Page 106 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

with increasing water content. Data illustrating these various effects are presented by
Vutukuri et al. (1974).
It must be recognised that, because of these effects, the uniaxial compressive
strengths of samples of rock having the same geological name, can vary widely.
Thus the uniaxial compressive strength of sandstone will vary with the grain size,
the packing density, the nature and extent of cementing between the grains, and the
levels of pressure and temperature that the rock has been subjected to throughout
its history. However, the geological name of the rock type can give some qualitative
indication of its mechanical behaviour. For example, a slate can be expected to exhibit
cleavage which will produce anisotropic behaviour, and a quartzite will generally be a
strong, brittle rock. Despite the fact that such features are typical of some rock types,
it is dangerous to attempt to assign mechanical properties to rock from a particular
location on the basis of its geological description alone. There is no substitute for a
well-planned and executed programme of testing.

4.3.2 Standard test procedure and interpretation
Suggested techniques for determining the uniaxial compressive strength and deforma-
bility of rock material are given by the International Society for Rock Mechanics
Commission on Standardization of Laboratory and Field Tests (ISRM Commission,
1979). The essential features of the recommended procedure are:
(a) The test specimens should be right circular cylinders having a height to diam-
eter ratio of 2.5–3.0 and a diameter preferably of not less than NX core size,
approximately 54 mm. The specimen diameter should be at least 10 times the
size of the largest grain in the rock.
(b) The ends of the specimen should be flat to within 0.02 mm and should not depart
from perpendicularity to the axis of the specimen by more than 0.001 rad or
0.05 mm in 50 mm.
(c) The use of capping materials or end surface treatments other than machining is
not permitted.
(d) Specimens should be stored, for no longer than 30 days, in such a way as to
preserve the natural water content, as far as possible, and tested in that condition.
(e) Load should be applied to the specimen at a constant stress rate of
−1
0.5–1.0MPa s .
(f ) Axial load and axial and radial or circumferential strains or deformations should
be recorded throughout each test.
(g) There should be at least five replications of each test.

Figure 4.3 shows an example of the results obtained in such a test. The axial force
recorded throughout the test has been divided by the initial cross-sectional area of
the specimen to give the average axial stress, a , which is shown plotted against
overall axial strain, ε a , and against radial strain, ε r . Where post-peak deformations
are recorded (section 4.3.7), the cross-sectional area may change considerably as
the specimen progressively breaks up. In this case, it is preferable to present the
experimental data as force–displacement curves.
In terms of progressive fracture development and the accumulation of deformation,
the stress-strain or load-deformation responses of rock material in uniaxial compres-
sion generally exhibit the four stages illustrated in Figure 4.3. An initial bedding down
and crack closure stage is followed by a stage of elastic deformation until an axial
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