Page 115 - Rock Mechanics For Underground Mining

BEHAVIOUR OF ISOTROPIC ROCK MATERIAL IN UNIAXIAL COMPRESSION

Figure 4.13 Axial force–axial dis-
placement curve obtained for Ten-
nessee Marble with post-peak unload-
ing and reloading (after Wawersik and
Fairhurst, 1970).

Marble which was unloaded and then reloaded from a number of points in the post-
peak range. Several points should be noted about the behaviour observed.

(a) On reloading, the curve eventually joins that for a specimen in which the axial
displacement increases monotonically with time.
(b) As displacement continues in the post-peak region, the proportion of the total
displacement that is irrecoverable increases.
(c) The unloading–loading loop shows some hysteresis.
(d) The apparent modulus of the rock which can be calculated from the slope of
the reloading curve, decreases with post-peak deformation and progressive frag-
mentation of the specimen.

If rock specimens are subjected to loading and unloading cycles in the pre-peak
range, some permanent deformation and hysteresis are generally observed. This is
often associated with ‘bedding-down’ effects, and for this reason, the ISRM Commis-
sion (1979) recommends that ‘it is sometimes advisable for a few cycles of loading
and unloading to be performed’.

4.3.9 The point load test
Sometimes the facilities required to prepare specimens and carry out uniaxial com-
pression tests to the standard described above are not available. In other cases, the
number of tests required to determine the properties of the range of rock types en-
countered on a project may become prohibitive. There may be still further cases, in
which the uniaxial compressive strength and the associated stress–strain behaviour
need not be studied in detail, with only an approximate measure of peak strength
being required. In all of these instances, the point load test may be used to provide
an indirect estimate of uniaxial compressive strength. This account is based on the
ISRM Suggested Method for determining point load strength (ISRM Commission,
1985).
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