Engineering Rock Mechanics

The inhence of geological factors on rocks and rock masses 21

Figure 2.14 Illustration of the complex nature of a rock mass due to successive

phases of superimposed fracturing.

likely mechanical characteristics. There are three ways in which a fracture
can be formed: one by pulling apart and two by shearing. These are
illustrated in Figs 2.15-2.17 showing that this leads to two fundamentally
different types of discontinuities: i.e. those which have been simply opened
and are termed joints (as in Fig. 2.15); and those on which there has been
some lateral movement and are termed shear zones or faults (as in Figs 2.16
and 2.17). Given that such features exist in all rock masses at a variety of
scales, it is hardly suprising that they will significantly affect the deform-
ability, strength and failure of rock masses. Moreover, other key char-
acteristics such as the permeability can be governed almost entirely by the
rock structure configuration.
It is found in practice that, indeed, the rock discontinuities have
implications for all engineering. Failure is very often associated directly
with the discontinuities, which are the weak links in our pre-existing,

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Figure 2.15 Tensile fracturing of rock (mode 1).