Page 166 - Rock Mechanics For Underground Mining

PRE-MINING STATE OF STRESS

induced by overcoring that part of the hole containing the measurement device. If
sufficient strain or deformation measurements are made during this stress-relief oper-
ation, the six components of the field stress tensor can be obtained directly from the
experimental observations using solution procedures developed from elastic theory.
The second type of procedure, represented by flatjack measurements and hydraulic
fracturing (Haimson, 1978), determines a circumferential normal stress component
at particular locations in the wall of a borehole. At each location, the normal stress
component is obtained by the pressure, exerted in a slot or fissure, which is in balance
with the local normal stress component acting perpendicular to the measurement slot.
The circumferential stress at each measurement location may be related directly to
the state of stress at the measurement site, preceding boring of the access hole. If
sufficient boundary stress determinations are made in the hole periphery, the local
value of the field stress tensor can be determined directly.
The third method of stress determination is based on the analysis and interpreta-
tion of patterns of fracture and rupture around deep boreholes such as oil and gas
wells. Although such ‘borehole breakouts’ are a source of difficulty in petroleum
engineering, they are invaluable for estimating the state of stress in the lithosphere.
For characterising the state of stress on a regional scale, a method which is funda-
mentally different from the three described above was formulated by Mukhamediev
(1991). It relies on the analysis in the domain of interest of stress trajectories, derived
from other types of stress measurement, to reconstruct the distribution of principal
stresses throughout the block. The method is discussed later in relation to the world
stress map.
The importance of the in situ state of stress in rock engineering has been recognized
by the documentation of ISRM Suggested Methods of rock stress estimation, reported
by Hudson et al. (2003), Sj¨oberg et al. (2003) and Haimson and Cornet (2003).

5.3.2 Triaxial strain cell
The range of devices for direct and indirect determination of in situ stresses in-
cludes photoelastic gauges, USBM borehole deformation gauges, and biaxial and
triaxial strain cells. The soft inclusion cell, as described by Leeman and Hayes (1966)
and Worotnicki and Walton (1976) is the most convenient of these devices, since it
allows determination of all components of the field stress tensor in a single stress
relief operation. Such a strain cell, as shown in Figure 5.4a, consists of at least three
strain rosettes, mounted on a deformable base or shell. The method of operation is
indicated in Figures 5.4b, c and d. The cell is bonded to the borehole wall using a
suitable epoxy or polyester resin. Stress relief in the vicinity of the strain cell induces
strains in the gauges of the strain rosettes, equal in magnitude but opposite in sign
to those originally existing in the borehole wall. It is therefore a simple matter to
establish, from measured strains in the rosettes, the state of strain in the wall of the
borehole prior to stress relief. These borehole strain observations are used to deduce
the local state of stress in the rock, prior to drilling the borehole, from the elastic
properties of the rock and the expressions for stress concentration around a circular
hole.
The method of determination of components of the field stress tensor from borehole
strain observations is derived from the solution (Leeman and Hayes, 1966) for the
stress distribution around a circular hole in a body subject to a general triaxial state
of stress. Figure 5.5a shows the orientation of a stress measurement hole, defined
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