Page 105 - Rock Mechanics For Underground Mining

BEHAVIOUR OF ISOTROPIC ROCK MATERIAL IN UNIAXIAL COMPRESSION

Yield occurs when there is a departure from elastic behaviour, i.e. when some of
the deformation becomes irrecoverable as at A in Figure 4.2a. The yield stress ( y in
Figure 4.2) is the stress at which permanent deformation first appears.
Failure is often said to occur at the peak strength or be initiated at the peak strength
(Jaeger and Cook, 1979). An alternative engineering approach is to say that the rock
has failed when it can no longer adequately support the forces applied to it or otherwise
fulfil its engineering function. This may involve considerations of factors other than
peak strength. In some cases, excessive deformation may be a more appropriate
criterion of ‘failure’ in this sense.
Effective stress is defined, in general terms, as the stress which governs the gross
mechanical response of a porous material. The effective stress is a function of the
total or applied stress and the pressure of the fluid in the pores of the material,
known as the pore pressure or pore-water pressure. The concept of effective stress
was first developed by Karl Terzaghi who used it to provide a rational basis for the
understanding of the engineering behaviour of soils. Terzaghi’s formulation of the
law of effective stress, an account of which is given by Skempton (1960), is probably
the single most important contribution ever made to the development of geotechnical
engineering. For soils and some rocks loaded under particular conditions, the effective

stresses, , are given by
ij

= ij − u ij (4.1)
ij
where ij are the total stresses, u is the pore pressure, and ij is the Kronecker delta.
This result is so well established for soils that it is often taken to be the definition of
effective stress. Experimental evidence and theoretical argument suggest that, over a
wide range of material properties and test conditions, the response of rock depends
on

= ij − u ij (4.2)
ij
where 1, and is a constant for a given case (Paterson, 1978).

4.3 Behaviour of isotropic rock material in uniaxial compression

4.3.1 Influence of rock type and condition
Uniaxial compression of cylindrical specimens prepared from drill core, is proba-
bly the most widely performed test on rock. It is used to determine the uniaxial or
unconfined compressive strength, c , and the elastic constants, Young’s modulus,
E, and Poisson’s ratio, , of the rock material. The uniaxial compressive strength
of the intact rock is used in rock mass classification schemes (section 3.7), and as
a basic parameter in the rock mass strength criterion to be introduced later in this
chapter.
Despite its apparent simplicity, great care must be exercised in interpreting results
obtained in the test. Obviously, the observed response will depend on the nature and
composition of the rock and on the condition of the test specimens. For similar miner-
alogy, c will decrease with increasing porosity, increasing degree of weathering and
increasing degree of microfissuring. As noted in section 1.2.4, c may also decrease
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