Living on shaky ground
AT LEAST 20% of Gauteng is perched precariously on dolomitic ground.
Dolomitic ground is prone to sinkhole formations, which can lead to damage to buildings and infrastructure and, in the worst case scenarios, to injury and perhaps death.
"Thirty-eight people have been killed by sinkholes over the past 50 years," said engineering geologist at the Council for Geoscience (CGS) Greg Heath.
He said about 2000 sinkholes had been recorded over the past 50 years. These were spread over the entire dolomite area and about half of them were concentrated on the far West Rand, where lowering of the water table by the mines had triggered several events.
"Because of this we need to be aware of this geo-hazard. The main effects involve avoiding land that is poor, using relatively expensive foundation solutions and implementing precautionary measures.
"These measures affect the amount of land that we have available, the density of housing that we can implement and the cost of housing [particularly for RDP houses]," he said.
Because dolomite occupies 20% of the land surface in the province, there is not much choice for new building with few alternative large open pieces of land still available.
"This means that the existing land has to be used better and alternative housing options considered such as multi-storey walk-ups that use less land and house more people," he said.
Heath said the council had produced guidelines for development on dolomite. Essentially, when buying a piece of land on dolomite, a dolomite stability geo-technical study must be undertaken on that land.
"It is vital that the geo-technical report has been independently reviewed by the CGS to ensure that the recommendations are appropriate. For people who already live on dolomite it is vital that they check their water-bearing services for leaks, as leaking services trigger sinkholes," Heath said.
He said homeowners should ensure that all rainwater drains away from their properties and does not pond against any walls or structures.
"Gutters and downpipes should be installed to lead water away from structures. Placing a concrete apron or paving around structures is also a good method to prevent water entering the soil.
"Normal rainwater has not triggered many sinkholes in open fields, however, when houses are built the natural surface flow paths are affected which causes water to pond and enter the surface below. Municipalities' water-bearing pipes also can be at fault here. This water carries the overlying soils into the voids in the dolomite below which makes the void bigger, leading to a sinkhole on the surface," Heath said.