Page 170 - Rock Mechanics For Underground Mining

PRE-MINING STATE OF STRESS

Figure5.6 (a)Coredrillingaslotfor measurement system seeks to restore the original, post-excavation local state of stress
a flatjack test and (b) slot pressurisa- at the experiment site. Such methods are intrinsically more accurate than those relying
tion procedure.
on positive disturbance of the initial condition whose state is to be determined.
Three prerequisites must be satisfied for a successful in situ stress determination
using flatjacks. These are:

(a) a relatively undisturbed surface of the opening constituting the test site;
(b) an opening geometry for which closed-form solutions exist, relating the far-field
stresses and the boundary stresses; and
(c) a rock mass which behaves elastically, in that displacements are recoverable
when the stress increments inducing them are reversed.
The first and third requirements virtually eliminate the use as a test site of an ex-
cavation developed by conventional drilling and blasting. Cracking associated with
blasting, and other transient effects, may cause extensive disturbance of the elastic
stress distribution in the rock and may give rise to non-elastic displacements in the
rock during the measurement process. The second requirement restricts suitable open-
ing geometry to simple shapes. An opening with circular cross section is by far the
most convenient.
The practical use of a flatjack is illustrated in Figure 5.6. The jack consists of
a pair of parallel plates, about 300 mm square, welded along the edges. A tubular
non-return connection is provided to a hydraulic pump. A measurement site is es-
tablished by installing measurement pins, suitable for use with a DEMEC or similar
deformation gauge, in a rock surface and perpendicular to the axis of the proposed
measurement slot. The distance d 0 between the pins is measured, and the slot is cut,
using, for example, a series of overlapping core-drilled holes. Closure occurs between
the displacement measuring stations. The flatjack is grouted in the slot, and the jack
pressurised to restore the original distance d 0 between the displacement monitoring
pins. The displacement cancellation pressure corresponds closely to the normal stress
component directed perpendicular to the slot axis prior to slot cutting.
Determination of the field stresses from boundary stresses using flatjacks follows
a procedure similar to that using strain observations. Suppose a flatjack is used to
measure the normal stress component in the direction OA in Figure 5.5c, i.e. the
plane of the flatjack slot is perpendicular to the axis OA. If A is the jack cancellation
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