Engineering Rock Mechanics

Tests on intact rock 179

discontinuity surfaces, the condition of the surfaces is of particular

importance. In Fig. 11.6, we show a series of empirically determined curves

relating the Schmidt hammer readings for various hammer types and

orientations to the unconfined compressive strength. Despite its apparent

simplicity, the Schmidt hammer has proved to be one of the most useful

indicators of rock strength.

The point load test is used on small pieces of rock which are either borehole

core or irregular lumps. The test is derived from the so-called Brazilian+ test

in which a disc is compressed diametrally between two loading platens and

the tensile stress at failure, ot/ calculated from the breakdown load as

P

Dt

(Tt = (2/n)-

where P is the load on the disc at failure,
D
t is the disc thickness.

is the disc diameter, and

When the effect of the geometry of the disc is studied, it is found that the

outside perimeter shape has little effect on the above formula: for example,

a square shape loaded across opposite corners gives a similar result for the

tensile strength. For this reason, and because the testing may be conducted

on irregular lumps (as shown in Fig. 11.5), a point load index, Is, was

developed by Franklin (1985):

P

D2

1, =-(units of stress).

This formula is direct1 related to the Brazilian test formula pen earlier,

except that the term D has replaced the term Dt because the specimen

could be an irregular lump. The reader should note that a coherent set of

units must be used in evaluating 1s: in order to produce a result with units

of MPa, units of N and mm are required for P and D, respectively.

The size effect was discussed earlier and the point load test is no

exception to this phenomenon. The standard test is conducted on a core

of 50 mm diameter, with correction methods being available to convert the

measured index to the Isso index if a different sized core or lump is used.

There are also methods of characterizing the results for tests conducted

parallel and perpendicular to the structure in anisotropic rock. In Fig. 11.6

the calibration curve and a set of results for converting the point load index

number to unconfined compressive strength (UCS) are given.

The point load test is useful because hundreds of tests can be easily

completed in a day with minimal sample preparation. Also, the prediction

of compressive strength has proved to be remarkably accurate over a wide

range of rocks. Bearing in mind that the sample can be irregular, that

elasticity theory is unlikely to be the correct predictive model, that failure

will probably occur under the loading platen, and that the compressive

J

*Readers may be interested to know the apocryphal tale surrounding the

Brazilian test-that it was developed from the observation of a church being moved

in Brazil on concrete rollers: when the rollers split, the idea of the test was born.