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Natech Accident Final
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Type
Natech Accident
Date
1994/02/22
Natural Hazard
Merriespruit rain, South Africa, 1994
Site
Harmony Gold Mine, South Africa
Status
Final

Units Involved

  1. Name
    No. 4 tailings dam
    Type
    Storage: Other
    Year of Construction
    1978
    Description
    The tailings dam was started in 1978. It is 154 ha rectangular dam and it is divided in three compartments: northern (4A), southern (4B), and small emergency (4C).

Event Sequences

  1. Name
    Tailings Dam Failure
    Unit
    1. No. 4 tailings dam
    Substances Involved
    1. Name
      Liquefied tailings
      Involved Quantity
      600000 m³
      Potential Quantity
      600000 m³
      Description
      Old-mine tailings were made largely by fine silica (quartz) and phyllosilicates but also they had significant amounts (up to 5%) of pyrite as well as residual cyanide.
    Initiating EventCritical EventMajor Event
    Component (Structural): Partial collapse
    The failure was triggered, during the early evening between 19:00 and 21:00, by the 50mm of rain that reduced in some places the freeboard to 150 mm. At approximately 21:00 there was a sudden collapse and a 150 m-wide breach formed in the dam wall. The berms overtopping has eroded slopes and buttress of the tailings dam, causing the buttress failure and the instability of the lower slope. When this has failed its resultant material is washed away, triggering a domino effect of local slope failures.
    Contributing Factors
    Equipment: Component failure:
    The division wall between compartments 4A and 4B was breached some time before the disaster. Hence, The liquid from compartment 4B has flowed into compartment 4A, increasing its level. Thus, the freeboard at the northern wall passed from the standard level of 1 m to a mere 300 mm.
    Human: Operator error: Non-existent
    The pool in compartment 4A was not located around the penstock outlet, but was lying against the northern wall. Thus the moisture has increased in the tailings along the northern wall for a considerable period of time.
    Organisational: Management organization: Inadequate
    In 1993 a change in the owner’s management structure occurred. People were promoted into positions of responsibility for the TSF without receiving appropriate training.
    In March 1993 the facility was inspected and seepage was noted on the north wall above the drainage exit point. Thus, a patch in the form of cladding was constructed. Moreover, after a discussion, the operator decided to stop all deposition on compartment 4A. Despite this, some deposition continued. In fact, during the night of the failure, slurry was pumped to the A4 compartment.
    Release: Liquid release to ground
    Approximately 600 000 m3 of liquid tailings escaped through the breach.
    Dispersion: Substance in / on ground
    The tide hit the adjacent township, causing extensive damage to property and the environment in its path of 4 km.

Weather Conditions

Precipitation
Rain
Description
The summer of 1993–1994 in the Free State was particularly wet. But, on the night of Tuesday 22 February 1994, the 50 mm of rain that have fallen in 30 minutes over the tailings dams were not unusual.

Emergency Response

Response Planning
Emergency response plan takes Natech events into consideration: Yes
Emergency response plan is sufficient in taking Natech events into consideration: No
Difficulties in Response to the Natural Hazard
Insufficient personnel and equipment to respond to both emergencies: Yes
Natech event prevents access of personnel to natural hazard affected area: No
Natech event prevents efficient operation of personnel in the natural hazard affected area: Yes
Difficulties in Response to the Natech Event
Insufficient personnel and equipment to respond to both emergencies: Yes
Damage to lifelines (e.g. water, power, communication, transportation): Yes
Response to the Natech Event
Senior management from the TSF operator arrived from Johannesburg just after midnight to inspect the breach. Helicopters were being used to illuminate the scene of the failure. Spontaneous rescues took place. Moreover, rescue teams have reported that houses were swept off their foundations.

Consequences

Off-site Fatalities
17
Human Health Impacts
There were 17 fatalities and extensive damage to property and the environment.

Remedial Activities

Remediation Activities
Spontaneous rescues took place.

Lessons Learned

Lessons Learned on Equipment
The cause of the failure was the moisture build up in the tailings along the northern wall and the liquid overtopping.
Lessons Learned on Organisational Aspects
Due to economic pressure at the time, personnel of the tailings dams was reduced. Moreover inexperienced staff was promoted to positions where they had the responsibility for the tailings dam. Tailing movements were not proper reported during the months leading to the failure. Therefore, the owner, the operator and six of their employees were subsequently found guilty of negligence and heavy fines were imposed.
Lessons Learned on Mitigation Measures
The Merriespruit disaster gave a very good reason to improve the safety of the tailings disposal. Hence, in 1995 the code had a fundamental change. Every tailings dam will be classified according to its hazard potential and certain mandatory procedures will be required, as the structural monitoring audited regularly by experienced, professional engineers.
ID: 56, Created: Vincenzo ARCIDIACONO, 2014-09-22 10:59:13 – Last Updated: Amos NECCI, 2021-11-09 13:36:52

Attachments

NoDescriptionFile Size
1.CAUSES AND CONSEQUENCES OF THE MERRIESPRUIT AND OTHER TAILINGS-DAM FAILURES 508.77KB
2.Failure of the Merriespruit TSF, South Africa, 1994. 722.68KB
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