Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 523

16.2.5 Borehole Slotter
The borehole slotter was developed by Bock & Foruria (1984) and later improved (Bock
1986, Azzam & Bock 1987, Bock 1993). It consists of a strain sensor mounted to the
wall of a borehole with a diameter of 96 - 104 mm and a pneumatically driven diamond
saw (Fig. 16.12). Using this saw, pairs of 1 mm wide, up to 25 mm deep, slots are cut in
opposite directions. The tangential strains due to stress relief as a result of slotting are
measured. After cutting the slots the borehole slotter is recovered.
Figure 16.12 Borehole slotter
Borehole slotter measurements are simple, can be carried out quickly and thus can
achieve a large number of measurements. However, it is applicable only in dry boreholes
to a depth of about 40 m.
Test evaluation is based on isotropic elastic stress-strain behavior of the rock mass,
assuming full stress relief after cutting the slots. Then the two-dimensional stress state
normal to the borehole axis can be evaluated.
Stress relief as a result of cutting the slot was investigated and verified by means of
FE analyses (Bock & Foruria 1984) and laboratory tests (Foruria 1987). The elastic
constants required for stress determination from measured strains are determined by
means of uniaxial compression tests on cylindrical specimens prepared from drill cores
that are taken near the test locations.
At least three pairs of slots, cut at axial distances of some 10 cm, are the minimum
requirement to calculate the two-dimensional stress state normal to the borehole axis
from measured strains (Bock & Foruria 1984). Normally, however, more slots are cut
to obtain both an over-determined system of linear equations for stress calculation and
redundancy of results.
Since, as in the case of doorstopper measurements, only the two-dimensional stress
state normal to the borehole axis can be determined, the results do not allow the
three-dimensional in-situ stress state to be determined without combining the results of