Failures of tailings dams – A growing trend
A simple analysis of the number of incidents affecting tailings dams (total failures, as at the Fundão tailings dam uphill of Bento Rodrigues in Brazil, or partial failures, as at the Merriespruit tailings dam in South Africa) shows us that there is a growing trend.
Since the systematic survey of failures of tailings dams started in 1961, there were 22 incidents up to 1971, with an average of 2.2 incidents per year. Then, from 2009 to 2019, 36 incidents occurred, with an average of 3.6 incidents per year.
This growing trend over recent years is witnessed by the fact that in 2009 there were 3 incidents (including the catastrophic failure of the Ajka tailings dam, upstream of the villages of Kolonár and Devecser in Hungary), whereas in 2019 there were 6 incidents (including the catastrophic failure of the Córrego de Feijão tailings dam, upstream of the village of Brumadinho in Brazil).
In order to understand this phenomenon properly, it must be remembered that tailings dams are impoundments for storing mine waste, unlike reservoirs for productive purposes, such as dams for the production of electric power or reservoirs for irrigation or artificial snow. Therefore, tailings dams do not generate any profit, but are only a mere cost for the company, which is not encouraged to invest in the safety of these structures.
In addition, tailings dams are structures in the making, which grow as mining activities proceed. Furthermore, they are built without respecting any limit. Indeed, mining societies often have to decide whether to set up a new reservoir or continue to feed the existing one, even when precise information on the amount of material still exploitable from the mine is lacking. Therefore, there are no reliable data on the economic returns in carrying on mining in the long term.
Consequently, the decision on whether to stop the accumulation of tailings in an existing reservoir and start a new one is often postponed to the detriment of the stability of old tailings dams, which should be closed and made safe.
This is what happened to the tailings dams upstream of the village of Stava: the embankment of the lower dam, which should have attained a maximum height of 9 m, was instead raised to a height of 25 m. Since the mine seemed depleted, an upper dam was built for storing the tailings resulting from the second working of the deposits contained in the lower dam. Nevertheless, when new fluorite seams were discovered, this new reservoir received the waste fragments of the freshly worked rocks. Additionally, it was also used to deposit the tailings resulting from processing the ore extracted from other mines. When failure occurred, the mine waste storage facility had reached a total height of about 60 m.
Given the above, monitoring activities on tailings dams by public authorities should be stricter than those on production structures, since the firms in charge already check the safety of the latter for their own interest.
The awareness of performing a hazardous activity from the environmental viewpoint should induce mining companies to implement concrete measures for risk mitigation and prevention. Indeed, these structures may be subject to catastrophic failure, considering their high water and mud content. Furthermore, their sizes increase considerably, which impairs long-term stability.
Among recent techniques available for managing mining waste, mechanical forced filtration seems to be very promising. This process allows the quick separation of the two phases of tailings. Thus, water is completely recovered and recycled and the construction of huge and hazardous disposal structures is no longer necessary.
Although the cost of this method is higher than traditional deposition in purposely-built reservoirs, the cost of treating tailings by means of forced filtration and their safe storage is immensely lower than the cost of damages and environmental restoration after failure.
In a strategic-type cost/benefit global analysis, it was demonstrated that in the long run the use of mechanical forced filtration methods makes much more economic sense than the use of tailings dams.
Another concrete measure could be drawing up insurance contracts for tailings dams, which in case of disaster would guarantee quick liquidation of damages. In addition, it would also involve insurance companies in the management of tailings dams, since their recurrent checks, which respond to the company’s own economic interest, would be more effective and thorough compared with those carried out by the tendering company itself or by public boards.
Unfortunately, the 120 incidents affecting tailings dams which have occurred from 1961 to date, demonstrate that mining societies have not yet acquired this awareness of their accountability. It would therefore be opportune to oblige mining companies to draw up insurance contracts for their tailings dams, since this would produce more efficient checks and safety for these earth structures.
Finally, it should be remembered that both the water necessary for working the ore and mining resources are public assets. Insurance obligation should therefore be included in the deed of assignment establishing mining concessions for the exploitation of water and mineral resources.