Mining in focus
plants increasingly used water as a medium in
the extraction process.
Tailings were initially placed behind a small
starter wall; deposition would be conducted in
a way that allowed the coarser tailings material
to settle on the outside (close to the deposition
walls), and the finer material to be stored on
the inside basin. This reflected the principles
of sand and clay deposition and sedimentation
along a river, and no specific under-drainage or
internal drainage was provided.
Probably not appreciated in these early years
were the implications of these dams growing;
they would commonly double in size every 10
years as the process plant capacities increased.
One of the results was that many failures
occurred, even in the late 1800s, leading to
the formation of Fraser Alexander in the early
1900s as a tailings dam construction specialist.
Among the factors affecting TSF engineering
since about 1980, has been the ongoing focus
on finer grind of material in the metallurgical
process, to reduce particle size to increase
extraction. This continuing focus on finer grind
has had a very significant impact on tailings
seepage flow, compressibility, and strength
behaviour. In the past decade, there has been
a renewed effort to grind finer — to improve
the extraction of the metals and minerals.
It is most likely that these latest finer grind
focus areas are possibly going to change the
fundamental designs of traditional tailings dam
engineering in South Africa.
In addition, efforts to save water by ongoing
and further re-use of the water decanted from
the TSFs have meant that mines’ tailings
water circuits are becoming more saline. This
increased salinity, combined with finer grind,
reduces density and therefore also the shear
strength of tailings material.
Another important reference point in TSF
design in South Africa was the failure of the
Merriespruit tailings dam near Virginia in the
Free State in 1994 — caused by overtopping as
a consequence of heavy rains and insufficient
water management at the facility. Seventeen
people died when 1.2 million tonnes of tailings
flowed out the impoundment to the town of
Merriespruit two kilometres away. This led to
a renewed and specific focus on the vertical
and beach freeboard requirements of TSFs,
and how to ensure that suitable freeboard was
available at any TSF at any time. SRK made a
significant contribution to this aspect of TSF
design, construction, tailings deposition, and
Factors affecting TSF
engineering
Continuous efforts are taking place to find ways of making tailings storage facilities safer.
water management as part of ongoing tailings
dam operations.
Since the 1990s, higher levels of urbanisation
have meant that less land is available for
TSFs, so these tailings dams had to cover a
smaller footprint and, therefore, be higher than
previously. Cyclone deposition was employed as
a strategy to allow for these designs.
Safety concerns
Today, the growing concern with TSF
safety has triggered a range of demands in
the field of tailings dam engineering. Legal
responsibility has become more onerous and
more focused; mines must now stipulate who
the responsible engineer (person or entity) is
for the design, construction, and operating of
the dam. Contractual clarity is required on
the respective responsibilities and duties of
the owner, designer, construction companies,
operating team, QA/QC and monitoring team,
and any other stakeholders.
The tailings dam site’s foundation conditions
must be fully investigated from a geotechnical
perspective, to inform the design, construction,
operation, and monitoring of the facility. It
should be remembered that the initial portion
of the Mount Polley failure was a foundation
shear strength failure condition. Foundation
failure conditions are also relevant to safe
operations of waste rock dumps, of which
the well-known case history of Bukwa rock
dump in Zimbabwe is but one example. More
recently, a rock dump failure caused by a shear
strength failure of the clayey soils in the rock
dump foundations, took place at a mine close
to Machadodorp in Mpumalanga.
Investigation of tailings behaviour has also
taken centre stage, as the Fundão failure, for
example, was related to inadequate drained and
undrained shear strength of the tailings. Once
again, the design, construction, operation,
and monitoring of TSFs now must take this
behaviour into account. This requires a better
appreciation of soil mechanics applied to
tailings behaviour, and a framework for how
we consider tailings properties using advanced
geotechnical field and laboratory testing
and then applying these tailings behavioural
properties in appropriate 2D and 3D
numerical models.
While effective stress analyses were part
of the previous requirement for TSF design,
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