WASTE GEOTECHNICS
Meeting Environmental Challenges for
Mine Waste Management
Michel Aubertin
Bruno Bussière
Introduction
Abstract
The mining industry contributes significantly to the Canadian and US economy.
Mining operations, however, produce various types of wastes that must be managed properly to protect the environment. In that regard, attention must be paid
to the geotechnical stability of surface works constructed for storage and confinement of solid and liquid wastes. Other areas of interest relate to the prevention and control of acid mine drainage (AMD) that results from the oxidation of
sulphidic minerals. The new industrial NSERC-Polytechnique-UQAT Chair, recently created in collaboration with mining companies and consulting firms,
will study various issues and solutions related to the environment and mine
wastes management. Emphasis will be placed on the development of tools and
techniques for better management of the various solid and liquid wastes produced, including waste rock from mine operations and tailings from the milling
process. To respond to specific needs expressed by the mining industry, the research program has been constructed around two broad projects: 1) the integrated management of wastes during mining activities, and 2) the reclamation
of potentially acid generating sites after closure. The program has been integrated with the different partners involved with the Chair and will aim at solving some of the remaining environmental challenges still being faced by the
mining industry.
Photo 1. Example of environmental impacts of acid mine drainage generated by a
tailings stack. Damages shown here are in part due to the acid generating tailings
that have escaped from the pond following a breach in the dyke.
The mining industry is an important
economic asset for many regions across
North America, particularly with respect to exports, employment and technology development. Mining nevertheless generates waste. Effective and
efficient waste management programs
are required to ensure long-term environmental stability. In that regard, considerable research efforts have been
made over the years, particularly in the
area of acid leachate. This leachate,
commonly known as acid mine drainage
(AMD), results from naturally occurring processes that occur when sulphides are exposed to water and air.
AMD can affect the ecosystems close to
disposal sites if not properly managed
(see Photo 1). Furthermore, recent
events have highlighted the importance
of dealing properly with the analysis,
design and construction of retaining and
confining structures for controlling the
disposal of solid and liquid wastes, in
both the short-term as well as the longterm. While exceptional, the possible
failure of dams and related works as a
result of various conditions can significantly affect the environment. Significant problems have occurred in recent
years in Spain, Italy, Guyana, Romania,
South Africa and the Philippines, and
also closer to us in Canada and in the
USA (but fortunately on a smaller
scale).
These environmental considerations
are important to the neighbouring communities, the mining companies and the
financial partners since the responsibilities and costs involved can be enormous.
Over the last fifteen years or so, the
mining industry, eager to continuously
improve its environmental performGeotechnical News,
September 2001
21
WASTE GEOTECHNICS
ance, has invested significant capital to
develop tools and techniques aimed at
better managing mine wastes. The wellknown Canadian MEND (Mine Environment Neutral Drainage) program is
a good example of the industry’s involvement in research, which has lead
to major progress in many areas
(MEND 2001). Despite the progress
made, it is generally recognized that
there are still significant gaps that need
to be studied and better understood, in
order to continue to improve the environmental performance of the mining
industry.
The industrial NSERC-Polytechnique-UQAT Chair on Environment and
Mine Waste Management (in French:
Environnement et gestion des rejets
miniers, or “Enviro-GeReMi”) has recently been created with Agnico-Eagle,
Aur Resources, Barrick Gold, DessauSoprin, Golder, Inmet, McWatters, Noranda, and SNC-Lavalin, in
collaboration with the Québec Department of Natural Resources. The Chair
will conduct research in these and related areas. In the following paragraphs,
a brief overview of the problems at hand
is presented (more details are given in
Aubertin et al. 2001), together with the
specific research program that will be
conducted by the Chair over the next
five (to ten) years.
Environmental Challenges for
the Mining Industry
A mine can be operated in various ways
including; open cast, underground
methods, surface stripping or hydraulic
leaching. The mining operation also includes milling. In turn, the mining and
milling methods influence the nature
and amount of wastes that are generated.
