Why Jagersfontein tailings dam failed

Public satellite and aerial imagery shows that the construction history of the tailings dam that collapsed in Jagersfontein last year deviated from best engineering practice.

A new study by civil engineers Luis Torres-Cruz and Christopher O’Donovan at Wits University’s School of Civil and Environmental Engineering used public remotely sensed data to gain insights into the construction history of the tailings dam, the precursors of collapse and the immediate consequences. 

When the tailings dam failed on 11 September, it unleashed grey sludge that poured across the Free State town, killing several people, injuring dozens, destroying more than 160 houses and inundating rivers and grazing land. 

Deviations

The study, which was published in the journal Scientific Reports on Wednesday, said the data suggests a construction sequence that is “inconsistent with sound tailings management practices”.

The deviations include mine waste deposits from predominantly one side of the dam, erosion on the dam wall, and significant amounts of ponded water at times positioned against the retaining structure. Based on the analysed images, it proposes that water moving down through the dam, followed by external erosion, ultimately triggered instability.

“These tailings dams have been failing globally and at a frequency that is of concern, some might even say alarming,” Torres-Cruz, corresponding author and a senior lecturer, who specialises in geotechnical aspects of tailings dams, told the Mail & Guardian. 

There is an ongoing effort to eliminate failures in these types of structures, he said. “Making progress towards that goal requires us to look at every failure and try to understand what went wrong so that it does not happen again. For every failure, ideally there should be lessons learned so that those lessons can help us ensure that these failures do not occur again.”

Another benefit of the research, Torres-Cruz said, is creating greater awareness about the value of publicly available images from satellites that orbit the Earth and capture an image every five days. 

“There is this record that is being created … and the resolution of these images oftentimes is adequate, so that we can see what is happening in large structures like a tailings dam. We can see how they are being built and they can help us detect whether some aspects, not all of them, of good engineering practice are being followed.”

O’Donovan, the co-author of the study and a Wits Master’s candidate, said in a statement that because the data is publicly available, anyone can independently verify their observations.

Millions of cubic metres of mine waste released

To assess the consequences of the dam collapse, the authors captured two high-resolution satellite images that illustrate the damage inflicted on the surrounding residential areas. Based on a three-dimensional reconstruction of the failed dam, they estimate that between four and six million cubic metres of mine waste was released. 

Additional satellite imagery showed that the mudflow travelled about 7km over dry land before reaching the reservoir of the Wolwas Dam. 

“Tailings continued to flow along [about] 56km of streams and rivers causing some of them to overflow and take on a pale colour. Pre- and post-failure natural colour images and sediment index renderings strongly suggest that the tailings had reached the Kalkfontein Dam reservoir by 12 September.”

Global standard

The study noted the “tragic failure” in Jagersfontein occurred at a time of heightened global concern about the safety of tailings dams, especially after the Brumadinho Dam disaster in Brazil that killed 270 people in 2019. 

The Brumadinho collapse prompted the creation of the Global Industry Standard for Tailings Management, which strives to achieve zero harm to people and the environment. “The Jagersfontein failure was a reminder that there is still much work to do to achieve this objective,” the authors said.

Long history

Mining activities in Jagersfontein date back to the 1870s. The De Beers Group acquired the mine in 1931 and operated it until 1971, when underground ore extraction ceased. The group sold the mine in 2010 and subsequent owners reprocessed the legacy mine waste already on surface without further ore extraction. 

“At least two more ownership changes happened after 2010, including one that took place only months before the failure of the dam. At the time of the failure, and during most of the post-2010 period, the mine was owned by the subsidiary company, Jagersfontein Developments.”

Unlike conventional water retention dams, tailings dams are often constructed progressively over their entire operational life, which can span decades. This poses the “double challenge” of ensuring that high construction quality standards are maintained over a long period of time and that it is documented.

Potential to monitor tailings dams

The researchers used the Google Earth Pro and the EO Browser platforms to analyse some of the satellite images. These are free for non-commercial purposes and have intuitive interfaces that require minimal training, and can be used to monitor tailings dams. 

They said that considering the significant uncertainties that remain in methods commonly used to assess the stability of tailings dams, a robust monitoring strategy should focus on their construction and operation. “That is, it is relatively easy to determine whether dam construction is following best practices, yet difficult to determine whether an improperly constructed dam is about to fail.”