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Geotechnical News • December 2012
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GEOTECHNICAL INSTRUMENTATION NEWS
pressure, indicating the onset of effec-
tive stress and therefore enhanced
backfill stiffness and strength. In
addition, exothermic binder reactions
are reflected in rising temperatures. It
is therefore desirable to see effective
stress and temperature rise occurring
simultaneously. An example of such a
data trend is shown in Figure 2. Note
that Figure 2 also includes vertical
total stress as a matter of interest,
although this would not generally be
required for barricade monitoring.
Interpretation of results
The monitored parameters are inter-
preted both quantitatively and quali-
tatively. The total pressure acting on
the barricade must remain below its
rated safe limit, otherwise backfill-
ing must be stopped so that pres-
sures can subside before backfilling
resumes. Ideally, the onset of effective
stress and temperature rise should be
observed before the backfill reaches
the full barricade height, as this condi-
tion indicates that the barricade is now
beginning to interact with solid to
semi-solid material.
Suggested instrumentation
strategy
Transducer types
Although measuring pore water pres-
sure is relatively straightforward, there
are many well documented issues
associated with measuring total pres-
sures in a granular material. One sig-
nificant issue is matching the stiffness
of the TEPC with that of the surround-
ing medium. This is all the more dif-
ficult when the stiffness of the material
is changing, as is the case for backfill.
The approach taken in this work was
to use TEPCs with the highest practi-
cal aspect ratio (diameter:thickness)
and stiffness possible. The merits of
this approach can be debated but such
details, while important, are beyond
the scope of the current article.
TEPCs and piezometers of the vibrat-
ing wire type have been used, supplied
by two leading manufacturers. The
TEPCs used have been about 250 mm
(10 in.) diameter with sensing sur-
face on one side (also called “contact
cells”) and about a 20:1 aspect ratio.
All of the transducers have thermistors
and provide a temperature data chan-
nel. During the initial filling stage,
while the backfill is still a fluid, both
manufacturers’ TEPCs have given
pressure readings consistent with the
piezometer up to the onset of effec-
tive stress, which is one of the critical
indicators of good backfilling prac-
tice. Subsequently, there appear to be
TEPC response differences that cannot
be currently adequately explained, and
therefore further research is needed
into the performance of these cells
in curing backfill where the stiffness
changes with time.
Transducer calibration
Manufacturers provide calibration
sheets for their vibrating wire piezom-
eters and TEPCs. The thermal and
fluid pressure calibrations have been
found consistently reliable for many
hundreds of transducers used in the
field to date. However, as explained
above, TEPC calibration is much
more problematic when the stiffness
of the material changes with time,
and it is therefore not advised that
mine-specific TEPC calibration be
attempted at present. Indeed, there are
other logistical considerations that can
be far more influential on the output
of TEPCs, and so attention to detail
in the construction and deployment
of the system is a more important
consideration.
Building and deploying a system
At least one TEPC and one piezometer
are recommended for routine bar-
ricade monitoring. These transduc-
ers should be installed at the same
elevation so that piezometer readings
can be directly subtracted from TEPC
readings to obtain effective stress.
Mounting the transducers directly to
the back of the structural barricade
is not recommended, as variations in
barricade stiffness and drainage condi-
tions make measurements there too
localized. Instead, it is recommended
that the transducers be placed within
the backfill about 2 m behind the bar-
ricade and at about one-third barricade
height. Ideally the mine should work
with the instrumentation supplier to
have an instrumentation module pre-
built so that the two transducers are
delivered on a frame that can be easily
Figure 2. Ideal monitoring data showing temperature increase and develop-
ment of effective stress (i.e., total pressure exceeding pore water pressure).