Features / 24 February 2022 / Albert du Toit

Gauteng’s sinkhole problem – And what it means for our roads

Sinkholes on major roads in Gauteng have made headlines recently, and can be attributed to the fact that around 25% of the province is affected by dolomite rock.

This is according to Progress Hlahla, South African National Roads Agency (Sanral) northern regional manager.

In 2022, Gauteng motorists have been affected by a sinkhole that formed in January on the edge of the N1 highway, leading to traffic delays.

On 14 February, two more sinkholes formed around 20km away from where the previous one did, on the sides of the R21 near Olifantsfontein.

These caused the popular route to be reduced to two lanes for the foreseeable future, leading to more delays for the unfortunate Gauteng drivers that make use of it.

In a 2011 report by the council for GeoScience, it said that of the 2,500 sinkholes recorded in the country at the time, approximately 98% of them were in Gauteng.

Speaking to 702 in a recent interview, Hlahla explained why these sinkholes form and what Sanral is doing to reduce damages.

Why sinkholes form

“The sinkholes are usually caused by [a] cavity underground or a hole underground, and these holes resort a lot from the dissolution of dolomite rock,” said Hlahla.

The dissolution of the rock could be due to water washing it away, such as the heavy rainfall recently experienced in Gauteng.

Human activities may also increase the risk of these events. When water from a burst pipe seeps into the bedrock for a prolonged period, it erodes it, said the Department of Water Affairs (DWA) dolomite guideline.

To reduce the occurrence of sinkholes on roads, Hlahla said Sanral first performs a process called “ground-penetrating radar” to scan for dolomite and cavities in areas where it wants to build a new road.

Sanral also employs multiple geotechnical techniques to identify at-risk areas when building roads. If building on top of dolomite is unavoidable, Hlahla said Sanral looks at “other solutions” to keep road users safe.

“If you look at Gauteng, 25% of the land is affected by dolomite one way or another – so it’s not always going to be possible to avoid entire areas that have dolomite,” he said.

The image by Scielo below shows the distribution of instability events and dolomite land across Gauteng.

Solving the problem

Hlahla said that repairing a sinkhole is a complex process and calls for the assistance of geotechnical experts.

The size of the sinkhole will then inform the type of solution, he said.

If it’s a small cavity, Hlahla said Sanral can fill it with rocks and compact the area.

Larger cavities need other options. This could entail building a reinforced concrete slab above the cavity which would allow a road to be laid on top.

It’s also important to keep water away from the sinkhole while remedial work is being done to avoid it growing, he said.

According to the DWA, underground concrete pillars can also be installed at the onset of a building project in a dolomite-affected zone to mitigate the risk of sinkholes damaging the finished structure.

This can be seen in the construction of the Gautrain bridge running over the highway around seven kilometres away from the N1 sinkhole, said the DWA.

However, the main challenge with sinkholes is that it’s almost impossible to predict the occurrence of one – and you can only react to it after it has happened, said Hlahla.

A new sinkhole in the road also calls for rapid reaction from Sanral to reduce the risk of motorists driving into it and getting hurt.

Hlahla said Sanral is consistently monitoring at-risk areas and that if a sinkhole occurs, it will not only survey the sinkhole but also a large part of the surrounding area to detect any other cavities that might have formed.

For Gauteng, dolomite structures and sinkholes are virtually unavoidable – but the good news is that the roads are being built in a way to reduce damages that might occur from future sinkholes.

Below are a few of the major sinkholes reported in Gauteng in recent years: