should be used in laboratory compression testing of rock. Figure 4.5 shows some
experimental data which illustrate this effect. When 51 mm diameter specimens of
Wombeyan Marble were loaded through 51 mm diameter steel platens, the measured
uniaxial compressive strength increased as the H/D ratio was decreased and the shape
of the post-peak stress–strain curve became flatter. When the tests were repeated with
‘brush’ platens (made from an assembly of 3.2 mm square high-tensile steel pins),
lateral deformation of the specimens was not inhibited; similar stress–strain curves
were obtained for H/D ratios in the range 0.5 to 3.0 However, ‘brush’ platens were
found to be too difficult to prepare and maintain for their use in routine testing to be
recommended.
It is tempting to seek to eliminate end effects by treating the specimen–platen
interface with a lubricant or by inserting a sheet of soft material between the specimen
and the platen. Experience has shown that this can cause lateral tensile stresses to be
applied to the specimen by extrusion of the inserts or by fluid pressures set up inside
flaws on the specimen ends. For this reason, the ISRM Commission (1979) and other
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