Page 118 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

4.4.2 Biaxial compression ( 1 2 , 3 = 0)
Biaxial compression tests are carried out by applying different normal stresses to
two pairs of faces of a cube, plate or rectangular prism of rock. The great difficulty
with such tests is that the end effects described in section 4.3.3 exert an even greater
influence on the stress distribution within the specimen than in the case of uniaxial
compression. For this reason, fluid rather than solid medium loading is preferred. An
alternative approach is to generate a biaxial state of stress at the inner surface of a
hollow cylinder by loading it axially with a fluid pressure applied to its outer surface
(Hoskins, 1969, Jaeger and Cook, 1979) in a triaxial cell (section 4.4.3). However, in
this case, the stresses at ‘failure’ cannot be measured, but must be calculated using the
theory of elasticity which may not be applicable at peak stress. The inner boundary of
the hollow cylinder is a zone of high stress gradient which could influence the result.
For these reasons, it is recommended that the use of hollow cylinder tests be restricted
to the simulation of particular rock mechanics problems such as the behaviour of rock
around a shaft, bored raise or borehole.
Brown (1974) carried out a series of biaxial compression tests on 76 mm square by
25 mm thick plates of Wombeyan Marble which were loaded on their smaller faces
through (a) 76 mm × 25 mm solid steel platens, and (b) brush platens made from
3.2 mm square steel pins. Figure 4.15 shows the peak strength envelopes obtained
in tests carried out at constant 2 / 1 ratios. The data are normalised with respect to
the uniaxial compressive strength of the plates, c = 66 MPa. The increase in peak
strength over c , associated with a given value of 2 , was greater for the solid platens
than for the brush platens. This was attributed to the influence of end effects. When
the brush platens were used, the maximum measured increase in peak strength over

Figure 4.15 Biaxial compression
test results for Wombeyan Marble (af-
ter Brown, 1974).

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