Geotechnical News • June 2015
27
WASTE GEOTECHNICS
that the cover profile (typically 1.5 m
thick) would be non-acid forming and
oxygen consuming. The details for the
design and the predicted performance
of the de-sulphurized cover at Detour
are fully described by Dobchuk et al.
(2013).
The design of dry tailings stacks
for both operation and closure will
advance as the BAT-PS principles are
coupled with the BAT-CS principles
for the prevention and control of ARD/
ML. The use of filtered tailings and
dry stacks will reduce (not eliminate)
the risk of failures such as we have
seen at Mount Polley, but at the same
time will create new risks that will
need to be managed. For mine sites
in the province of British Columbia,
ARD/ML will likely become one of
the most significant risks for long-term
environmental impacts and closure
costs. New methods of risk assess-
ments to optimize the design of stacks
that maintain the best combination
of BAT-PS and BAT-CS principles
and requirements will be necessary.
There is extensive experience in Brit-
ish Columbia and internationally in
tailings impoundment design with risk
assessment using failure modes and
effects analyses (FMEA). FMEA does
not eliminate risk, but is a tool that
allows identification and quantifica-
tion of risk and therefore the selection
and application of mitigation measures
to reduce risk. Shaw and Robertson
(2015) demonstrate that FMEA meth-
odology can be applied also to assess
the risk of ARD/ML and to identify
mitigation measures to better man-
age uncertainty and errors associated
with ARD/ML prediction and control.
By including both PS and CS failure
mechanisms, likelihood and conse-
quences in the FMEA, one is better
able to optimize the tradeoffs between
PS and CS measures and therefore
minimize overall risk. Surely the mini-
mization of overall risk is what we
strive for in defining ‘Best’ in BAT.
In the view of the authors, filtered
tailings dry stacks are not the only
method for BAT-PS. We are certain
the panel is simply not recommending
the use of filtered tailings dry stacks,
but more importantly they are recom-
mending BAT Principles. Alternatives
may include paste and thickened tail-
ings, cycloned sand or even conven-
tional slurries with extended beaches,
underdrains and compacted lifts.
Similarly, wet disposal of ARD tail-
ings should not be considered the only
method of BAT-CS. Rather BAT is
that combination of technologies, that
when combined, result in the least risk
of physical and chemical instability
that could potentially lead to failure.
BAT may include filtered dry stacked
tailings or wet disposal of ARD tail-
ings if their inclusion in the structure
results in least overall risk.
A transition to filtered tailings, where
appropriate, may also offer additional
new opportunities for mine waste
management. An immediate benefit
may be the reduction of the footprint
required for the tailings impoundment.
Progressive closure with construction
of the stack should also be possible.
Co-disposal of reactive mine waste
rock, for which ARD can be very dif-
ficult to prevent and control, may be
possible. For example, reactive waste
rock layers may be co-mingled with
filtered tailings to serve as sealing lay-
ers that prevent advection of oxygen in
the waste rock. Vertical ribs of waste
rock may also be constructed in the
profile of the filtered tailings stack
to improve drainage and stability.
In addition, filtered tailings may be
blended with waste rock and mixed at
an optimum ratio to form a dense high
strength paste rock.
Scale may also present of the addi-
tional challenges and difficulties in
the implemention of BAT-PS and CS
principles. The implementation of
filtered dry stacked tailings at large
mines may be prohibitively expensive
or require logistics that we are not
yet capable of managing. While the
technology and capacities for filtration
systems are rapidly developing, metal
mines keep getting bigger with tail-
ings production rates often exceeding
120,000 tpd, and the largest currently
under construction at 360,000 tpd. We
now have a number of tailings dams
under construction with ultimate toe
to crest heights exceeding 300 m, and
the highest exceeding 400 m. The
elimination of ponds may be difficult
in very wet climates where runoff
control has to be practiced, and the
construction of stacks may be difficult
in high rainfall regions. Clay-rich tail-
ings may prove difficult to dewater by
filtration sufficiently to construct and
maintain stable stacks. In many cases,
BAT for these mines may not include
filter pressed stacked tailings, but this
does not preclude or lessen our need to
strive for BAT that includes the most
optimum combination of technolo-
gies needed to reduce risk to socially
acceptable, very low values.
Conclusions
We see value arising from the fail-
ure at Mount Polley. We expect the
drive to evaluate, test and implement
emerging and new technology will add
knowledge that will generate opportu-
nities for improved methods of design,
construction, monitoring and regula-
tion that will create earth structures
on mines that are both physically and
chemically stable. These new earth
structures will be easier to reclaim
and transition to land uses that are
environmentally secure and socially
acceptable.
References
Cash, A.E., Urrutia, P., Wilson, G.W.,
Robertson, J. and Turgeon, M.H.
2012. A 2011 Update for the
Single-Layer Desulphurised Tail-
ings Cover Completed in 1999 at
Detour Gold. Proceedings Mine
Closure 2012. Australian Centre
for Geomechanics, Perth, ISBN
978-0-9870937-0-7.
Dobchuk, B., Nichol, C., Wilson, G.W.
and Aubertin, M. 2013. Evaluation
of a Single-Layer Desulphurized
Tailings Cover. Canadian Geotech-
nical Journal, 50(7): 777-792.
