Sinkholes threat to houses in Gauteng

2011-10-05 22:07
Johannesburg - Gauteng authorities have warned of an increasing risk of dolomitic sinkholes opening up around Johannesburg, which has cost 38 lives in the last 50 years.

"The dolomitic situation is satanic," says Gauteng MEC for local government and housing, Humphrey Mmemezi.

"[Residents don't realise] it comes during the night... People can wake up and the section [of the township] is not there."

Greg Heath, an engineering geologist at the Council for Geoscience (CGS), says dolomite forms a loose belt around Johannesburg and makes up a quarter of the province. It stretches from Westonaria in the west to Centurion in the north and Thokoza in the east.

At least 2 600 sinkholes have been recorded.

"The damage they have caused to development and infrastructure is estimated at a very conservative R1.5bn.

Unstable land


"At least 38 people have died as a result of sinkholes and the number will increase if people do not move off unstable land," says Heath.

About 110 informal settlements exist on dolomitic land in Gauteng.

Heath says sinkholes are almost always man-induced and develop when communities move into an area with water-bearing services. In townships and informal settlements, leaking taps or burst pipes could trigger dolomitic erosion, which expands underground cavities.

In areas of mining or farming, dolomitic ground could become unstable through changes in the water table.

Two weeks ago, part of a street in Centurion collapsed as a sinkhole opened up. The 7m x 5m hole stretched over a lane of Jean Avenue, revealing water pipes and a dark depth below.

In Westonaria, an ever-growing sinkhole the size of a small dam and about 100m deep, lies a metre away from a tee-off at the local golf course.

Excuse


In Thembelihle, an informal settlement near Lenasia, protesting residents who want electricity infrastructure were told they need to relocate as the dolomitic land beneath their shacks is unstable.

Many residents are refusing to move, even though new housing developments are available for them at Lehae and Vlakfontein. They believe dolomite is being used as an excuse for removing informal settlements.

Heath says with an increasing population and a limited amount of land, the government will have to review dolomitic areas and assess the risk of erosion.

"We can't always avoid dangerous ground, but [the question is] how can we use it? People have gravitated towards Johannesburg [and surrounding areas]. We have to use what land we've got."

Unpredictable

Geologists can work out where "good" dolomite and "bad" dolomite lies.

"Good" dolomite is more stable and may allow development while "bad" dolomite is filled with voids and should be avoided.

Heath says dolomite is unpredictable over short distances and extensive surveys are needed to map it.

Engineering geologists and geotechnical engineers use a gravity surveying machine to determine the density of underground materials. A team then drills percussion boreholes at small intervals to survey what is up to 100m below.

The downside of this technology is that it is slow and expensive, costing about R250 per metre.

With few other options, the provincial government has mandated the CGS to help with surveys on populated dolomitic land.

Immediate intervention

Besides Thembelihle, seven areas in Gauteng have been prioritised for immediate intervention.

They are Khutsong and Bekkersdal on the West Rand; Bapsfontein informal settlement, Thokoza, Vosloorus, Katlehong on the East Rand; and the Centurion CBD in Tshwane.

Most have been targeted because of leaking and deteriorating infrastructure.

In Bapsfontein, the abstraction of dolomite groundwater by farmers is causing sinkholes. In Centurion, anticipated development in the CBD will increase the risk of dolomitic exposure.

Precautions have been taken for part of the Gautrain, which runs through Centurion, traversing about 16km of dolomitic ground.

The CGS's risk management strategy involves precaution, monitoring and working with property owners and developers.

For established properties, gutters and aprons - a ground covering of concrete or other material surrounding a property - are used to protect underlying dolomite from water erosion.

Water-bearing services are also monitored periodically for leakages. If a leakage is found, it should be repaired quickly.

Mmemezi has urged residents to be responsible with taps, pipes and buckets, or face the consequences.

"If they don't care and ignore our warnings, they'll be buried."
- SAPA
Read more on:    humphrey mmemezi  |  johannesburg  |  environment
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