Anisotropy and
70 inhomogeneity
We have already emphasized the natural history of the rock material which
is being used for engineering purposes. A consequence of the millions of
years of mechanical, chemical and thermal processes to which the rock
mass has been subjected is that it may well be anisotropic and inhomo-
geneous. In this chapter, we will first define these two terms in the context
of rock engineering and then explain two useful acronyms, CHILE and
DIANE, representing respectively the assumptions required for modelling,
as opposed to the actual rock properties. We then discuss the concepts of
anisotropy and inhomogeneity, and conclude with a section on the
ramifications of the ideas for rock engineering analysis and design.10.1 Definitions
An anisotropic rock has different properties in different directions. These
properties may be of any type: for example, deformability modulus,
strength, brittleness, permeability and discontinuity frequency. In some
cases, the ability to deal with anisotropy is built into the fundamental
definition of the property itself, e.g. the compliance matrix for intact rock
can contain up to 21 elastic constants which represent all possible types of
elastic anisotropy. In other cases, for example, the compressive strength,
there is no such in-built capability and the engineers are left to decide on
the optimal characterization of anisotropy commensurate with their
requirements. If we measure the compressive strength parallel and perpen-
dicular to the laminations in a metamorphosed rock, is this sufficient to
characterize the anisotropic variation?
An inhomogeneous rock has different properties at different locations.
Again, this refers to any of the properties which we may be measuring.
However, the ability to characterize inhomogeneity is not built-in to any of
the fundamental definitions of the properties and we have to have recourse
to statistical techniques. Later on, we will distinguish between ’point
properties’ and ‘volume properties’, the former being essentially the
properties of intact rock, and the latter being essentially the properties
governed by the structure of the rock.