50 In situ stress
which may be used in the laboratory to determine the elastic properties
directly at the site where the test was conducted. Given the validity of the
assumptions, the USBM gauge and its homologues are efficacious because
they are reusable, permit measurements to be made many times within a
borehole and are relatively cheap and robust. In Fig. 4.8, the raw data
recorded during a USBM overcoring stress measurement test are shown.
It can be seen that the effect of removing the pre-existing stress
components has been to cause an expansion along all three diameters, with
one of the deformations, u3, in the figure, being more than the other two
deformations.
The measurement of a diametral displacement is analogous to the use
of a flatjack for measuring the normal stress component. In a similar way
to the flatjack, each measurement of displacement effectively permits cal-
culation of one normal strain. Through the use of the stress transformation
equations, it is possible to calculate the principal components of the biaxial
stress state and their orientations. There is, however, the added com-
plication of the presence of the borehole, which perturbs the stress state
from its natural in situ state.
4.3.4 The CSIRO overcoring gauge
This device operates on a similar principle to the USBM torpedo except that
it is a gauge which is glued into the borehole and can measure normal
strains at a variety of orientations and locations around the borehole wall.
The gauge is glued into position within the pilot hole, initial readings of
strain are taken and the gauge is then overcored. This destresses the
resulting hollow cylinder and final strain gauge readings are taken. TheThe traces are the electrical output from the device
plotted against time during overcoring and hence
illustrate the evolution of diametral change during
overcoring.-O.OO~ r
\measurement
for each curve
\.-.-.- u3
0.020
0 100 200 300 400 500
Depth of overcoring bit (mm)
Figure 4.8 Data obtained during a USBM overcoring test.