Merriespruit tailings dam disaster

The Merriespruit tailings dam disaster occurred on the night of 22 February 1994 when a tailings dam failed and flooded the suburb of Merriespruit, Virginia, Free State, South Africa. Seventeen people were killed.

Late in the afternoon on the day of the failure, a thunderstorm occurred and about 50 mm of rain fell in 30 minutes. When the dam failed, 600 000 m³ of liquid flowed 4 km. The nearest houses were located 300 m downslope of the dam; when the wave of water and tailings reached them, it was 2.5 m high.

Seventeen people were killed and there was widespread devastation and environmental damage. Eighty houses were destroyed.

When gold-bearing rock is processed and the gold is extracted, the remaining material is moved to tailings dams as waste, as was the case at the Merriespruit No. 4 facility.

As is typical with gold tailings in South Africa, the Merriespruit tailing dam was constructed using the "upstream semi-dry paddock" method, where a "daywall" perimeter is constructed and allowed to settle and dry out (typically carried out during the day and supervised) before slurry is placed in the "nightpan" (typically carried out at night without supervision). A is ideally located centrally in the dam and drains water (including stormwater).

Number 4A in the Harmony Gold Mine tailings complex failed. The dam was designed in the early seventies by the mine's metallurgical manager and a representative from the tailings dam contractor. The town of Merriespruit, a 250-house suburb of the Goldfields town of Virginia, was already established when the No. 4 dam was started in 1978. The northern wall of the dam was only 320 m from the nearest houses in Merriespruit.

In the early years the slurry had a low relative density that led to difficult construction conditions with seepage and sloughing on the northern wall. A drained tailings buttress was constructed against the face of the northern wall. A single ringmain around the dam was not used, contrary to current practice. A return water dam that could accept water from the dam itself was not provided.

In March 1993 an inspection noticed seepage on the north wall and it was agreed to stop deposition into compartment 4A. According to the contractor the freeboard at this time was an acceptable 1.0+ m. The division of compartments 4A and 4B was breached some time before the disaster, resulting in drainage from 4B to 4A. The extra drainage led to a freeboard of 300 mm. Despite the termination of daywall construction, deposition of excess plant water containing tailings continued, with the water decanted by the penstock and the remaining tailings using up the remaining freeboard.

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