Page 120 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

Figure 4.18 Results of triaxial com-
pression tests on an oolitic limestone
with volumetric strain measurement
(after Elliott, 1982).

It is necessary to have available for use with the triaxial cell a system for generating
the confining pressure and keeping it constant throughout the test. If the confining
pressure is generated by a screw-driven pressure intensifier, it is possible to use
the displacement of the intensifier plunger to measure the volumetric strain of the
specimen (Crouch, 1970). Figure 4.18 shows some results obtained using such a
system in tests carried out at three different confining pressures on specimens of an
oolitic limestone. An important feature of the behaviour of rock material in triaxial
compression is illustrated by Figure 4.18. When the specimen is initially loaded it
compresses, but a point is soon reached, generally before the peak of the axial stress–
axial strain curve, at which the specimen begins to dilate (increase in volume) as
a result of internal fracturing. Shortly after the peak strength is reached, the nett
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