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Geotechnical News • December 2012
29
GEOTECHNICAL INSTRUMENTATION NEWS
Paolo Mazzanti
Chief Executive Officer, NHAZCA S.r.l.
and
Researcher, “Sapienza” Università di
Roma,
Via Cori snc, 00177, Rome, Italy.
T: +39 346 977 6508
E: paolo.mazzanti@nhazca.com
Figure 1. Vertical cross section
through a mined area of the
mineralized zone.
barricade
draining / curing
backfill
outline of
stope
overcut
undercut
slurry backfill
Field monitoring for improved mine backfill systems
M.W. Grabinsky, B.D. Thompson, W.F. Bawden
Introduction
The large voids created by under-
ground mining are backfilled to
provide regional ground support. Our
understanding of backfill behaviour
has improved significantly using
elaborate field monitoring techniques
1
;
however this article will instead focus
on simplified systems for routine
monitoring. A brief explanation of the
engineering problem is first provided
1
see cover photos on the September 2009 and
June 2012 issues of Geotechnical News, and the
free technical article at http://www.nrcresearch-
press.com/doi/abs/10.1139/t2012-040
for those readers unfamiliar with min-
ing processes and terminology.
Brief overview of underground
mining procedures and terms
The mineralized zone to be exploited
is called a stope (Figure 1). Under-
cut and overcut access tunnels are
created so that the ore in the stope
can be drilled and blasted, with the
blasted ore being extracted through the
undercut. A steel reinforced shotcrete
barricade is then constructed within
the undercut and slurry backfill is
delivered through the overcut. The
backfill typically contains silt to sand
size granular material at up to 70%
solids content, and also contains
Portland cement binder. Some of the
water in the slurry must drain, and the
binder must cure (hydrate) so that the
backfill gains the stiffness and strength
required to support the surrounding
rock mass during subsequent mining
of adjacent stopes.
Purpose and approach of the
monitoring program
Design concerns and what needs to
be monitored
The immediate mine design concerns
are (i) to determine the pressures act-
ing on the barricade, and (ii) to assess
if the backfill is properly curing. These
concerns are addressed by monitoring
total pressure, pore water pressure,
and temperature. It is also necessary
to estimate backfill height within the
stope as a function of time. This is
done by conducting a cavity moni-
toring survey (CMS) to determine
stope geometry prior to filling, and
then using the volume-rate of backfill
delivery to calculate the average back-
fill elevation as a function of filling
time. Instrument locations within the
void must also be determined using
standard survey techniques.
Expected results
Backfills deposited as slurries will
initially generate an isotropic total
pressure equal to the unit weight of the
backfill x depth below the deposition
surface. In this case both piezometers
and total earth pressure cells (TEPCs)
will register the same total pressures.
The primary mechanisms believed to
be responsible for pore water pressure
dissipation are drainage and water
consumption during binder hydration
(i.e. chemical shrinkage or self-
desiccation). When either mechanism
occurs the measured pore water pres-
sure will become lower than the total
such as high information density,
monitoring historical deformation,
simultaneously viewing large areas
without interaction with the ground/
structure are very important if you
use deformation as a tool for “under-
standing” geotechnical or geological
processes. In this way, monitoring of
deformation can be a useful additional
tool for use during the preliminary
design phases of projects.
However, adoption of these fascinat-
ing opportunities can lead to expensive
equipment, complex data processing,
difficult interpretation of results, and
some limitations that may lead to
misleading conclusions. To repeat,
we need to carry out a vast amount of
observational work [also
using remote
methods], but what we do should be
done for a purpose and done well
.