Engineering Rock Mechanics

160 Anisotropy and inhomogeneity

overlapping footprints in the geometry and mechanical properties that
we see today; this is the realm of structural geology analyses. However,
we do wish to capture the geometrical and mechanical essence of the
rock mass in so far as it is required for engineering. As highlighted by
both William Judd and previous chapters in this book, an understanding
of the geological setting, the in situ stress, strain, the intact rock, the
fractures, the nature of rock masses, and permeability is essential for
engineering analysis.
In this chapter, we now consider the consistency of the rock mass
properties. Is the rock deformability and strength the same everywhere?
Is the rock strength the same when measured in different directions?
Are the fractures in sets of parallel fractures? In short, what is the
orientational and spatial consistency of the rock mass in question, and
how do we characterize any variations that occur? To emphasize the
fact that the rock is not a perfect material for modelling, we use two
acronyms, as introduced in the Frontispiece.

CHILE: Continuous, Homogeneous, Isotropic, Linearly Elastic (a model

DIANE Discontinuous, Inhomogeneous, Anisotropic, and Not Elastic

The terms in these acronyms have the following meanings.

Continuous

material).

(the rock reality).

is mechanically continuous; there can be variations

in the mechanical property values but there are no

mechanical breaks.

does contain mechanical breaks having effectively zero

tensile strength.

Discontinuous

Homogeneous * has the same property values at all locations.

Inhomogeneous (or heterogeneous) has different property values at

different locations.

Isotropic has the same property values in different directions.

Anisotropic has different property values in different directions.

Linearly Elastic the stress-strain curve is a line with a constant slope,

all strains are instantaneous, and all energy can be

recovered.

on unloading not all energy input can be recovered

and strains may be time dependent.

In the past, the rock mass was often modelled as a CHILE material.

Nowadays, there is recognition that the rock reality is DIANE and

that numerical modelling should be able to incorporate all the DIANE

aspects as required by the rock engineering design problem in hand.

Often, the pragmatic approach to characterizing a rock mass is achieved

by dividing the rock mass into structural domains, each having different

property values. These domains can be the lithological domains, or they

can be more detailed and be directly based on geotechnical changes.

Not Elastic

* Greek homos = same, gems = kind, isos = equal, tmpos = way.