Intact rock 73loading, through a relatively linear elastic zone, to the compressive
strength (the peak of the curve), and then downwards as the specimen
structure disintegrates and the load can no longer be sustained.
It is helpful to remember that this type of complete force-displacement
curve occurs for intact rock under any of the conditions of compression,
tension and shear (or any combination of these). Also, polyaxial and
dynamic stress states can be applied to a rock specimen. For all these
cases, different engineering stress-strain curves will be obtained, de-
pending on the stress or strain states applied, and different curves will
be obtained if the tests are programmed for stress control, strain control,
or some other type of control.
The linear response in the ascending pre-peak region is used to
provide the elastic constants discussed in Chapter 5. The strength of
the rock in any of the loading conditions is given by the peaks of the
curves, e.g. the compressive, tensile and shear strengths2. The form of
the curve in the descending post-peak region indicates the nature of
failure but the shape of the post-peak curve will depend on how the test
is programmed: for example, if stress is programmed to increase as the
independent variable, the test will become uncontrollable at the peak
because the specimen cannot sustain a higher stress than its strength.
Three rock failure criteria are explained in Chapter 6 of ERM 1 (namely,
the Mohr-Coulomb, plane Griffith, and Hoek-Brown criteria) and all
these predict failure as a function of the applied stress components.
In addition to the changes caused by direct mechanical loading, the
intact rock can be degraded by or be resistant to exposure to water,
freeze-thaw cycles, and chemical effects, as illustrated in Fig. 6.3 for
the case of the rocks at Stonehenge in the UK. The standing stones are
sandstone with a siliceous matrix which explains why they have been
able to resist 5000 years of British weather.
At the surface of the Earth, there is a dynamic weathering environment
within which rocks are subjected to a variety of thermal, hydrological,
mechanical and chemical (THMC) gradients. The term 'weathering' is
used to describe the effect of these processes on the intact rock as it
responds, changing towards a state which is in equilibrium with its
environment. All exposed intact rock is subjected to these processes and
will only retain its mechanical integrity if it has a high resistance to the
processes, such as the granite used for building veneers, paving stones
and gravestones.
The THMC processes also operate on unexposed underground rocks
and on the fractures within rocks, although at a slower rate because
the gradient changes and associated energy changes cannot occur as
rapidly as on the surface. However, the natural processes have had
millions of years in which to operate and so underground rocks can
often be significantly weathered, especially in the vicinity of fractures
*The rock properties measured in such tests should be regarded as engineering,
rather than material, properties. A material property does not depend on the specimen
geometry and loading conditions of the test, but the strengths and failure properties of
rocks do depend on these factors, and therefore they are not material properties.