Engineering Rock Mechanics

Time-dependency in rock engineering 22 1

Pande et al. (1990). Because time-dependency is ubiquitous in rock

mechanics problems, these developments and the ability to incorporate the

techniques in design are critical to the advancement of rock engineering.

13.4 Time-dependency in rock engineering
Engineers have found it convenient to consider phenomena as either
associated with very high strain rates or very low strain rates. This is
because the process of rock excavation (e.g. by blasting) occurs rapidly,
whereas deformation (e.g. displacement occurring throughout the life of
an excavation) occurs slowly. In the high strain rate category we include
blasting, vibrations and fatigue; in the low strain rate category we
include creep, subsidence and long-term displacements. We noted that the
strain rates can be spread over 15 orders of magnitude, with the result
that it is debatable whether any generic time-dependent model can be
valid over such a large range. In the newer applications of rock
engineering, such as radioactive waste disposal, the specified design lives
can be large, of the order of 1000 years. Thus, not only is there concern
with the time-dependent behaviour but we have to consider whether all
the rock properties and mechanisms can be considered to be uniform
over such an extreme time period. This is exacerbated by the fact that
we can only conduct testing procedures in the range of medium to high
strain rates.
If the rock properties are determined by geophysical means, at very high
strain rates, we should ask ourselves how valid is it to apply these to
engineering applications of, say, a billion times greater duration than the
test period? This question has profound implications for the validity of
theoretical models, test results and the interpretation of field measure-
ments. We are led to the conclusion that engineering judgement must
still play a large part in determining the type of time-dependent analysis
that is used.