Page 117 - Rock Mechanics For Underground Mining

BEHAVIOUR OF ISOTROPIC ROCK MATERIAL IN MULTIAXIAL COMPRESSION

investigators show that the value of I s determined in a test of equivalent diameter, D e ,
may be converted to an I s(50) value by the relation

0.45
D e
I s(50) = I s × (4.6)
50
Beginning with Broch and Franklin (1972), a number of investigators have developed
correlations of the Point Load Index with the uniaxial compressive strength, c . The
most commonly used correlation is

(4.7)
c ≈ (22 − 24)I s(50)
Caution must be exercised in carrying out point load tests and in interpreting the
results, especially when correlations such as that given by equation 4.7 are used. The
test is one in which fracture is caused by induced tension, and it is essential that a
consistent mode of failure be produced if the results obtained from different specimens
are to be comparable. Very soft rocks, and highly anisotropic rocks or rocks containing
marked planes of weakness such as bedding planes, are likely to give spurious results.
A high degree of scatter is a general feature of point load test results and large numbers
of individual determinations (often in excess of 100) are required in order to obtain
reliable indices. For anisotropic rocks, it is usual to determine a Strength Anisotropy
Index, I a(50) , defined as the ratio of mean I s(50) values measured perpendicular and
parallel to the planes of weakness.

4.4 Behaviour of isotropic rock material in multiaxial compression

4.4.1 Types of multiaxial compression test
A basic principle of the laboratory testing of rock to obtain data for use in design
analyses, is that the boundary conditions applied to the test specimen should simulate
those imposed on the rock element in situ. This can rarely be achieved. General
practice is to study the behaviour of the rock under known uniform applied stress
systems.
As was shown in Chapter 2, a general state of three-dimensional stress at a point
can be represented by three principal stresses, 1 , 2 and 3 , acting on mutually
orthogonal planes. No shear stresses act on these planes. A plane of particular interest
is the boundary of an underground excavation which is a principal plane except
in the unusual case in which a shear stress is applied to the boundary surface by
the support. The rock surrounding an underground excavation is rarely in a state
of uniaxial compression. In the general case, away from the excavation boundary
or on the boundary when a normal support stress, 3 , is applied, there will be a
state of polyaxial stress ( 1 = 2 = 3 ). The special case in which 2 = 3 is called
triaxial stress. It is this form of multiaxial stress that is most commonly used in
laboratory testing. On the boundary of an unsupported excavation, 3 = 0, and a
state of biaxial stress exists. The behaviour of intact, isotropic rock materials under
each of these applied stress conditions will be discussed briefly in the following
sections.
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