22 Geological seffing
Figure 2.16 Shear fracturing of rock (mode 2).Figure 2.17 Shear fracturing of rock (mode 3).natural, engineering material. Furthermore, the distinction between joints
and faults is important. If the two sides of the fracture have been pushed
over one another, as in Figs 2.16 and 2.17, the discontinuities are likely to
have a low resistance to any additional shear stresses induced by engi-
neering activities. For these and other reasons, it is most helpful if the
engineer has a knowledge of structural geology and rock structure.
Some examples of the way in which the discontinuity genesis leads to
differing mechanical properties are illustrated in Figs 2.18-2.20. In Fig. 2.18,
an open joint is shown; this is clearly a break in the continuum. As can be
seen in the figure, stresses cannot be transmitted across this discontinuity
because the two sides are not connected. Moreover, this aperture within
the rock mass is an open conduit for water flow with a permeability many
orders of magnitude greater than the adjacent intact rock. In Fig. 2.19, a
particular type of discontinuity is shown which occurs in limestone and
dolomitic rocks and which has a high resistance to shear because of the
connecting material across the discontinuity, although this resistance will
still be less than the intact rock. Also, such a discontinuity will have a
permeability higher than the intact rock. In Fig. 2.20, there is a sketch of
the surface of a slickensided fault, i.e. a discontinuity on which there has
been slip movment under stress causing the discontinuity surfaces to
become altered and, in particular, to have a slippery surface. In some cases,
such discontinuities can be pervasive throughout the rock mass with the
result that the engineer must expect that, in near surface regions, failure
will always occur along the discontinuity surfaces.