Specimen geometry, loading conditions and environmental effecn 105AK and BK which represent time-dependent unloading along the stiffness
line of the loading configuration, be it a laboratory testing machine or an
in situ rock structure. The reader should note that the lines AK and BK are
the same machine stiffness lines as those shown in Fig. 6.7. Thus, failure
along the line BK can also be interpreted as a time-dependent effect,
because the specimen cannot sustain the loads associated with BK for any
significant length of time.
Furthermore, at high stress levels, creep has often been studied and
divided into three types of behaviour: primary, secondary and tertiary
creep. These are indicated by the letters A, B and C in the inset diagram in
Fig. 6.16. Primary creep is an initial period during which creep occurs at a
high rate; secondary creep is a period during which the creep rate is very
much diminished; and tertiary creep is a period during which the creep
rate accelerates until failure occurs. These periods can be interpreted as the
line ABC crossing from the pre-peak portion of the complete stress-strain
curve to the post-peak failure locus. In other words, there is an initial period
of rapid creep as the displacement moves away from the pre-peak curve;
there is a quiescent period next; and finally the creep accelerates as the
displacement approaches the post-peak curve.
Finally, fatigue, whether the cycles are in stress or in strain, is a complex
process in which the previous types of time-dependent, microstructural
cracking described are occuring incrementally at different levels of stress
and strain during the cycling process.
In terms of long-term in situ structural stability, we would anticipate that
for engineering purposes there is a long-term stability curve as indicated by
the dotted complete stress-strain curve in Fig. 6.16. We know that
underground excavations can remain open for thousands of years without
any apparent time-dependent collapse. In this case, the stresses and strains
associated with the rock around the excavation are on the long-term
stability curve, and will have approached it through a combination of creep
and relaxation over the years. We would expect different rock types to
have different forms of long-term stability curves: the curve for a granite
might be similar to the one obtained at relatively high strain rates in
the laboaratory, whereas the curve for a rock salt could be very much
lower than that obtained in the laboratory. Also, some rocks will suffer
mechanical and chemical degradation which will be superimposed on
the direct time-dependent effects. Conversely, if stresses applied to a
rock structure in the short term are sufficiently high to cause the line AK
in Fig. 6.16 to be above the long-term stability curve, then failure will be
the inevitable consequence. The consequences for enpeering design are
manifold.
It is for all these reasons that some degree of standardization is essential
in laboratory testing, not only to provide coherency for comparative
purposes, but also to be able to extrapolate to field strain rates from a
constant worldwide measurement base. This is because the behaviour of
rocks differs widely depending on the strain rate to which they are
subjected-because of wide variations in the microstructure of rocks. For
example, a limestone may exhibit brittle behaviour when subjected to the
high strain rates developed by explosives, say 1 x lo5 , typical Class I