Laboratory Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 415

Figure 14.6 Influence of height-to-diameter ratio H/D on the strength σ f measured in uniaxial
compression tests (Wittke 1990)

Figure 14.6 (center) illustrates the shear stresses existing at the specimen's end faces due
to friction. The stress state in the middle part of the specimen is not influenced by these
end effects and thus is homogeneous and uniaxial.
Fig. 14.6 (right) shows the influence of the height-to-diameter ratio H/D on the failure
strength σ f measured in uniaxial compression tests (Wittke 1990). It follows that σ f

does not deviate from the unconfined compressive strength σ cIR of the intact rock if

H/D ≥ 2.5 (Vutukuri et al. 1974, DGEG 1979b) and σ cIR can be directly read off as the

maximum axial stress from the axial stress versus axial strain plot.

The stress state in specimens of considerably lower H/D ratios is influenced by the
friction at their end faces and thus no longer homogeneous and uniaxial. This leads
to a failure stress σ f that is higher than the unconfined compressive strength σ cIR (Fig.

14.6, right).
The determination of the unconfined compressive strength of intact rock with planar
grain structure requires the carrying out of tests on specimens with different inclination
angles of the isotropic plane with respect to the specimen's end faces as illustrated in

Fig. 3.8 (Section 3.2.2).
In ISRM (1999a) and DGGT (2004b) instructions are given for measuring the com-
plete stress-strain curve of intact rock including post-failure behavior by means of
uniaxial compression tests. However, this is only possible using special testing devices
that fulfill the highest requirements with regard to machine stiffness, strain rate con-
trol and data acquisition. It should therefore be noted that brittle specimens even
under the most favorable test conditions (stiff testing frame, low strain rate and cir-
cumferential strain rate control) may fail abruptly when tested without confinement
(cf. Fig. 14.6, left). Accordingly, the corresponding recommendations given in ISRM
(1999a) and DGGT (2004b) may be applicable only to ductile rocks such as shale,
mudstone and rock salt.

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