These wastes include the soils excavated
to reach the rock mass (where the minerals are usually located), the rock
wastes that have no economical value
that must be withdrawn to reach the ore,
and the mill tailings (a mixture of fine
particles and water) produced at the concentration plant. To these, one can add
mine waters pumped to the surface,
sediments produced by clarification of
waters from the mine or the mill, and the
sludge produced by the treatment of
contaminated water (especially acidic
22
Geotechnical News, September 2001
waters). These wastes must be managed
properly during the life of the mine and
after, at the reclamation stage (eg.
Aubertin et al. 2001).
Waste management during mine
operation
Waste rock produced by the rock mass
excavation required to reach the ore is
primarily composed of coarse-grained
materials that are, in most cases, hauled
to the surface and disposed of in stockpiles. The large size of waste piles raises
the risk of them becoming geotechnically unstable, particularly along slopes
or within the foundations. This risk
however is not easily analysed because
of the strongly heterogeneous nature of
the material stored in waste dumps. The
deposition methods used favour material segregation, which in turn influences the hydrogeological and geotechnical behavior of the waste piles.
Analysis is particularly difficult when a
varying degree of saturation exists. This
is often the case for waste rock where
saturation is influenced by material
grain size, porosity, hydraulic properties
and by the ever changing water budget
conditions (precipitation, evaporation,
runoff, seepage, water retention, etc).
This aspect of waste rock behavior has
not been well characterised, and is not
well understood. Furthermore, the evolution of material properties (mechanical, hydrogeological, geochemical)
over time is also an element that requires
follow-up as the dumps created by mining operations have to remain safe for
very long periods. More work is also
needed on the methods used to evaluate
the safety of waste rock piles. The industry as a whole would benefit from research into the different instability
modes that can occur. We must take into
account the material variability, the
treatment of recurrent events (extreme
precipitation and earthquakes for instance), and the durability and degradation of the exposed particles. Alternatively, mine planners could devise
different deposition methods that would
minimize the generation of AMD (for
example, by mixing or layering coarse
waste rocks with fine grained tailings),
so that closure and reclamation of the
dump would be facilitated at the end of
the mine. Because rock waste material
is usually very coarse, the dump can
exhibit high permeability (to water and
gas) and low capacity for water retention by capillarity. As a result, when the
reactive minerals are exposed to natural
conditions, ideal conditions can be created for generating acid mine drainage.
Once the critical reactions have been
initiated, it has proven to be a difficult
process to stop. The amount of AMD
generated, depending on the dump conditions, can be significant. The process
can last for centuries. Of course, it seems
possible to reduce the amount of precipitation that could reach the wastes, by
constructing a cover for instance, but for
large waste rock dumps this is not a
simple nor economic undertaking. More
work is equally needed in this area to
improve our understanding of largescale cover response over time, and also
to find ways to minimize the cost and
maximise efficiency for various climatic
conditions.
The surface disposal of milling
wastes, or tailings, requires further attention. Tailings, consist of fine particles (mostly in the silt fraction, smaller
than about 80 microns), that are transported hydraulically and deposited in
specially designed impoundments surrounded by retaining structures such as
dams, dykes and related components.
The transportation and disposal method
via a slurry condition induce a rather
loose state initially, with high water content and low mechanical strength. Over
time, tailings consolidate.The insitu
density and strength increase. Unfortunately, the time required to reach acceptable properties to ensure their stability
may exceed the operating period.
Hence, the retaining structures must be
designed in such a way that they can
withstand the stress and pore water pressures induced by the loose and consolidating materials. Designing the
retaining structures to accommodate
these conditions requires great care,
even under the most routine conditions.
The task is more complicated when extreme events such as floods and earthquakes are considered. The problem is
further amplified when the dams and
dykes are themselves built with noncompacted and heterogeneous tailings.
WASTE GEOTECHNICS
Slope stability, surface and internal
erosion, excessive settlements and
crest cracking, flood water overtopping, and loose tailings liquefaction
are some of the problems that can be
encountered. Work is still needed in
these areas to reduce the financial and
environmental risks associated with
such critical issues.
At the same time as we look at actual
operating conditions, one must also
look for ways of minimising the risks.
