Geotechnical News • June 2018
37
WASTE GEOTECHNICS
Effective tailings disposal and storage design using
instrumented column testing
David Williams
Introduction
The average tailings dam failure rate
over the last 100 years is over 1%
or over 2 per year, which is more
than two orders of magnitude higher
than that for water retention dams
of 0.01%. Particular focus is given
to tailings dam failures that occur
in developed countries (e.g., Mount
Polley, Canada in 2014, and Cadia,
Australia in 2018), or those that
involve global mining companies
(e.g., Samarco, jointly owned by BHP
Billiton and Vale, Brazil in 2015).
These recent, high profile tailings dam
failures are threatening the mining
industry’s financial and social licence
to operate.
While tailings dams themselves are
subject to detailed design, construction
quality assurance and quality control,
and their operation is managed and
monitored, less attention is paid to
designing tailings disposal and storage
to best accommodate the often chang-
ing production rate and nature of the
tailings. The tailings production rate
and geometry of the tailings storage
facility (TSF) dictate the rate of rise
of the tailings, the possible cycling of
deposition, the management of tailings
supernatant water, the final dry density
and shear strength achieved, and the
ultimate closure of the facility. The
climatic setting and topography of the
site also impact significantly on the
TSF.
Conventional tailings disposal
Impact of accounting approach
Tailings disposal and management has
been based on minimising short-term
capital and operating costs, with future
operating and rehabilitation costs
reduced by a high discount factor
(typically 10%, several times higher
than the inflation rate, and hence dif-
ficult to justify) in a net present value
accounting approach. This has led to
the widespread adoption of surface
TSFs to store tailings slurry, which
are delivered by robust and relatively
inexpensive centrifugal pumps and
pipelines to small storages raised
incrementally. In turn, this leads to
soft and wet tailings deposits, TSFs
that primarily store entrained and
supernatant water, and rehabilitation
difficulties.
Impact of climate and topography
In a dry climate or during extended
dry seasons, advantage can be taken
of the desiccation of the tailings by
solar and wind-induced evaporation
of moisture from the tailings surface.
Desiccation results in an increase in
the dry density of the tailings, and
hence a reduced stored volume, and
to an increase in their shear strength.
However, desiccation takes place to
only limited depth and decreases expo-
nentially with depth below the surface.
Hence, to maximise the effectiveness
of desiccation in dewatering, densify-
ing, and strengthening the tailings, the
tailings should be deposited in layers
of limited thickness, supernatant
water should be efficiently removed
to expose the tailings surface, and the
tailings deposition cycled to allow
sufficient time for desiccation. In a
wet climate, exposure and desiccation
of the tailings surface may not be pos-
sible, and it is more important to maxi-
mise dewatering in the plant. Sulfidic
tailings may need to be maintained
underwater to limit their oxidation.
Desiccation in a dry climate is
enhanced by a surface TSF with a
large footprint, which is likely in a
topographic setting of low relief, such
as in Australia. In a wet climate, a
large tailings footprint will result in
large rainfall runoff being captured.
In a flat topography, a dam will be
required around the entire perimeter
of the TSF, and the cost of raising
the dam will become prohibitive at
relatively low heights, requiring that
a new or extended storage be con-
structed. At sites with high topo-
graphic relief, such as in the Andes, a
high dam of limited length will be suf-
ficient and a small TSF footprint will
result. This increases the rate of rise of
the tailings, and makes exposure and
desiccation of the tailings difficult,
limiting their dewatering, densification
and strengthening. In a wet climate,
rainfall incident on a small TSF foot-
print will be limited, although clean
runoff from the reporting catchment
may be large and would need to be
diverted around the TSF.
Tailings disposal and storage
design
Effective tailings disposal and storage
design should take account of the cli-
matic and topographic settings of the
site, the tailings production rate and
