Final report : Study into the causes of the Jagersfontein Fine Tailings Storage Dam failure on 11 September 2022

Abstract

This report presents an investigation into the geotechnical causes of the Jagersfontein Fine Tailings Storage Dam failure which occurred at approximately 06h00 on the morning of Sunday 11 September 2022. The dam is registered with the Department of Water and Sanitation as the “Jagersfontein Fine Tailings Storage Facility Dam”, a Category III dam with a safety risk. It is owned by Jagersfontein Developments (Pty) Ltd who focuses on reprocessing legacy tailings dumps from previous mining operations. The De Beers mining company operated the Jagersfontein mine until it ceased operations in 1971. The mine and associated tailings dam remained dormant until 2010 when reprocessing of legacy tailings dumps, without any further underground activities, commenced. Jagersfontein Developments (Pty) Ltd developed two new tailings deposition compartments adjacent to an old tailings dam built by the De Beers Group during their tenure of the Jagersfontein Mine. Construction of the first compartment began in 2010 immediately to the west of the old dam. Construction of the second compartment began in 2015, located to the south and east of the old dam. The new eastern compartment merged with the old tailings dam between 2017 and 2018. At that point the tailings facility consisted of two main areas, a western and eastern compartment. These two compartments merged between 2020 and 2021 to create a facility that consisted of a single deposition area. It was the southern wall of the eastern compartment, of which construction began in 2015, that failed on 11 September 2022. The Investigation Panel comprised geotechnical engineers from the Universities of Pretoria and the Witwatersrand appointed by the Directorate: Dam Safety Regulation of the Department of Water and Sanitation. A second and parallel study, documented in a separate report, involved a dam breach study by hydraulic engineers of the University of Pretoria. The geotechnical study included three site visits: The first involved a site walkover and collection of a limited number of samples. During the second site visit a piezocone (CPTu) investigation was carried out as well as sample collection. The third visit comprised a seismic CPTu investigation, Mostap sampling and further collection of samples for index testing. Due to legal reasons the Investigation Panel was not permitted to discuss the tailings dam failure with personnel of Jagersfontein Developments (Pty) Ltd. The investigation included a desk study of documentation provided to the Investigation Panel via the Department of Water and Sanitation, as well as a study of aerial and satellite images, some freely available and some purchased. Available data also included several surveys of the dam carried out from time to time. The design report (Robinson, 2015) for the eastern compartment of which the southern wall failed indicated that the compartment was constructed on land already impacted by legacy fine tailings deposits. This was supported by aerial and satellite images. The design report indicated that the fine tailings had a low shear strength, recommending a 20° effective friction angle and a very low permeability of 1 x 10-12 m/s. In brief, the design for the eastern compartment comprised outer walls constructed from coarse tailings (grits), to contain the fine tailings slurry (slimes) deposited hydraulically in the tailings dam. The outer walls were raised via upstream construction. The satellite images showed signs of instability along the eastern third of the southern wall of the tailings dam starting in Feb 2019. This eastern third of the tailings dam wall was adjacent to a water storage dam known as Dam 10. The first signs of instability were seen just after Dam 10 filled up rapidly from rainfall after being empty during the dry first half of the 2018 - 2019 summer. Based on available information, the slope showing signs of instability was constructed at what is believed to be the natural angle of repose of the coarse tailings (grits) (1:1.5 or approximately 33°). This is steeper than the recommended design angle available to the Investigation Panel (1:2 or approximately 27°). It appears that this section of the wall is underlain at least partially by an old tailings dump, referred to as Dump 10, which the Investigation Panel believes represents a foundation of inadequate shear strength. Satellite imagery showed a large amount of southward movement taking place along the toe of the affected section of the dam wall from February 2019 until the failure in September 2022. Changes in the alignment of the access road directly downstream of the southern wall of the dam show how an initially straight road became curved around the affected part of the wall, eventually deviating 87 m from its original alignment. Satellite images showed substantial increased construction activity along the affected section of the wall, which is interpreted to have been efforts to maintain stability while the dam was kept in use. The affected section of the wall had a footprint width of approximately twice the footprint width of any other part of the dam wall at the time of the failure. Stability analyses show that the section of the wall that first showed signs of distress was unstable when assuming drained strength parameters. The first slope instability was likely triggered by the rapid filling of Dam 10. Thereafter, near continuous movement seems to have occurred, which the Investigation Panel believe would have resulted in undrained conditions along an interface between the dam wall and its foundation. Such undrained conditions would have reduced the strength to levels insufficient to ensure stability. Satellite images indicate a reduction in the rate of movement at the toe in the months preceding the failure, but some acceleration in the rate of inward widening of the embankment crest along parts of the southern wall during this time. As the dam did not have a decant facility, no beach could form and water and/or fine tailings slurry often extended right up against the inner slopes of the crest. Upstream raising of the dam therefore had to be constructed on poorly consolidated tailings. An eyewitness account indicates that, on the morning of the failure, part of the crest failed to the inside. This was followed by slumping of the crest, overtopping and the uncontrolled release of approximately 5.1 million m3 of tailings slurry and water. The Investigation Panel believes that the failure at the inside of the crest provided a trigger to remobilise undrained conditions at the base of the dam wall, reducing the shear strength along the zones where movement had been occurring for a long time, resulting in the failure of the Jagersfontein Fine Tailings Storage Dam.