Page 174 - Rock Mechanics For Underground Mining

PRE-MINING STATE OF STRESS

strength properties allow an estimate of the state of stress immediately outside the
zone of influence of the borehole. Widely used in the petroleum industry for estimating
the state of stress in deep reservoirs, the method is reviewed in detail by Zoback et al.
(2003). An example of its application in hard rock is provided by Paillet and Kim
(1987).
Development and application of a method for in situ stress measurement based on
the Kaiser effect is described by Villaescusa et al. (2002). The Kaiser effect is an
expression of the immediately preceding maximum stress to which a specimen of
rock has been subjected, and provides a method of estimating the recent stress history
of a core sample recovered from a borehole. For a specimen containing microcracks,
such as brittle rock, uniaxial loading is aseismic until a stress threshold is reached
characteristic of the earlier stress magnitude it experienced. Application of the Kaiser
effect in estimating the complete stress tensor relies on a capacity to determine re-
liably the magnitudes of the preceding normal stresses applied to the specimen in
various directions. This is done by monitoring the emission of acoustic pulses during
loading of small undercores recovered from the larger, oriented drill core taken from
the ground. By Kaiser-effect gauging of preceding stresses in six undercores in six
mutually independent orientations, it is possible to invert the normal stresses to re-
cover the field stresses which the large core experienced in situ. The advantage of the
method is the relative convenience and ease of application and the related low cost.
Anelastic strain recovery (Voight, 1968) exploits the relaxation which a rock core
experiences after it is isolated from the stressed host rock mass. The time-dependent
strains are measured with strain gauges, the principal strains calculated, and the total
strains are estimated by assuming direct proportionality between total strains and the
anelastic strains.
Differential strain curve analysis (Roegiers, 1989) or deformation rate analysis
(Villaescusa et al., 2002) are similar in principle to Kaiser effect gauging of the
recent stress history of a sample of rock. When a core is taken from the ground, re-
laxation of normal stress allows microcracks to open. In axial loading of undercores
taken from a larger core, closure of open microcracks is indicated by a clear change
in the slope of the axial stress-normal strain plot. The related value of the applied
normal stress is taken to correspond to the normal stress existing in the ground imme-
diately prior to recovery of the core. If closure stresses are determined for undercores
taken in six mutually independent orientations, then as for Kaiser-effect gauging, it
is possible to invert the stress data to recover the field principal stresses and their
orientations.

5.4 Presentation of in situ stress measurement results

The product of a stress measurement exercise is the set of six components of the field
stress tensor, usually expressed relative to a set of local axes for the measurement
hole. These yield the stress components expressed relative to the global axes by
a simple transformation. The principal stress magnitudes and orientations are then
determined from these quantities using the methods described in Chapter 2. If a single
determination is made of the field stress tensor, the orientations of the principal stress
axes can be plotted directly on to a stereonet overlay as shown in Figure 5.9. The
value of this procedure is that the required mutual orthogonality of the principal
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