On 25 January, a tailings dam suffered a catastrophic failure and collapsed releasing 11.7 million cubic metres of toxic mud at the Córrego do Feijão iron ore mine, 9 kilometres (5.6 mi) east of Brumadinho, Minas Gerais, Brazil.
Earlier this year, Brasil has suffered from the reflection of the rupture of tailing dam in Brumadinho, in the state of Minas Gerais. Therefore, not only the geotechnical community but also several engineers have discussed the construction methods of the tailings dam. So, this article gathers information on tailing, types of tailing dams and types of dam failure.
Tailing definition and storage
In Mining, ore extraction occurs through a set of activities in the earth’s crust involving physical and chemical treatments to separate the material that present economic potential from the low earned value material. This low earned value material can be stored in dry form and in liquid form.
When it’s dry, the material is composed of rocks and soils that result from the stripping process. These dry Tailings are stored in the stockpiles on thalwegs and slopes near to the mining site, and they present geometry previously defined to avoid problems of stability and ruptures in the stockpile.
Another form of storage is in the liquid form, which the tailings are composed of the same materials of dry form but with the addition of water in proportions ranging from 30 to 50%. This is the most common method of storing tailings that result from iron ore since the extraction of the ore already occurs with water. Therefore, the transport is carried out by tailings pipelines and deposited by gravity in the tailings dam.
What are the types of tailing dams?
Different from what occurs in other types of dams, the tailing dam doesn’t present at the beginning its maximum volume, but gradually increases its capacity with the advance of the tailings deposit, this growth of the dam is called raise.
The first method of construction of the tailings dam occurs with the construction of the raise to downstream. Among the advantages of this construction, the method is the greater load resistance, low susceptibility to liquefaction and seismic shocks, and also the possibility of better technological control in the compaction of the layers, waterproofing and drainage. Although considered the safest method, the method of downstream is also the most expensive.
The second method is called the centre line, which consists of building the raise following a vertical axis. This method tries to balance the advantages and disadvantages between downstream and upstream method and is considered an intermediate version.
Finally, the third one method is called upstream and is the most commonly used by mining companies due to the lower cost and consists of raises that grow to upstream and being part of the raises supported on the mining tailings. This method is considered the least safe because it presents low technological control and because the tailings have liquefaction potential. Figure 1 illustrates these 3 methods of tailing dams.
Methods of tailing dams. Reference: D. Kossoff et al, 2014
But, are all these methods safe?
According to Rico, Benito, Salgueiro, Díez-Herrero e Pereira (2008) who carried out a study on the occurrence of failures in dams and related to the construction methods, which the upstream method is responsible for more than 70% of dam failures. The Brumadinho tailing dam contributes to the statistics presented by cited authors and demonstrates the insecurity of a constructive method which there is no adequate technological control or that get the risk of liquefaction.
So how can a tailing dam break?
The fault or failure in dams can be classified into 3 groups, which are caused by structural failures, hydraulic failures or infiltration.
Structural failures are those characterized by the rupture of the embankment material or foundation, leading to sliding upstream or downstream slope. As it occurs in other structures, the structural defects begin to show signs of problems with the appearance of cracks, which increases with the time and if the error is not identified, the rupture occurs.
Brumadinho’s dam break
Source: Rádio Itatiaia, Youtube.
The Hydraulic failures are those that occur due to the unforeseen water action on the dam structure, leading to an erosive action on the slopes. Among the several types of faults that are in this group, we can highlight the overtopping that is when the water exceeds the height of the dam, the erosive actions that can occur in the upstream reducing the embankment area, or even the erosive action caused by rainfall, movement of people and vehicles in the dam.
The Faults by leaks are those in which the correct drainage of the dam didn’t occur, thus the infiltration causes erosion of fine materials. The fine materials are carried and results in the leak, known as piping, and consequently in the dam failure. Furthermore, the excess of pore water pressure that resulting from the failure of the drainage can lead to soil liquefaction when occurs a seismic action, for example.
The concept of tailing liquefaction consists in the loss of shear strength of the granular materials when they are requested by static or dynamic loads, and thus, with this reduction of the effective tension the material behaves as dense liquid. This behaviour of the tailings may be due to seismic activities or even explosions that generate a dynamic loading in the tailings.
Coming in the Next article –>>
But after all, how can Global Road Technology help to prevent the dam’s failure? Or even how can we help to reduce the environmental impacts generated by failures as occurred in Brumadinho in Brazil? In the next article, we’ll talk about it!
Reference: Rico, M., Benito, G., Salgueiro, A. R., Díez-Herrero, A., & Pereira, H. G. (2008). “Reported tailings dam failures: a review of the European incidents in the worldwide context”. Journal of Hazardous Materials.
Kossoff, D; Dubbin b, W.E; Alfredsson, M; Edwards, S.J; Macklin, M.G; Hudson-Edwards, K.A (2014). “Mine tailings dams: Characteristics, failure, environmental impacts, and remediation”. Elsevier, Applied Geochemistry.
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