Geological setting
2
In this chapter, we will set the scene for the consideration of engineering
in the natural material, rock. Most of our engmeering materials (with the
exception of timbers and soil) are manufactured and tested to specification.
Subject to correct quality control, we can then be sure that the engineering
material in question has a given set of properties which are used in the
design process. However, rock is extremely old compared to all other
engineering materials: its age is measured in millions of years and it has
undergone significant mechanical, thermal and chemical activity.
We will describe in the following sections the ramifications of the rocks
history for engineering, starting with rock as an engineering material and
following with a discussion of the conditions in natural rock environments.
In Section 2.3, we will discuss explicity the influence of geological history
on five of the most important aspects of rock mechanics; and, later on, we
will explain more directly (in the context of stresses and strains) the
concepts of continuity, homogeneity and isotropy of rock material and rock
masses.
2.1 Rock as an engineering material
One of the most important, and often frequently neglected, aspects of rock
mechanics and rock engneering is that we are utilizing an existing material
which is usually highly variable. This is demonstrated in Figs 2.1-2.3. The
rock will be used either as a building material so the structure will be made
of rock, or a structure will be built on the rock, or a structure will be built
in the rock. In the majority of civil engineering cases, rock is removed to
form the structure as in, for example, the excavation of rock for a
hydroelectric machine hall chamber. In this sense, we are dealing with a
reverse type of construction where the rock material is being taken away,
rather than added, to form a structure. On the mining side, rock may be
excavated in an open pit and we will then be concerned with the stability
of the sides of the open pit.
In these examples and all others in rock engineering, the material is
natural. As engineers and in the context of mechanics, we must establish