88 lnkrctrockconvenient and can be established by unloading and reloading the
specimen from any point on the curve. This is illustrated in Fig. 6.4.
The factors discussed so far have been concerned with the complete
stress-strain behaviour and the link with Chapter 5 on the deformability
of materials. Another important parameter highlighted in Fig. 6.2 is the
maximum stress that the specimen can sustain. Under the loading
conditions shown in the diagram, the peak stress is the uniaxial
compressive strength, 0,.
It is important to realize that the compressive strength is not an intrinsic
material property. Intrinsic material properties do not depend on the speci-
men geometry or the loading conditions used in the test: the compressive
strength does. If a microphone is attached to a specimen during the gener-
ation of the complete stress-strain curve, significant acoustic emission is
found to occur, starting from a stress level of about 50% of the compressive
strength. Through these observations, and by obtaining sections through
specimens that have been taken to various points on the complete
stress-strain curve, it is observed that microcracking continually increases
from this 50% 0, stress level until the specimen is completely destroyed. The
compressive strength is an arbitrary stage in this continual microstructural
damage process, representing the maximum sustainable stress. At the peak
of the curve, the specimen has had many axial cracks induced within it, but
macro-shear (i.e. on the scale of the specimen itself) does not take place until
about halfway down the descending portion of the curve.
In other forms of engineering, for example, the strength of concrete in
structural engineering, if the applied stress reaches the compressive
strength, there can be catastrophic consequences. This is not necessarily the
case in rock engineering, which is why we are concentrating the
discussion on the characteristics of the complete stress-strain curve, as
opposed to the specific value of the compressive strength. However, the
compressive strength is probably the most widely used and quoted rock
engineering parameter and therefore it is crucial to understand its nature.
Also, whether failure beyond the compressive strength is to be avoided at
457000 t
40,000
35,000
30,000
2 25,000
0 20,000
15,000
10,000
5,0009
40 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24
Figure 6.4 Repeated loading and unloading to illustrate the variation in secant
modulus throughout the complete stress-strain curve (from Bieniawski, 1968).