Page 116 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

Figure 4.14 Point load test appara-
tus (photograph by ELE International
Ltd).

In this test, rock specimens in the form of core (the diametral and axial tests), cut
blocks (the block test) or irregular lumps (the irregular lump test) are broken by
a concentrated load applied through a pair of spherically truncated, conical platens.
The test can be performed in the field with portable equipment or in the laboratory
using apparatus such as that shown in Figure 4.14. The load should be applied at least
0.5D from the ends of the specimen in diametral tests, where D is the core diameter,
and equivalent distances in other tests as specified by the ISRM Commission (1985).
From the measured value of the force, P, at which the test specimen breaks, an
Uncorrected Point Load Index, I s , is calculated as
P
I s = (4.5)
D 2
e
where D e , the equivalent core diameter, is given by the core diameter, D, for diametral
tests, and by 4A/ for axial, block and lump tests, where A is the minimum cross
sectional area of a plane through the specimen and the platen contact points.
The index, I s , varies with D e and so size correction must be applied in order to
obtain a unique point load strength index for a particular rock sample for use for
strength classification. Wherever possible, it is preferable to carry out diametral tests
on 50–55 mm diameter specimens. The size-corrected Point Load Strength Index,
I s(50) , is defined as the value of I s that would have been measured in a diametral test
with D = 50 mm. The results of several series of tests carried out by a number of
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