The failure and resulting flow slide at the tailings dam B-I at Vale S.A.’s Córrego do Feijão Iron Ore Mine in January 2019 was the result of flow liquefaction within the tailings in the dam, an Expert Panel report has concluded, with six issues highlighted as contributing factors to the failure.
The panel, made up of Peter K. Robertson, Ph.D. (Chair), Lucas de Melo, Ph.D., David J. Williams, Ph.D. and G. Ward Wilson, Ph.D, found that the following issues created the conditions for instability in the dam, resulting in its failure:
- A design that resulted in a steep upstream constructed slope;
- Water management within the tailings impoundment that at times allowed ponded water to get close to the crest of the dam, resulting in the deposition of weak tailings near the crest;
- A setback in the design that pushed the upper portions of the slope over weaker fine tailings;
- A lack of significant internal drainage that resulted in a persistently high water level in the dam, particularly in the toe region;
- High iron content, resulting in heavy tailings with bonding between particles. This bonding created stiff tailings that were potentially very brittle if triggered to become undrained; and
- High and intense regional wet season rainfall that can result in significant loss of suction, producing a small loss of strength in the unsaturated materials above the water level.
The Panel said the history described above created a dam that was composed of mostly loose, saturated, heavy, and brittle tailings that had high shear stresses within the downstream slope, resulting in a marginally stable dam. Laboratory testing showed that the amount of strain required to trigger strength loss could be very small, especially in the weaker tailings. These were the main components that made flow liquefaction possible.
The sudden strength loss and resulting failure of the marginally stable dam were due to a critical combination of ongoing internal strains due to creep, and a strength reduction due to loss of suction in the unsaturated zone caused by the intense rainfall towards the end of 2018, said the report. This followed a number of years of increasing rainfall after tailings deposition ceased in July 2016.
Look out for our February 2020 print edition where we will cover the report in more detail.
