noun
the gradual caving in or sinking of an area of land.
"the race was abandoned because of subsidence of the track"
synonyms:
collapse, caving in, falling in, giving way, sinking, settling
"subsidence due to coal mining is a notorious problem"
As per the first hit from Google, subsidence is
a) a problem if you have property, infrastructure or assets on an area of land that is subsiding and
b) it’s often linked to mining
In South Africa it’s also linked to dolomite areas and dewatering but more on that later.
The ground is sinking a bit - so what?
This post was initially meant to be a light read, but unfortunately the consequences of some of the more notable subsidence related incidents have been pretty serious. About 2500 subsidence and sinkhole related incidents have occurred in South Africa. Here are just 5 notable accounts:
1. The Sinkhole farm - Wonderfontein spruit valley (about 50km West of Johannesburg)
Gold miners pumped out water that was interfering with mining operations.
The area is still afflicted with sinkholes, damaging water supply pipelines and sewage infrastructure leaving 1000’s without basic services.
2. West Driefontein No.2 Shaft
Truly tragic, the sinkhole swallowed a crusher plant with 29 people inside it. This incident occurred in the same general area as the above ‘sinkhole farm’ i.e. West of Johannesburg.
About half of all South African subsidence and sinkhole related incidents have been in this general area.
3. Bapsfontein (near Pretoria)
Farms in the area have massively drawn water resulting in the collapse of underground caverns. This has led to large subsidence in the area requiring the relocation of 3000 families.
There have been violent protests at the prospect of relocation.
4. The Devil's Territory* of eMalahleni (formerly Witbank)
The area has been extensively mined underground for coal. Subsidence allows air underground which results in the spontaneous burning of the remaining coal left in the supporting pillars. The surfaces can reach up to 630 degrees celsius and toxic gases as well as sulphur smells (hence the name) are released. Powerlines and railways have been damaged.
*The "Devil's Territory" name was coined by HOMEF in describing the Witbank Coalfields.
5. Centurion south of Pretoria
30 sinkholes have appeared in the area in 2018 alone due to Centurion being built on permeable dolomite. Roads have been damaged with some properties cracking.
As much as R25 million will be used to fix the pictured sinkhole.
Sinkholes and subsidence in South Africa seem to be predominantly triggered by:
Mining activity
Dewatering or pumping out of underground water
Dolomite rock being dissolved by acidic water (acid rain or acidic mine water)
Looking at two maps of South Africa, one a map of dolomite, the other a map of mining activity, it appears that Mpumulanga, Gauteng, Northern Cape and North West provinces are likely to continue facing challenges with subsidence and sinkholes. Property and infrastructure in these areas such as roads, rail, pipelines, electricity pylons, cell phone towers, etc must be closely monitored from a safety, health and environmental perspective. Insurance is probably a good idea too.
References:
Sinkholes and subsidence in South Africa, A.C Oosthuizen and S. Richardson
The Lower Wonderfontein Spruit: an expose ´ C. J. U. Swart Æ E. J. Stoch Æ C. F. van Jaarsveld Æ A. B. A. Brink
Risk mapping of eMalahleni municipal area with focus on coal mining impacts, Schoeman, A.
Hazard of sinkhole formation in the Centurion CBD using the Simplified Method of Scenario Supposition, A C Oosthuizen, J L van Rooy
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