The influence of geological factors on rocks and rock masses^17Figure 2.7 Tunnel deformation caused by high rock stresses at the Underground
Research Laboratory, Manitoba, Canada.properties and its susceptibility to water penetration and weathering
effects.
The most useful single description of the mechanical behaviour is the
complete stress-strain curve in uniaxial compression. This curve will be
explained fully in Chapter 6, but is introduced here briefly to illustrate the
very significant effect of the rock microstructure and history on the
mechanical behaviour. In Fig. 2.8, a general complete stress-strain curve is
shown for a sample of rock being compressed in one direction, i.e. in
uniaxial compression. The reader should note that: the horizontal axis is
strain, which is the relative change in length of the specimen; and the
vertical axis is stress, which is the load per unit area.
There are several features of interest, the first of which is the modulus
of the rock, represented by the letter E on the diagram. For a high-modulus
(i.e. stiff) material, this initial part of the complete stress-strain curve will
be steep; for a low-modulus (i.e. soft) material, it will be gentle.
The next feature is the compressive strength which is the maximum
stress that can be sustained; this is illustrated by the dotted line in the
figure.
The third feature is the steepness of the descending portion of the curve
which is a measure of the brittleness, as illustrated in Fig. 2.9. The two main
cases shown are the behaviour after the compressive strength is reached