Page 167 - Rock Mechanics For Underground Mining

METHODS OF IN SITU STRESS DETERMINATION

(a) by dip, , and dip direction, , relative to a set of global axes x, y, z. Relative
to these axes, the ambient field stress components (prior to drilling the hole) are
p xx , p yy , P zz , p xy , p yz , p zx . A convenient set of local axes, l, m, n, for the borehole
is also shown in Figure 5.5a, with the n axis directed parallel to the hole axis, and
the m axis lying in the horizontal (x, y) plane. The field stress components expressed
relative to the hole local axes, i.e. p ll , p ln , etc., are readily related to the global
components p xx , p xz , etc., through the stress transformation equation and a rotation
matrix defined by
⎡ ⎤ ⎡ ⎤
− sin cos sin cos cos
xl xm xn
⎣ − sin sin − cos
[R] = ⎣ yl ym yn ⎦ = cos sin ⎦
cos 0 sin
zl zm zn
In Figure 5.5b, the location of a point on the wall of the borehole is defined by
the angle
measured clockwise in the l, m plane. Boundary stresses at the point

(b)

Figure 5.4 (a) A triaxial strain
cell (of CSIRO design), and (b),
(c), (d) its method of application.

(c) (d)

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