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Geotechnical News • December 2012
31
GEOTECHNICAL INSTRUMENTATION NEWS
and quickly erected and anchored
(Figure 3). The transducers should be
pre-wired with a single connector that
attaches to a portable data acquisi-
tion system. The data acquisition
system should then be configured to
the mine’s data backbone so that the
information is fed to the backfill plant
on the surface. Plant operators must
be trained to interpret these results and
decide if and when a plant shutdown is
required.
Visual monitoring
In addition to the pressure and tem-
perature monitoring, it is valuable to
provide a camera feed to the backfill
plant so that the operators can also
visually monitor the overall barri-
cade response to backfilling (Figure
4). There have been instances where
a small construction defect has led
to localized barricade failure and
release of backfill, and such local-
ized response would probably not be
picked up by the instrumentation sys-
tem. Had the barricade been monitored
visually the operators would have seen
cracks developing and leakage from
these cracks, and a plant shutdown
to investigate and possibly remediate
the barricade could have prevented its
ultimate failure.
Lessons learned
In addition to the recommendations
already mentioned, the follow-
ing should be considered by mines
embarking on routine backfill monitor-
ing programs.
Have a supplier build a system
The essential components of the
system have already been mentioned:
instrumentation module; dedicated
data acquisition system; data network-
ing to surface; camera feed. Ideally
the mine should work with a supplier
who can build a suitable system to the
mine’s specification and then sup-
port that system in the field. It has
been extremely valuable to have the
supplier’s technician on site for the
first instrumentation installation and
monitoring, to train mine personnel
in verifying system performance and
trouble-shooting any problems prior to
backfilling.
Transducer range, resolution, and
accuracy
Barricades typically have a safe pres-
sure rating in the range of 100 – 200
kPa, although the trend is towards
better barricades with increased safe
pressure ratings. One of the manufac-
turers supplies a 1 MPa vibrating wire
TEPC with a quoted resolution of 0.25
kPa minimum which is certainly suf-
ficient for barricade monitoring. One
must be careful when interpreting a
manufacturers’ claims of TEPC accu-
racy, however, as such figures do not
reflect the performance of the entire
TEPC installed in the field, where the
accuracy of the transducers output can
be influenced by factors such as stiff-
ness of the surrounding medium.
Protect the data cables
Once the instrumentation has been
installed, the connecting data cables
need to be covered with a protective
sand berm. The sand berm can extend
through the base of the structural bar-
ricade and will actually act as a drain/
filter which is marginally beneficial to
barricade performance.
Zero the instruments
A TEPC that is built and calibrated
(zeroed) near sea level will register an
initial positive pressure underground,
reflecting the increased air pressure
arising from the mine’s ventilation
system. This initial reading needs to
be zeroed out for engineering calcula-
tions that are based on gauge pressures
(i.e. relative to the ambient pressure).
Also, the piezometer tip needs to be
Figure 3. A pre-built instrumentation
module with single connection to
the data acquisition system.
Figure 4. Backfill plant operator monitoring barricade pressures and video
feed in real time, in order to optimize stope filling.