Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 524

measurements obtained in boreholes with different orientations. From experience, this
can lead to incorrect and meaningless results. Particularly if the results attained in one
borehole are subject to large scatter, they are uncorrelated, so that neither redundancy
nor consistency of the determined stress states is achieved.
To overcome this disadvantage different borehole slotter designs have been developed
for determining the three-dimensional in-situ stress state (Yuen 1989, Lawrence 1992,
Corthesy et al. 1999). Corthesy et al. (1999) proposed a slotter prototype that allows
the determination of the in-situ stress tensor by sawing inclined slots, in addition to the
slots parallel to the borehole axis. However, this technique has not yet been applied.

16.2.6 Borehole Wall Stress Relief Method
The “borehole wall stress relief method” (BWSRM) allows the three-dimensional in-si-
tu stress state to be determined (Ge et al. 2006, Ge & Hou 2010). At the wall of an ex-
ploration borehole three strain gauge rosettes are fixed, each equipped with three strain
gauges. Subsequently these rosettes are overcored in radial directions to obtain cylindri-
cal specimens. Then the strains due to stress relief of the three overcored specimens are
measured. The elastic constants of the specimens are determined by means of uniaxial
compression tests. The three-dimensional stress state can then be determined from the
recorded strains, assuming isotropic elastic stress-strain behavior.

16.3 Stiff Inclusion

Stiff inclusions, also referred to as “stress meters” or “pressure cells”, are intended to
measure stress changes in rock (Leeman 1964). Stiff inclusion devices are tightly insert-
ed or grouted into boreholes so that any stress change in the surrounding rock mass is
transmitted to the inclusion.
Stress change measurements are performed either using flat jacks (Fecker 1997) or by
devices operating according to the vibrating wire principle (Hawkes & Hooker 1974,
Sellers 1977). The operating principles of both devices will be described in Section 17.6.

The accuracy and sensitivity of stress change measurements depend on the shape of
the inclusion as well as the moduli ratio of the inclusion and the surrounding rock mass
(Filcek & Cyrul 1977). If the inclusion modulus is more than 3 to 4 times higher than
that of the surrounding rock, stress variations in the inclusion become insensitive to the
Young's modulus of the rock mass (Leeman 1964). However, there is an upper limit of
the inclusion modulus to produce a measurable effect in the inclusion. The adequate
stiffness of inclusion devices applied in isotropic elastic rock mass was investigated by
Bois et al. (1994).
Stiff inclusions are often used in ductile and creeping rock to measure the absolute
stress state. Theoretically, in the course of time the stress state existing in these rocks
will be completely transferred to the stiff inclusion due to relaxation. However, with