OPERATIONS
REDUCE THE RISK
OF TAILINGS
The risk of tailings dam failures can be reduced significantly by using
accurate pore pressure monitoring, writes Dr Kym L. Morton.
T
he January 2019 Brumadinho tailings dam failure
in Brazil which killed over 250 people has created
worldwide focus on what can be done to reduce
tailings dam failures. South Africa has had its own tailings
dam failures; notably the Merriespruit failure in Virginia
in 1994 where 17 people were killed and over 300 houses
destroyed.
Modern techniques for tailings dam monitoring emphasise
the measurement of movement of the slopes using radar,
LIDAR and prisms. However, these techniques only measure
the reaction of slopes to instability factors and do not address
the causes or assist with reducing risk.
Once the slopes of a tailings dam have been established the
only factor that can be controlled to reduce the risk of failure
is the weight (measured as pressure) of water in the slopes of
the tailings dams. The presence of water within the tailings
dam slopes reduces the stability of the slopes of the tailings
dam by modifying the shear stresses on the potential
failure surfaces.
Pore pressure monitoring is an accurate method to measure
the weight of water in a slope or dam wall and enables
early intervention to delay or prevent failure. Open pit slope
stability techniques can be used to manage dam wall stability.
The success of accurate monitoring design depends on the
location, construction and management strategy for a tailings
dam. The best monitoring systems are designed for each
specific tailings dam.
Figure 1 shows the near construction of a tailings dam nearing
completion.
The figure below illustrates the pond in the centre of the
tailings dam and the pressure surface of the seepage face
within the tailings dam wall. There is also an additional
pressure surface under the dam created by the groundwater
below the dam.
Older tailings dams were often constructed by depositing
tailings into depressions or into riverbeds, they therefore can
have weak unstable foundations with no lining and hydraulic
connection to underlying aquifers. The base of the tailings
dam can often be recharged from buried streams fed from
upstream runoff.
Failure in a slope, known as volume deformation, will occur as
three possible scenarios:
• Compression of water in the pores of the material;
• Compression of individual particles (sediments etc.); or
• Re-arrangement of particles, usually to a more compact
configuration.
Figure 1: Final construction of a tailings dam (Adapted from GARD Guide).
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African Mining February 2020
www. africanmining.co.za