Improving material properties is a
promising avenue. For instance, using
densification techniques (such as those
developed for thickened tailings discharge and paste backfill) may well become a practical alternative that will
improve hydrogeotechnical properties
of tailings and reduce potential for instability. The use of tailings as underground backfill is another way of
disposing of these wastes. This can be
beneficial for ground control and for
surface protection (where the required
areas may be reduced), but optimisation
and integration of the techniques with
the mine and mill operations, taking into
account the hydrogeotechnical and geochemical characteristics of the materials, still require much more
investigation before one can establish a
systematic design scheme.
Water is another waste produced by
mining. The amount and composition of
water being used in a mine or a mill
depends on a number of factors, including the nature of the mineral ore and
surrounding rocks, the drilling process
and use of explosives (soluble ANFO
for instance), the type of backfill (with
or without cement, presence of sulphides, etc), and the process employed
for ore concentration. It is usually recognised that acid mine drainage is the
main source that affects water quality.
AMD (also called acid rock drainage, or
ARD) is encountered in base and precious metal operations, as well as with
coal and uranium mining. It ensues from
the natural reactions that occur when
sulphide minerals are exposed to atmospheric oxygen in the presence of water.
The oxidation of the reactive minerals
reduces the pH of the leachate, often to
values between 2 and 3, and this in turn
increases the solubility of metals in the
host rock. To ensure that the contamination does not adversely affect the environment, mining operations often use
chemical treatment to re-establish acceptable pH levels and precipitate metals before releasing the effluent (in part
or in its entirety) to nature. In this case,
alkaline agents (such as lime) are used
to neutralise the acidity and decrease
metal solubility. The metals and other
elements then precipitate and form a
sludge that must be disposed of in a safe
and economic manner. The treated effluent that meets discharge standards
and regulations can then be returned to
the neighbouring surface water system.
Although the chemical reaction and
treatment processes have been studied
extensively and improved in recent years,
the hydro-mechanical behavior of the
sludge (in terms of sedimentation, consolidation, permeability, and strength) requires more investigation. This would
allow the engineers who must design a
management system for sludge, to provide a more optimal design.
It is thus the goal of
many active groups
to optimize their design
and ensure their long-term
performance and integrity.
Waste rock and tailings, together
with other types of wastes produced by
mining operations, could also benefit
from recycling programs. For instance,
waste rock can be used as construction
material for dams, roads, and foundations, while naturally alkaline or desulfu rised tailing s have been used
successfully for creating hydrogeological barriers (especially covers). Various
other waste materials available at the
mine site could be valorised (ex. large
size tires). Ongoing projects at École
Polytechnique and Université du
Québec en Abitibi-Témisamingue will
continue through the joint Chair which
will conduct further investigation on
such possibilities.
Reclamation of mine sites producing AMD
One of the most important needs of the
industry relates to the development of
long term, ideally walk-away type of
mine closure and reclamation plans.
This is especially challenging when acid
generating materials are involved. In
this case, the reactive wastes disposed of
within surface facilities must be controlled for long periods (at least one hundred to two hundred years according to
some regulations), without the possibility of impacting the environment
through effluent release or solid migration. Apart from treating the acid water
(which in some places is not recognised
as an acceptable long term closure and
reclamation option), the measures taken
usually aim at restricting the availability
of water and/or oxygen to the sulphide
minerals. This is by no mean a simple
task. Water covers, oxygen consuming
(organic) covers, and covers with capillary barrier effects (CCBE) are some of
the available options that have been at the
core of national and international research
activities over the last decade or so (including at Polytechnique and UQAT). It
is thus the goal of many active groups to
optimize their design and ensure their
long-term performance and integrity.
These complex issues require more work
that must involve simultaneous experiments in the lab, the field and with numerical tools, as it will be necessary to make
predictions for long periods.
New techniques and new materials
must be evaluated. For instance, a project is underway to investigate using a
perched water table with a phreatic surface located at a depth such that capillary rise (corresponding to the air entry
value) maintains saturation in the reactive tailings. The advantage of this alternative to a water cover is that the use of
water retaining structures is minimized.
