Engineering Rock Mechanics

The influence of geological factors on rocks and rock masses 23

Figure 2.18 Open joint which will allow free flow of water.

Figure 2.19 Stylolitic discontinuity with high shear resistance.

These are some examples of the way in which the discontinuities can
have a dramatic effect on rock behaviour. A comprehensive explanation of
the engineering approach to the geometry and mechanical behaviour of
discontinuities is presented in Chapter 7. Later on, in Chapter 10, we will
be discussing inhomogeneity and anisotropy with reference to the rock
structure. It is quite clear from Fig. 2.20 that a slickensided feature in the
rock mass will have a very significant effect on the local rock mass
properties. In particular, it will cause the rock to have different properties
in different directions and hence be a dominant factor causing anisotropy
of the rock mass. These topics of inhomogeneity and anisotropy
have ramifications throughout the book and for rock engineering in
general.

2.3.3 In situ pre-existing rock stress

In a mechanics problem, one considers a body with certain mechanical
properties and the effect of loading the body with certain forces or stresses.
In Sections 2.3.1 and 2.3.2, we have discussed the material properties of the
rock in terms of the intact rock and the overall rock structure. We remember
the point that rock is a natural material. We now consider the loading
conditions and again emphasize that there may already be an in situ pre-
existing state of stress in the rock. In some cases, such as a dam or nuclear
power station foundation, the load is applied as in a conventional
mechanics problem (Fig. 2.21). In other cases, such as the excavation of a
tunnel or mine stope, no new loads are applied in unsupported
excavations: it is the pre-existing stresses that are redistributed by the
engineering activity (Fig. 2.22). In all cases, this will result in the stresses
being increased in some areas, and decreased in others. Finally, there could