Sunday, 3 November 2019

Sinkhole causes closure of Snake Road in Benoni area of Ekurhuleni in Gauteng Province, South Africa.

Part of the Snake Road between Pioneer Drive and Golden Drive at Benoni in city of Ekurhuleni in Gauteng Province, South Africa, has been closed following the appearance of a large sinkhole. The sinkhole first appeared beside the road on Friday 1 November 2019, and continued to expand over the next two days, causing city officials to take the step of closing off the road.

Sinkhole on the Snake Road in Benoni, Gauteng. Jacob Mamabolo/Twitter.

Sinkholes are generally caused by water eroding soft limestone or unconsolidated deposits from beneath, causing a hole that works its way upwards and eventually opening spectacularly at the surface. Where there are unconsolidated deposits at the surface they can infill from the sides, apparently swallowing objects at the surface, including people, without trace.

Sinkholes are a problem in many parts of eastern South Africa, with Gauteng Province considered to be the most afflicted area. They are principally caused by karstification (erosion) of the Malmani Dolomite of the Chuniespoort Group (part of the Transvaal Supergroup). These dolomites are typically 60-100 m beneath the surface and covered by less well consolidated sediments. 

Distribution of instability events and dolomitic land across Gauteng in the different District and Metropolitan Municipalities. Constantinou & van Rooy (2018).

Dolomite is a form of high magnesium limestone which is usually fairly impermeable to water, but with, like other limestones, is highly susceptible to erosion by acidified water.  Rainfall can be acidified by carbon dioxide (a component of the atmosphere), which dissolves in the water to form carbonic acid, which presents no immediate health threat to humans but which attacks limestone vigorously. In areas where extensive coal burning occurs, such as the industrial zones of Gauteng, this can be made worse by emissions from the burning of coals with high sulphur content, which can lead to the formation of weak sulphuric acid, and even more efficient eroder of limestones.

Historic sinkhole and subsidence size distribution across Ekurhuleni. Constantinou & van Rooy (2018).

The Malmani Dolomites is actually made up of layers of acid soluble dolomite and insoluble chert (amorphous silica). Acidified water peculating through the ground can erode the dolomite layers away over time, with the chert layers providing support until to much dolomite is lost, when they give way abruptly, leading to the sudden appearance of sinkholes at the surface.

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