Another option under consideration for
insitu application is the use of bentonite,
either in geocomposites (GCL) or
mixed with sand or tailings, to create a
layered cover system that would be an
efficient oxygen barrier that could resist
the effect of freeze-thaw cycles encountered across North America.
The effect of climate is particularly
important as techniques developed for
Geotechnical News,
September 2001
23
WASTE GEOTECHNICS
Figure 1. Schematic representation of project 1; taken from the Proposal by Polytechnique and UQAT (2001) submitted to
NSERC (unpublished)
certain conditions (ex. relatively wet locations) may not be entirely appropriate
for different situations (ex. under arid or
semi-arid climate). Researchers at
Polytechnique and UQAT have worked
on the use of covers with capillary barrier effects (CCBE) to control the production of AMD. In eastern Canada,
where the water budget is largely positive, covers serve as an oxygen barrier
to reduce molecular diffusion. In other
parts of western Canada and the USA,
on the other hand, the technique has to
be adapted for semi-arid conditions
where the evaporation potential exceeds
the precipitation; the cover then becomes a “store-and-release” system. In
this case, however, one has to consider
that precipitation can occur as extreme
events that may temporarily exceed the
storage capacity of a cover. This is necessary to design a system that is able to
divert the excess water laterally (without direct vertical flow) using recently
developed saturated/unsaturated flow
models. Such type of cover is being
investigated by the Chair, in collaboration with industrial partners (in particular at the Barrick Goldstrike Mine
complex, in Nevada)
Evaluation of the various alternatives
for closure and reclamation of disposal
sites with reactive materials requires
that proper prediction techniques be
available. Static tests, kinetic tests, and
geochemical models have been developed and used on a regular basis, to
establish if and when a given material
would be generating acid. Some of these
techniques provide useful results but
very few can be used to establish the best
way of controlling the acid generation
in the long term.
24
Geotechnical News, September 2001
The Polytechnique-UQAT Chair
on Environment and Mine
Waste Management
The main objective of the new industrial
NSERC Chair is to ensure that ongoing
and new research activities will continue to enhance environmental performance for various types of wastes
generated by mining activities. The development and implementation of the
Polytechnique-UQAT Chair has been
designed to match industry prioritized
needs. It will also allow the synthesis
and consolidation of the research activities already underway. Considering the
significance of the content of the research program, the Chair will focus on
the development of new projects over
the next five years that will push the
frontiers of the innovative tools. The
development of a strong and stable research team will accelerate the process
of innovation. The main researchers involved, particularly the authors of this
article who will act as Chair and Junior
Chair respectively, will thus collaborate
closely with industry, including mining
companies, consulting firms and various R&D organizations in Québec and
in Canada. Such collaboration is considered a fundamental requirement at this
stage of the research. Thus, the Chair
will strongly promote exchanges with
colleagues from the two universities and
from other research groups in Canada
and elsewhere.
The Chair’s program has been centered around two main projects: 1) the
integrated management of liquid and
solid wastes during the mine operation,
and 2) the reclamation of acid generating sites after closure.
Project 1 relates more specifically to
the integration of waste management
methods with the different components
of mining activities such as geology, ore
and waste extraction, ore processing and
waste disposal facilities on either surface or underground. This project deals
specifically with the behavior of materials and design criteria for engineering
works associated with tailings ponds,
where the effects of anisotropy, heterogeneity and varying saturation on the
hydrogeotechnical response will be
taken into account. Research will also
be conducted on waste rock piles and
related structures, using innovative approaches for insitu characterization
such as the combination of geophysical
surveys, borehole and trenches sampling, natural and artificial tracer tests,
in order to define the internal structure
of piles. Furthermore, new constitutive
modeling and numerical modeling approaches will be developed to better represent water and gas flow within the
dumps. New waste management strategies and techniques will be investigated
and developed in order to improve on
environmental efficiency and reduce the
associated costs of mine wastes disposal. These include the development of
various techniques to densify the tailings in order to improve insitu properties, as well as the development of new
design criteria relating to piles geometry
and layering scenarios for waste rocks
disposal. A new method for characterizing the hydro-mechanical behavior
of treatment sludge will be studied. The
recycling of different types of solid
wastes such as oversize tires and alkaline crushed rocks will also be an area
of interest to the Chair. Figure 1 presents
a schematic diagram of Project 1.
