Methods OF stress determination 49If the elasticity assumptions are made, we find that the stress
concentration factors do not depend on the elastic constants of the rock nor
the diameter of the borehole. We do, however, need to know the tensile
strength of the rock and this is a subject fraught with controversy. Suffice
it to say, the best way to measure the tensile strength is under the
conditions for which it is required, i.e. by pressurizing a hollow cylinder
of rock. This is because the tensile strength (i.e. the stress causing tensile
failure) is not a material property. A material property does not depend on
the specimen geometry and loading conditions of the test: the tensile
strength does.
Against this background of many potential difficulties, a great deal of
research effort is being expended on improving confidence in stress
measurements made using this technique. There are ways of overcoming
all the difficulties mentioned above, see Cuisiat and Haimson (1992).
4.3.3 The USBM borehole deforrnution guuge
As indicated in part 3 of Fig. 4.3, the USBM technique allows the complete
stress state in a plane to be determined from three measurements of
the change in different diameters of a borehole when the stresses are
released by overcoring the borehole. The instrument is shown in Fig. 4.7.
When the torpedo is inserted in a borehole, six 'buttons' press against
the borehole wall and their diametral position is measured by strain gauges
bonded to the sprung steel cantilevers supporting the buttons. When this
borehole is overcored by a larger drill, the stress state in the resulting
hollow cylinder is reduced to zero, the diameter of the hole changes, the
buttons move, and hence different strains are induced in the strain gauges.
From previous calibration exercises, the actual diametral changes are
deduced. From these changes, and with the use of elasticity theory, the
biaxial stress state in the plane perpendicular to the borehole axis is
deduced.
In this test, as in hydraulic fracturing, we are determining far-field
stresses which have been concentrated around the measurement borehole.
A useful aspect of the USBM technique is that it produces an annular core
Figure 4.7 The USBM borehole deformation gauge.