Engineering Rock Mechanics

Questions and answers: strain and the theory of elasticity 67

convenient to assume that the rock is isotropic and that the laboratory
values, perhaps adjusted by some factor, are representative of the rock
mass. However, many rock masses are significantly anisotropic because
of their contained fractures, and so the isotropy assumption will often be
invalid, as illustrated in Q and A 5.4 and 5.7.
(b) Because we do not have solutions to elastic problems for the fully
anisotropic case, and because measuring the elastic properties is time-
consuming and expensive, no one has yet felt justified in attempting
to measure and use all twenty-one elastic constants for engineering
application.
(c) In the future, it is likely that there will be a move towards
transversely isotropic and orthotropic characterization where the elastic
constants of the rock mass will be estimated either from the properties
of the intact rock and the fractures or by back analysis from in situ
displacement measurements. This is because there is increasing experi-
ence that the isotropic assumption does not adequately represent reality.
For some years now, the strain values obtained using the CSIRO gauge
for stress measurement have been interpreted using a transversely iso-
tropic model for the intact rock, as appropriate. There is little doubt
that this trend towards improved rock property characterization will
continue.

45.9 (a) How do you know if a material is elastic?

(b) How is time taken into account in the theory of elasticity?

A5.9 (a) A material is elastic if, after deforming, it returns to its original
shape and size and all the mechanical energy used in deforming the
material can be recovered.
@) Time is not taken into account in the theory of elasticity. Time-
dependent effects and their interactions are complex but can be modelled
to a first approximation by the theory of visco-elasticity. The complex-
ities of time-dependent rock behaviour are described in the book by
Cristescu and Hunsche (1998) '.

Q5.10 When a rock specimen fails in a compressive test in the labor-

atory or when an in situ rock mass fails due to natural compression,

is 'stress' or 'strain' the cause of failure?

AS. 10 When a rock specimen is loaded in a laboratory compression test,
a testing machine is used and nowadays these are servo-controlled test-
ing machines. This type of machine is programmed to correct a feedback
signal via a hydraulic jack (or actuator) so that either the force, and
hence the stress on the specimen, is increased at a constant rate, or the
displacement, and hence the strain of the specimen, is increased at a con-
stant rate. Other feedbacks are also possible, in particular circumferential
strain.

4Cristescu N. D. and Hunsche U. (1998) Time Eficts in Rock Mechanics. Wiley, New

York, 342pp.