238 Rock mechanics interactions and rock engineering @ems
actions can provide a systematic approach to hazard analysis and hence
to the establishment of optimal construction sequences.
474.7 Assume that you have been asked to undertake a technical
audit of a rock engineering design for a DIANE rock mass, where a
CHILE numerical code has been used to determine the elastic rock
stresses and displacements around an underground excavation. List
briefly the likely sources of error in the determination of the rock
stresses and displacements.
A14.7 We recall the meaning of the two acronyms, DIANE and CHILE:
DIANE: Discontinuous, Inhomogeneous, Anisotropic, and Not-Elastic
CHILE: Continuous, Homogeneous, Isotropic, and Linearly Elastic
The sources of error in modelling a DIANE rock with a CHILE numerical
code will be the following.
(1) Specific influences of fractures are not taken into account: the rock
has been assumed to be continuous when it is not.
(2) Variations in intact rock properties with location are not taken into
account, e.g. in bedded strata.
(3) Variations of rock properties in different directions are not taken
into account.
(4) Differences between the mechanical properties of the rock mass
when it is being subjected to loading or unloading cycles are
not taken into account, nor are time-dependent effects taken into
account, such as strain rate, creep, relaxation and fatigue.
Thus, we can see that the rock stresses and displacements are not
likely to be correct. There may well be other factors, such as the influence
of water, which have not been taken into account and which will lead to
further errors.
The engineering question is whether such CHILE solutions are of
assistance for engineering design. Do they provide information and un-
derstanding compatible with the available resources and the engineering
design objective? Are they a sufficiently good approximation for the
problem in hand? Much of the seminal book by Jaeger and Cook (1979)
is based on the CHILE theory of elasticity and the associated applications
have been successful, especially in those excavations around the world
where at depth high rock stresses have closed the fractures and the rock
mass is relatively homogeneous and isotropic. However, in near-surface
excavations, where the rock stresses are lower, the fractures more fre-
quent, and the strata more disturbed and weathered, there should be
more concern about the validity of a CHILE model.
In short, algorithms in numerical codes should include the necessary
interactions required to capture the essence of the problem as required
by the engineering objective.Jaeger J. C. and Cook N. G. W. (1979) Fundamentals of Rock Mechanics. Chapman and
Hall, London, 3rd edn., 593pp.