Page 173 - Rock Mechanics For Underground Mining

METHODS OF IN SITU STRESS DETERMINATION

The minimum tangential boundary stress is obtained by superimposing this stress on
that given by equation 5.11, i.e.

(5.12)
min = 3p 2 − p 1 − p 0
and the minimum effective boundary stress is

min = 3p 2 − p 1 − p 0 − u (5.13)

The crack re-opening pressure p r corresponds to the state of borehole pressure p 0
where the minimum effective boundary stress is zero, i.e. introducing p r in equation
5.13

3p 2 − p 1 − p r − u = 0

or
p 1 = 3p 2 − p r − u (5.14)

Because p 2 = p s , equation 5.14 confirms that the magnitudes of the major and minor
plane principal stresses p 1 and p 2 can be determined from measurements of shut-in
pressure p s and crack re-opening pressure p r . The orientation of the principal stress
axes may be deduced from the position of the boundary fractures, obtained using a
device such as an impression packer. The azimuth of the major principal stress axis
is defined by the hole diameter joining the trace of fractures on opposing boundaries
of the hole.
Although hydraulic fracturing is a simple and apparently attractive stress measure-
ment technique, it is worth recalling the assumptions implicit in the method. First,
it is assumed that the rock mass is continuous and elastic, at least in the zone of
influence of the hole and the hydraulically induced fractures. Second, the hole axis
is assumed to be parallel to a field principal stress axis. Third, the induced fracture
plane is assumed to include the hole axis. If any of these assumptions is not satisfied,
an invalid solution to the field stresses will be obtained. A further limitation is that it
provides only plane principal stresses, and no information on the other components
of the triaxial stress field. The usual assumption is that the vertical normal stress
component is a principal stress, and that it is equal to the depth stress.

5.3.5 Other methods of estimating the in situ state of stress
Compared with overcoring, flatjacks and hydraulic fracturing, some other methods
of estimating the in situ state of stress are attractive by virtue of the relative ease
with which the raw data for the stress determination can be recovered. Of particular
interest in this regard are methods based on borehole breakouts, stress history gauging
through the Kaiser effect, and differential strain curve analysis or deformation rate
analysis.
The borehole breakout method described by Zoback et al. (1985) relies on the state
of stress around a borehole being sufficient to cause compressive fracture at preferred
locations around the hole boundary. Assuming a principal stress direction is parallel
to the borehole axis and that the principal stress field in the plane perpendicular to the
hole axis is anisotropic, the Kirsch equations and a knowledge of the rock material
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