6 Intact rock
Having introduced the concepts of stress and strain, we can now consider
how the rock reacts to given loads. It is convenient to consider first the
intact rock, then the discontinuities and, finally, to consider how they com-
bine to determine the properties of rock masses. Thus, in this chapter, we
will discuss the properties of intact rock; in Chapter 7, discontinuities, and
in Chapter 8, rock masses.
6.1 The background to intact rock testing
In rock mechanics, since the early 1960s when it began, more attention has
been paid to intact rock than to any other feature of the rock mass. This
occurred for two main reasons: the subject has relied heavily on the general
topic of the mechanics of solid materials (evolving from rock physics); and
the main way in which samples are obtained remote from human-access is
by diamond drilling to produce cylindrical cores of rock (which are emi-
nently suitable for testing). These two factors caused a concentration of work
on intact rock testing because of the ’convenience’ of a well-established back-
ground and readily available specimens. The circumstances were probably
further reinforced by the general practice of engineers to establish the
properties and behaviour of the materials with which they work.
In this chapter, we will be concentrating on the deformability, strength
and failure of intact rock. The early emphasis on this subject culminated
in 1966 with the ‘discovery’ of the ability to obtain the complete
stress-strain curve. The curve provided previously unknown information
on the behaviour of rocks after their peak strength has been reached. The
failure region has special significance in rock mechanics and rock engi-
neering because, in some circumstances, we can design an underground
structure knowing that the rock will pass into the post-peak region. Such
a design is very different from the traditional approach in all other forms
of engineering, where the material must be kept in the pre-peak region,
i.e. behaving essentially elastically.
In situ, the high stresses that can lead to the material entering the post-
peak region either occur directly, as a result of excavation, or indirectly at