WASTE GEOTECHNICS
Photo 2a. Laboratory instrumented column to evaluate the behavior of CCBE
(covers with capillary barrier effects) to limit oxygen flux.
Photo 2b. Intermediate scale instrumented test cells to evaluate the actual
response of CCBE in the field (constructed at the Manitou site, Quebec; Project
MEND 2.22.2c)
Project 2 of the Chair’s program involves different aspects related to the
closure and reclamation of acid generating waste disposal sites at the end of
mine life. In order to better address these
challenges, the long-term behavior of
layered (“dry”) covers placed over reactive mine residues to reduce AMD production will be evaluated in more detail.
More specifically, the research will focus on the effects of detrimental factors
either being mechanical, geochemical,
climatic, or hydraulic, on the long-term
performance of different types of cover
systems. Covers with capillary barriers
(CCBE) have been shown, through
laboratory column tests (Photo 2a) and
insitu test cells (Photo 2b) to be an effective means of controlling the production of acid mine drainage. Ensuing
from such types of experimental evaluations, large scale layered covers have
been designed and built at various locations in recent years, including the LTA
and Lorraine mine sites in Quebec.
While the use of water as an oxygen
barrier is currently considered among
the best available techniques used by the
industry to mitigate and control AMD,
studies show that there is still a need to
do more research work to obtain optimal
conditions with such control measure.
Further efforts will be focussed on water
covers placed over oxidized materials
and on the relatively new technology
known as elevated water table. While
AMD predictive methods have been
studied extensively in the past, the integration of the mineralogical and rock
lithology components of the materials
will be further studied to improve existing tools. Better evaluation of the net
neutralization potential and prediction
Figure 2. Schematic representation of project 2; taken from the Proposal by Polytechnique and UQAT (2001) submitted to
Natural Science and Engineering Research Council of Canada (NSERC) (unpublished)
Geotechnical News,
September 2001
25
WASTE GEOTECHNICS
of the acid production rate are expected.
The main goal is to provide better input
data for the selection and design of optimized rehabilitation techniques. An
overview of Project 2 is presented in
Figure 2.
The numerous research activities described above will involve the Chair
holder (located at Polytechnique) and
the Junior Chair holder (located at
UQAT) that will lead the research efforts conducted by both universities.
Also involved will be two new professors, at least four post-doctoral researchers, over 20 graduate students
(over 5 years), a technical support staff
for the laboratory and field work, and
the contribution of various collaborators
within and outside the two universities.
More information on the Chair can
be obtained by contacting the authors.
References
Aubertin, M. Bussière, B., Bernier, L.,
2001. Environnement et gestion des
rejets miniers. Presses Internationales de Polytechnique, CD-Rom (to
appear, Fall 2001).
MEND Secretariat CANMET, 2001.
MEND Manual, Report 5.4.2, Volumes 1 — Summary, Volume 2 —
Sampling and Analysis, Volume 3 —
Prediction, Volume 4 — Prevention
and Control, Volume 5 — Treatment,
Volume 6 — Monitoring.
Michel Aubertin, Professor, NSERC
Chair Enviro-GeReMi, École Polytechnique, Campus Université de Montréal,
Montréal, Québec, H3C 3A7
[email protected]
Bruno Bussière, Professor, NSERC
Chair Enviro-GeReMi, Université du
Québec en Abitibi-Témiscamingue,
Rouyn-Noranda, Québec, J9X 5E4
[email protected]
Editor
G. Ward Wilson
Chair of Mining and the Environment
Department of Mines and Mineral
Process Engineering
Room 517, 6350 Stores Road
University of British Columbia
Vancouver, B.C.
Tel: 604-822-6781
Fax: 604-822-5599
email: [email protected]
26
Geotechnical News, September 2001