Methods of stress determination 45in Fig. 4.3, the complete state of stress can be determined from measure-
ments of strain in six or more different directions taken during one
application of the method. The material properties of the rock are also
required for this method: a device which is equipped with nine or 12 strain
gauges can determine the state of stress in a transversely isotropic rock with
five elastic parameters.
It is emphasized that the understanding of how the components of the
stress tensor are established by these four different methods is crucial to
the planning of an optimal strategy for stress measurement. There are other
complicating factors which we will be discussing later, but the key is to
understand the fundamental basis of the tests as described here. In this
context, none of the indicator methods, with the possible exception of
differential strain analysis, can estimate the complete stress tensor. It
follows that invariably our strategy for stress determination will comprise
of integrating all the information to hand.
In the following sub-sections, the four main ISRM methods are outlined
and discussed. The diagrams are those used in the ISRM Suggested
Methods document.
4.3.1 Flatjack
In Fig. 4.4, the basic principle of the flatjack test is shown. Two pins are
drilled and fixed into the excavation boundary. The distance, d, between
them is then measured accurately. A slot is cut into the rock between the
pins, as shown in the diagram. If the normal stress is compressive, the pins
will move together as the slot is cut. A flatjack, which is comprised of two
metal sheets placed together, welded around their periphery and
provided with a feeder tube, is then grouted into the slot. On pressurizing
the flatjack with oil or water, the pins will move apart. It is assumed that,
when the pin separation distance reaches the value it had before the slot
was cut, the force exerted by the flatjack on the walls of the slot is the same
as that exerted by the pre-existing normal stress. There will be some error
in this assumption, mainly due to jack edge effects, but these can be taken
into account if the jack is suitably calibrated. The test provides a good
estimate of the normal stress across the flatjack.
The major disadvantage with the system is that the necessary minimum
number of six tests, at different orientations, have to be conducted at six
different locations and it is therefore necessary to distribute these around
the boundary walls of an excavation. Invariably, these tests will be
conducted under circumstances where the actual stress state is different at
each measurement location. Hence, to interpret the results properly, it is
also necessary to know the likely stress distribution around the test
excavation.
4.3.2 Hydraulic fracturing
The hydraulic fracturing method of stress measurement basically provides
two pieces of information via the breakdown pressure and the shut-in
pressure (cf. the introductory text in Section 4.3 and part 2 of