Page 104 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

Figure 4.1 Idealised illustration of
the transition from intact rock to a
heavily jointed rock mass with in-
creasing sample size (after Hoek and
Brown, 1980).

4.2 Concepts and definitions

Experience has shown that the terminology used in discussions of rock ‘strength’ and
‘failure’ can cause confusion. Unfortunately, terms which have precise meanings in
engineering science are often used imprecisely in engineering practice. In this text,
the following terminology and meanings will be used.
Fracture is the formation of planes of separation in the rock material. It involves
the breaking of bonds to form new surfaces. The onset of fracture is not necessarily
synonymous with failure or with the attainment of peak strength.
Strength,or peak strength, is the maximum stress, usually averaged over a plane,
that the rock can sustain under a given set of conditions. It corresponds to point B
in Figure 4.2a. After its peak strength has been exceeded, the specimen may still
have some load-carrying capacity or strength. The minimum or residual strength
is reached generally only after considerable post-peak deformation (point C in
Figure 4.2a).
Brittle fracture is the process by which sudden loss of strength occurs across a
plane following little or no permanent (plastic) deformation. It is usually associated
with strain-softening or strain-weakening behaviour of the specimen as illustrated in
Figure 4.2a.
Ductile deformation occurs when the rock can sustain further permanent defor-
mation without losing load-carrying capacity (Figure 4.2b).

Figure 4.2 (a) Strain-softening; (b)
strain-hardening stress–strain curves.

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