164 Anisotropy and inhomogeneityThus, the type of anisotropy and/or inhomogeneity variation could be
gradual within the intact rock or sudden as a discontinuity is crossed. In
fact, there can be variation on all scales: within grains or crystals, within
the microstructure, within laboratory-sized samples of intact rock, within
engineering structure-sized volumes of rock, and so on. These scales will
be incorporated in our discussions in the succeeding three sub-sections.
However, an immediate distinction can be made now between the
assumptions that are traditionally required for modelling and the real
properties of the rock. This can be remembered by two acronyms: CHILE
and DIANE.
A Continuous, Homogeneous, Isotropic and Linearly-Elasfic (CHILE) mate-
rial is one that is most commonly assumed for the purposes of modelling.
Traditional stress analysis techniques are formulated in terms of these four
attributes, simply for necessity andor convenience for obtaining closed-
form solutions. In the past, limited computational techniques precluded
any more sophisticated analysis. Nowadays, however, especially in
consulting and research organizations, there are computer codes available
which will routinely deal with violation of any of these traditional
assumptions. This leads directly to the second acronym.
A Discontinuous, Inhomogeneous, Anisotropic, Non-Elastic (DIANE) rock is
the material with which the engineer has to deal." We should therefore
consider the significance of the difference between the CHILE material
being modelled and the DIANE rock being engineered, and the likely error
arising from the direct application of a model based on a CHILE material.
Alternatively, the specific attributes of the DIANE rock can be modelled.
Superb examples of the latter procedure are the development of block
theory and the use of distinct element techniques in numerical analysis.
There is a connection between each of the characteristics of discon-
tinuousness, inhomogeneity, anisotropy and non-elasticity. For example,
in a cyclothem of repeating sandstone, mudstone and limestone strata,
containing bedding plane separations and exhibiting time dependency,
there will be inter-relations between the attributes-because of the
physical characteristics and mechanisms. The rock is discontinuous
because of the bedding plane separations and any other fracturing that may
be present. It is inhomogeneous because of the existence of the different
rock types. It is anisotropic because of its sedimentary nature. It is not elastic
because there is hysteresis and time dependency, perhaps related to the
presence of the bedding planes. Moreover, the rock is anisotropic because it
is inhomogeneous, e.g. the deformation modulus for a suite of strata will
be different pardel and perpendicular to the bedding planes. Also, the rock
is anisotropic because it is discontinuous. The matrix showing example
connections between these four main attributes is shown in Fig. 10.1.
There is little chance of any modelling based on CHILE assumptions
being realistic. There are a few circumstances where all the assumptions
would apply simultaneously, to the extent where the modelling would
produce results of value in rock engineering analysis and design.
"The acronym DIANE was coined by Branko Vukadinovic of Energoprojekt,
Belgrade, Yugoslavia.