Fractures and
hemispherical
projection
7
7.1 Natural, pre-existing fractures
During its geological history a rock mass has been lithified and subjected
to a variety of tectonic forces and fluid pressures. When the naturally ap-
plied stresses, strains and fluid pressures cause failure, the rock fractures
in a brittle or semi-brittle manner. Initially, one or two sets of fractures
develop. The fractures in each of the initial sets are usually planar,
persistent and parallel because they were caused by tensile stresses and
hydraulic pressures. As subsequent geological disturbances occurred,
especially the major orogenic events, a suite of fracturing arrays was
introduced into the rock mass by tensile, compressive and shear stresses
(Fig. 7.1). The exact fracturing mode or combination of modes in each
case would have been a function of the applied stresses and the frac-
tures already present. Moreover, ductile rock displacements resulting
in folding will also have occurred and these will have complicated the
picture.
Thus, when today an engineer surveys a rock mass, it contains frac-
tures. The fracture pattern can be either clear or rather daunting from
the viewpoints of structural geology analysis and engineering charac-
1 1
Figure 7.1 Fracturing induced by tectonic stresses. (a) tensile failure; (b) compressive
failure; (c) shear failure.