Engineering Rock Mechanics

Anisotropy 165

Continuous

fracture

Discontinuities

i

i

Inhomogeneous

-64

-32

*96

Anisotropic:

9+65

83

u+ Non-elastic

Discontinuous-

ness

1

Inhomogeneity

Modulus is

rectiori dependen

Anisotropy

Discontinuit

rn

ftt

When

discontinuities

are present we

may find that

Si, * Sjl

Non-elasticity

Figure 10.1 Connections between the attributes discontinuousness, inhomo-
geneity, anisotropy and non-elasticity.

Examples are the South African goldmines and well-bores at great depth:
in these circumstances the high stresses can effectively close all discon-
tinuities and the rock is more or less homogeneous and isotropic within
the scale of the engineering being considered.
It follows, therefore, that engineers must always question the results of all
modelling in rock mechanics and rock engineering to consider for themselves
to what extent they consider the DIANE rock is well represented by the
CHILE material in the model. In some circumstances such representation
may be valid, in others, it may be wrong, misleading and dangerous
to use. The following discussion is designed to provide the reader with
some background knowledge in order to address this problem with
confidence.

10.2 Anisotropy

The word 'anisotropy' is derived from the two Greek words anisos (mean-
ing unequal) and tropos (meaning turning or direction). Directionality is one
of the key aspects of rock engineering. If, for example, we conduct a site
investigation using a vertical borehole, will the fracture data be useful for
designing a horizontal tunnel? The answer to this question is that if the rock
is fractured isotropically, the results will be most useful. However, if the
rock is anisotropic, the results could well be misleading if used without care.
We have already demonstrated in Chapter 7 that the discontinuity
frequency can vary significantly with direction, and so the value derived
from measurements made in the vertical borehole might be different to that