26
Geotechnical News •June 2015
WASTE GEOTECHNICS
Mount Polley failure shows why
physical stability must remain fore-
most and cannot be compromised”.
We also recognized that the case of
‘filtered stacked tailings’ at Greens
Creek was given by the panel as an
example for the application of BAT
principles, and that other methods of
tailings disposal can achieve these
principles and should also be evalu-
ated. Adoption of the BAT principles
will create new challenges as well as
new opportunities for the designers of
new surface tailings impoundments.
The writer’s note that in the specific
case of ‘dry stacking of filtered tail-
ings’ this method should not be con-
sidered a panacea for the elimination
of failure potential. One of the writers
is currently reviewing two such dry
stacks where the design, operating and
site conditions have lead to an urgent
need for remedial modifications to
avoid failure conditions.Furthermore,
the implementation of BAT principles
for physical stability (BAT-PS) is not
necessarily BAT for chemical stability
(BAT-CS). The tradeoff in risk is real
and must be addressed in light of all
stages of environmental assessment.
We note that on March 19, 2015 the
Environmental Assessment Office
of the British Columbia Ministry of
Environment released an Informa-
tion Bulletin „Province Implements
Post-Mount Polley Requirements in
Environmental Assessment“ outlining
information that companies propos-
ing to build new tailings dams will
be required to include. For example,
selected options for tailings manage-
ment will need consider other options
that address the potential for adverse
effects on the environmental as well as
on health, social, heritage and eco-
nomic issues. Furthermore, companies
will need to present and compare best
practices and best available technolo-
gies for tailings management.
Having just attended the 10th Inter-
national Conference on Acid Rock
Drainage (ICARD), it is apparent
many geochemists will resist the
implementation of the BAT-PS princi-
ples. Geochemists dedicate themselves
to chemical stability, as opposed to
physical stability, for the prevention
and control of acid rock drainage and
metal leaching (ARD/ML). A key
design principle for the prevention of
ARD for tailings is to maintain full
saturation within the profile, and in
many cases water covers are used
to prevent surface oxidation. Water
covers are routinely employed as a
preferred strategy for the long-term
closure of reactive tailings in Canada,
a practice that has been embraced
worldwide as outlined by the global
acid rock drainage (GARD) guide. In
summary, the general principles laid
out in the GARD guide (BAT-CS) con-
tradict the BAT-PS principles, leav-
ing mine waste professionals faced
with competing or conflicting design
criteria. The IEEIRP, along with geo-
technical engineers and geochemists
recognize this “Catch 22” situation.
The province of British Columbia and
its technical community of experts in
ARD/ML are world renowned. British
Columbia aquatic standards are often
used as objectives for water manage-
ment by mining companies worldwide.
The tradition of expertise in British
Columbia dates back several decades
as can be seen in the publication of
the Draft Acid Rock Drainage Techni-
cal Guide that was prepared for the
British Columbia Acid Mine Drain-
age Task Force in 1989. Furthermore,
the Annual BC-MEND ML/ARD
Workshop has enjoyed more than 20
years of success, regularly attracting
experts from around the world. We
do not want to overstate the expertise
and capacity in British Columbia to
address and manage the chemical
stability of reactive mine rock, but it is
certainly world class. Furthermore, we
believe the expertise available within
the technical community of British
Columbia will be successful in the
integration of the BAT-PS principles
with the well-established principles for
the prevention of acid rock drainage
and metal leaching (BAT-CS).
New opportunities
The development of new technolo-
gies and improved methods for the
management of reactive mine tailings
lie ahead of us. A key component
for the design of dry tailings stacks
will be the degree of water saturation
within the tailings profile established
over the long term. For example,
tailings profiles with water saturation
levels less than 85% are considered
resistant to liquefaction. Conversely,
tailings profiles with water saturation
levels greater than 85% are considered
resistant to oxygen diffusion and sub-
sequent ARD. Successful implementa-
tion of the BAT principles for both PS
and CS will now rely on a comprehen-
sive understanding of unsaturated soil
mechanics. The Soil-water characteris-
tic curve (SWCC) of the tailings con-
trols water saturation as a function of
matrix suction (i.e. negative porewater
pressures) along with the unsaturated
hydraulic conductivity of the tailings.
The design of cover systems to control
infiltration rates and oxygen fluxes for
the prevention and control of ARD
in reactive mine tailings is based on
coupling the hydraulic properties of
the SWCC with microclimatic condi-
tions associated with precipitation and
evapotranspiration. This technology
is considered well developed and can
be extended to predict the unsaturated
hydraulic performance of dry tailings
stacks. For example, the design of the
cover system used for final closure of
the stack will need to provide infiltra-
tive fluxes that optimize unsaturated
flow conditions in the tailings.
De-sulphurization and the production
of clean tailings will also provide new
opportunities. De-sulphurized tailings
were applied as clean cover for closure
of the tailings beach at Detour Mine in
1999 (then owned by Placer Dome).
A recent study reported by Cash et al.
(2012) demonstrated that that cover
has performed very well more than
10 years since closure. A residual
sulphur content of less than 0.5% was
allowed to remain in the tailings so
