Tests on discontinuities 1 8 1strength is predicted from a calibration curve, then we should be grateful
as engineers that the test does indeed prove to be useful. Over the years,
the point load test has become the most widely used test for measuring the
strength of intact rock.
Other examples of index tests are the measurement of acoustic velocity
and slake durability. The acoustic velocity can be measured easily using
portable equipment, the P- and S-wave velocities either being used as an
index in their own right (indicating anisotropy and/or inhomogeneity), or
Young’s modulus and Poisson’s ratio can be estimated using relations
developed from the theory of elasticity. Since the stress wave velocities
depend on the elastic moduli of the rocks and there is an empirical
correlation between rock moduli and rock strength, the UCS can also be
estimated from acoustic testing.
The degradability of rock is important when enpeering in ’soft’ rocks.
A typical material that degrades is shale: a tunnel excavated in shale may
initially be stable, only to collapse a few days later. It is useful, therefore, to
be able to assess the degradability of the rock, for which purpose the slake
durability test was developed by Franklin (1979). A piece of rock is placed
in a copper cage which is rotated under water under specified testing
conditions. The loss of sample weight is a measure of the susceptibility of
the rock to the combined action of slaking and mechanical erosion.
Fuller descriptions of these and other tests on intact rock are given in the
standardized testing procedures published on behalf of the ISRM and
National Standards bodies, such as the American Society for Testing and
Materials (see Table 11.1).
11.4 Tests on discontinuities
The discussion in Section 11.3 is concerned with the rock material, i.e. the
solid blocks in the rock exposure shown in the top photograph and the
solid core pieces shown in the middle photograph of Fig. 11.1. Now we
discuss testing the breaks in the continuum, the discontinuities.
7 7.4. I Geometricd attributes
Referring to the middle photograph of Fig. 11.1, certain geometrical
properties of the discontinuities can be measured easily from a length of
core. The properties can be determined more accurately using a scanline
set up on a rock surface, as in the top photograph of Fig. 11.1. A purpose-
designed logging sheet for borehole core, which can also be used for
scanline work, is presented in Fig. 11.7. The specific contents of this logging
sheet which should be noted are:
(a) the fact that there is a photograph of the core;
(b) there is an associated sketch of the discontinuties present within it;
(c) the discontinuities are numbered;
(d) the distance to each discontinuity is measured;
(e) the angle of each discontinuity to the core axis is measured;
(f) and there are comments on the discontinuity type (genesis) and surface
condition and/or coating.