City of Tshwane on high alert amid surge of sinkholes in Centurion and other parts of the city

After the prolonged rains in most parts of the country, sinkholes have emerged in Centurion, with the City of Tshwane confirming that it has 61 sinkholes recorded on the priority listing. 

According to an expert, prolonged rains may have triggered sinkholes to develop.

A large sinkhole that formed over the weekend in Lyttelton Manor, Centurion, is one of many. 

According to Prof J Louis van Rooy of the department of geology at the University of Pretoria, the dolomitic soil in Centurion is a significant factor in the frequency of sinkholes.

“The dolomite bedrock is not only pure dolomite, but there are other rock types like chert interlayered which don't dissolve and other elements are also left after the dolomite dissolves. These materials are now loosely filling the voids and the cavities may also be filled with soil falling in from the surface,” said Van Rooy.

“This is then a metastable system, and if there is a cave or large void deeper down heavy rains or concentrated water inflow from a burst pipe may mobilise the loose void infill into the deeper cavity, which then creates a sinkhole when the void appears on the surface.”

Van Rooy said if concentrated water inflows occur in a metastable karst environment, it may trigger sinkholes if there is a void in the rock, and the water erodes the loose material into a deeper cave or cavity. 

He said the main action should be to stop water leaking into the possible sinkhole area, prevent any further inflows from stormwater and reroute waterborne services.

Van Rooy said areas where sinkholes occur were developed many years ago and over time waterborne services deteriorate. 

“Residents need to look out for surface cracks that may indicate soil movement, not overwater gardens, look out for leaking taps or wet soil patches that may indicate subsurface water leakages and so on. The municipality needs to mitigate risk by keeping services up to the standards as prescribed, replacing ageing and leaking services, and generally manage surface stormwater to prevent concentrated inflows,” he said.

Van Rooy said there is a link between urban development and sinkhole occurrence due to the disturbance of the karst environment, the addition of waterborne services that may leak, concentrated stormwater runoff, additional loads on the surface and so on.

“Sinkholes are natural phenomena or geological hazards and the natural process is in some instances accelerated by human actions. The sinkhole may still have developed naturally but over a long time without any human influence. Sinkholes are natural hazards similar to earthquakes, volcanoes, floods, slope failures and urban development needs to be properly and responsibly managed. Engineering geologists and geotechnical engineers are involved in all development on dolomite land today to ensure safe living conditions in these environments,” he said.

The city said the affected road intersections are closed off with barricades for the safety of residents.

City of Tshwane spokesperson Lindela Mashigo said the sinkhole is due to a combination of internal stormwater drainage and a subsequent water leak.

“The long-term plan is to upgrade all the ageing wet services, especially water pipes, to dolomitic standards to minimise the occurrences of leaks and potential sinkhole formation. The main cause of the sinkholes is leaking water pipes, however, sinkholes also occur due to leaking sewer and stormwater pipes, pounding of surface water and leaking irrigation pipes,” he said.

The city has advised affected property owners and residents on safety aspects around the sinkhole and on safeguarding measures within the property. 

“Affected property owners are advised to contact their respective insurance companies,” Mashigo said.

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