Publication
Warnings Unheeded: The Tale of the 1994 Merriespruit Tailings Dam Disaster
With Riley Manwaring and Irfan Alvi. ASDSO Decade Dam Failure series, September 25, 2024.
- Author
- Meghan Walter
- Year
- 2024
- Topics
- Failure case historiesDam safetyEngineering ethics
- Source
- See the link at the end
A talk in ASDSO's Decade Dam Failure series on the 1994 Merriespruit disaster, given thirty years after the event. The Harmony Gold Mine No. 4 tailings dam failed on the evening of February 22, 1994, and killed seventeen people in the town below. The warnings had been there for years. The talk walks the timeline from construction to breach, shows what the satellite imagery revealed in court, and draws out the human factors behind an ordinary storm that became a disaster.
| Item | Detail |
|---|---|
| Speakers | Meghan Walter, P.E.; Riley Manwaring, EIT; Irfan Alvi, P.E. |
| Series | ASDSO Decade Dam Failure series |
| Presented | September 25, 2024 |
| Subject | Harmony Gold Mine No. 4 tailings dam, Merriespruit, Virginia, South Africa |
| Format | Slide presentation, 28 slides |
Despite knowledge gained from, on average, two tailings dam failures every year in South Africa, the Merriespruit disaster still occurred.
Wagener (1997), quoted in the talk.
The dam and the town
Mine tailings are the waste left after minerals are extracted and processed: a mixture of sand, silt, metals, chemicals, and water, solid, liquid, or a slurry of fine particles, often toxic and potentially radioactive. They are usually discharged as a slurry to a storage facility such as a tailings dam. In South Africa they are also called slimes.
The Harmony Gold Mine No. 4 dam was raised by the upstream method to a height of 102 feet, standing about 1,000 feet from the houses of Merriespruit. It had two main compartments, 4A of 72 hectares to the north and 4B of 60 hectares to the south, separated by a dividing wall, with a penstock to decant water.
Fifteen years of warnings
| When | What happened |
|---|---|
| 1979 | Construction begins. The contractor's head office supervisor reports difficult dam-building conditions because the slurry arrives at low relative density |
| 1979 to 1994 | The No. 1 delivery pipe gives trouble the whole life of the dam: high friction head, frequent choking, bursts, and organic growth inside |
| 1985 | Consulting engineers assess the dam and conclude it is in good condition. No seepage or stability issues noted |
| 1986 | The mine is exempted for five years from the Water Act, which would have classified the dam Category III on size and hazard and required maximum safety precautions |
| 1991 | Sloughing begins at the north wall; a rock buttress proves ineffective. The Minerals Act requires capacity for the 100-year, 24-hour storm with 0.5 m of freeboard. Merriespruit is exempted |
| 1992 | The sloughing on the northern wall of 4A is repaired with a hydraulically placed buttress at the toe, 90 m long and 9 m wide |
| 1993 | A change in management structure cuts personnel, including managers at the dam. Seepage is noted again on the northern wall and repaired. Operator and manager decide to stop deposition in 4A |
| 1993 | Despite the stop-work order, deposition into 4A continues. The exemption is extended two more years, to 1995 |
| Unknown | The wall dividing 4A and 4B breaches. Continued deposition into 4B moves slurry into 4A, enlarging the pool, pushing it against the north wall and away from the penstock, and reducing freeboard |
| Oct 1993 and Feb 1994 | Satellite images show the pool ponded against the north wall |
| Feb 22, 1994 | A storm drops 1.26 inches, about a five-year event. Witnesses see water overtopping the crest of the northern wall at about 7 p.m. At 9 p.m. the wall fails |
The failure
The breach was 450 feet wide and released about 500 acre-feet of tailings slurry downstream into the town. Seventeen people were killed and more than 200 injured. Eighty properties were destroyed. Damages came to $17 million in 1994 dollars.
The first responders waded through the chest-high depth of mud tailings, which contained cyanide. The team could feel the chemicals in the mud burning their skin as they continued doing their part in the search party. They were ordered not to touch parts of their faces such as the mouth and eyes to avoid any further burns or contamination.
Ryan Ellis, a warrant officer stationed in Welkom at the time, quoted in the talk.
Inquest and inquiry
For the first time in South African history, satellite imagery was used in the forensic investigation and entered as scientific evidence in court. It showed that the pool in compartment 4A was not around the penstock outlet but ponded against the northern wall, and had been for a considerable time. With the eyewitness reports, the imagery established overtopping as a critical contributing factor: overtopping eroded the slope, which is believed to have caused progressive slope failures, which triggered a static liquefaction flow failure of the metastable tailings.
Six employees of Harmony Gold Mine and of the company that managed the dam were found criminally liable. Three mine employees were convicted of culpable homicide, though the homicide charges were later dropped. The fines against the two companies totaled less than $15,000 in 1994 dollars.
Human factors
- Siting the dam immediately upstream of a town.
- An industry assumption that gold mine tailings were not liquefiable.
- Economic pressure and a reduction in staff.
- Lack of experience and expertise.
- Violation of the stop-work order on deposition in compartment 4A.
- Warning signs unheeded.
Lessons
- The failure of the Merriespruit No. 4 tailings dam can ultimately be attributed to human factors.
- It is essential to recognize and respond to warning signs in a way that can be clearly communicated to all operational and management personnel.
- Public safety should be the first priority.
A fuller account of this failure, with the engineering detail and the sources, is in the case study: Harmony Gold Mine No. 4 Tailings Dam (South Africa, 1994).