Tailings safety

The January 2019 Brumadinho tailings dam failure in Brazil that 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.

AUTHOR: Kym L Morton, Water strategist and consulting mining hydrologist, KLM Consulting Services

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.

This article first appeared in Mining Review Africa Issue 12, 2019
Read the full digimag here or subscribe to receive a print copy here

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.  

The figure 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 scenaria:

  1. Compression of water in the pores of the material
  2. Compression of individual particles (sediments etc.)
  3. Re-arrangement of particles, usually to a more compact configuration.

Once pore pressures, measured by piezometers,  are plotted as lines of equal head (h), known as  equipotentials, the pressure gradient of the slope will be like figure 3.

Pore pressures can be measured using pressure transducers, typically  the sealed vibrating -wire point type piezometers, grouted into coreholes drilled into critical sections of the slopes of the tailings dam retaining walls.

The water balance of a tailings dam  is affected by the rainfall input to its pond and any surface runoff entering the dam foundations from  upstream. 

The groundwater levels around, below and upstream impact on the water balance of the tailings dam and  also need to be monitored and managed to increase safety. 

Each dam is unique and therefore requires a bespoke monitoring network, designed by a qualified hydrogeologist,  based on a full understanding of the water inputs , both surface and underground. 

Figure 4 shows the layout of an accurate monitoring network around an individual tailings dam in plan.

An accurately instrumented tailings dam will include monitoring of:

  1. Rainfall;
  2. Pond levels;
  3. Upstream surface and groundwater flows and pressures;
  4. Downstream surface and groundwater flows and pressures; and
  5. Pore pressure monitoring within all the tailings dam’s slopes.

This data is converted to information and then resultant plots of pressures can be used within dashboards to review the actual pressures compared to the required pressures. 

Trends that show increasing pressure can be used to implement pressure reduction actions such as drainage or pumping.  Dashboards can shared to inform decision makers of increasing pressures which can then be reduced by increasing drainage of the unstable slope.  

The information can be transmitted by satellite to local and remote decision makers.

Conclusion

Accurate and well distributed pore pressure monitoring of a tailings dam is essential to the understanding of the main factor which controls tailings dam stability. 

Reducing pore pressures by drainage of pumping will increase safety and if implemented early will prevent failure.

This strategy is far  more accurate and successful than the simple monitoring of slope movement. When a slope moves it is often too late to reverse the imminent failure,  intervention by monitoring and then active management of  water pressure can prevent failure  before a catastrophic event.  

Dr Kym Morton is an international expert in mining hydrology. Her qualifications include an MBA and PhD from Imperial College, London, an MSc Hydrogeology from University College and a BSc honours from Kings College. All three universities are in the world’s top 20. Her specialisations are mine dewatering design, integrated water management for slope stability, open pit to underground transition and tailings dam risk mitigation.

Based in South Africa and the UK she consults worldwide on mine water risk reduction and business improvement. Over 39 years she has worked on over 300 mines, including 18 years for De Beers Group, covering all continents. Block caving dewatering design is a specific skill.    

References

Morton et al 2008 Importance of pore pressure monitoring in high walls

Morton KL 2019 The use of pore pressure monitoring for accurate tailings dam monitoring