Page 121 - Rock Mechanics For Underground Mining

BEHAVIOUR OF ISOTROPIC ROCK MATERIAL IN MULTIAXIAL COMPRESSION

Figure 4.19 Complete axial stress–
axial strain curves obtained in triaxial
compression tests on Tennessee Mar-
ble at the confining pressures indi-
cated by the numbers on the curves
(after Wawersik and Fairhurst, 1970).

volumetric strain of the specimen becomes dilational. Dilation continues in the post-
peak range. The amount of dilation decreases with increasing confining pressure.
At very high confining pressures, often outside the range of engineering interest,
dilation may be totally suppressed with the volumetric strains remaining contractile
throughout the test.
Figure 4.19 illustrates a number of other important features of the behaviour of
rock in triaxial compression. The axial stress ( a )–axial strain (ε a ) data shown were
obtained by Wawersik and Fairhurst (1970) for the Tennessee Marble giving the
uniaxial stress–strain curve shown in Figure 4.11. These and similar data for other
rocks show that, with increasing confining pressure,
(a) the peak strength increases;
(b) there is a transition from typically brittle to fully ductile behaviour with the
introduction of plastic mechanisms of deformation including cataclastic flow
and grain-sliding effects;
(c) the region incorporating the peak of the a –ε a curve flattens and widens;
(d) the post-peak drop in stress to the residual strength reduces and disappears at
high values of 3 .

The confining pressure at which the post-peak reduction in strength disappears and
the behaviour becomes fully ductile ( 3 = 48.3 MPa in Figure 4.19), is known as the
brittle–ductile transition pressure and varies with rock type. In general, the more
siliceous igneous and metamorphic rocks such as granite and quartzite remain brittle
Figure 4.20 Effect of pore pressure at room temperature at confining pressures of up to 1000 MPa or more (Paterson,
(given in MPa by the numbers on the
1978). In these cases, ductile behaviour will not be of concern in practical mining
curves) on the stress–strain behaviour
problems.
of a limestone tested at a constant
confining pressure of 69 MPa (after The influence of pore-water pressure on the behaviour of porous rock in the triaxial
Robinson, 1959). compression test is illustrated by Figure 4.20. A series of triaxial compression tests
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