Page 175 - Rock Mechanics For Underground Mining

PRESENTATION OF IN SITU STRESS MEASUREMENT RESULTS

stress directions can be determined by direct measurement. It therefore provides a
useful check on the validity and consistency of the solution for the principal stresses.
Other points of confirmation of the correctness of the solution are readily established
by considering the magnitudes of the various stress invariants, calculated from the
several sets of components (expressed relative to different sets of reference axes) of
the field stress tensor.
In the case where redundant experimental observations have been collected, many
independent solutions are possible for the in situ stress tensor. The methods suggested
by Friday and Alexander (Brady et al., 1976) can then be used to establish mean
values of the components and principal directions of the stress tensor. An example of
the way in which an over-determination of experimental parameters can be used is
illustrated in Figure 5.10. More than 1000 independent solutions for the field stresses
allowed construction of histograms of principal stress magnitudes, and contour plots
Figure 5.9 Presentation of principal
stress data on lower hemisphere stere- of principal stress directions. Presumably, greater reliability can be attached to the
ographic projection. mean values of principal stress magnitudes and orientations obtained from these plots,
than to any single solution for the field stresses. The usual tests for consistency can
be applied to the mean solution in the manner described earlier.

Figure 5.10 Histogram plots of A number of other assessments, in addition to those to test the internal consistency
principal stress frequencies and con- of a solution for the field stresses, can be performed to take account of specific site
tour plots and principal stress orienta-
conditions. A primary requirement is that the ambient state of stress cannot violate
tions obtained from redundant strain
the in situ failure criterion for the rock mass. As was noted in Chapter 4, establishing a
observations.
suitable rock mass failure criterion is not a simple procedure process, but an essential
proposition is that the field stresses should not violate the failure criterion for the intact
rock material. The latter rock property may be established from standard laboratory
tests on small specimens. Since the in situ strength of rock is typically much less than
the strength measured on small specimens, the proposed test may not be a sensitive
discriminant of the acceptability of a field stress determination. However, it ensures
that widely inaccurate results are identified and re-examined.
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