Centurion Sinkhole in Lyttleton Manor

14 November 2017

Lessons from Tshwane sinkhole repair

Although standards are in place to guide the repair of sinkholes in South Africa, local authorities can reduce the impact and cost of these events by making available an accurate and complete set of municipal records, and by acting promptly when signs are detected.

Having worked with the City of Tshwane municipality to repair a large sinkhole on a suburban street in Centurion, Gauteng in November 2012, SRK Consulting highlighted the need for local authorities to build management capacity and commit the necessary budget to deal with these events.

Context of sinkholes

Areas underlain by dolomite – such as the Malmani Subgroup of the Transvaal Supergroup in the southern districts of Tshwane – are most susceptible to sinkhole activity.

“Since 2012, development on dolomite land has been controlled by standards promulgated by the South African standards authority, but instability events will continue to occur,” said SRK Consulting principal engineering geologist and associate partner John Stiff. “This puts municipal infrastructure particularly at risk, due to the vast network of bulk water-bearing services such as high pressure water, stormwater and sewerage networks.”

Stiff said karst-related instability events are well documented and pose significant challenges to infrastructure, causing major disruption to the provision of municipal services and to the access to neighbourhoods.

What had to be fixed?

“The 2012 sinkhole was one of the largest to have occurred in the Centurion area over the last 25 years” said Ashika Sudu Deputy Director at the City of Tshwane Roads and Transport Department. Early causes of the sinkhole included a leak in a domestic water connection which began in April 2012 – some seven months before the event. By late November of that year, a widening of cracks in the surface was observed, and within a day this very large sinkhole had formed.

“The sinkhole measured 16 metres by 35 metres and up to 9 metres deep – affecting the roadway, private property, and the sport facilities of the adjacent school,” said Stiff. “It also disrupted municipal water, sewer and stormwater services to the area.”

How it was solved

“Our solution began with a services audit, to establish the nature and location of municipal services in the area, following which these services could be diverted to allow our investigation and repair work to continue,” he said. “We conducted rotary percussion drilling to a depth of about 30 metres, as well as dynamic probe super heavy (DPSH) tests to establish the presence of near surface voids and weak zones in soil horizons blanketing the bedrock.”

Through these investigations, the bedrock depth could be ascertained and subsurface extent of the sinkhole could be established; results from the probes also revealed the risk of ground instability to adjacent structures and facilities.

“It is important to conduct an adequate investigation to guide the appropriate rehabilitation design,” said Stiff. “Once the rehabilitation method is adopted, however, it may still require some adaptation to ensure the optimal result.”

In executing the rehabilitation process, SRK’s role involved supervising the contractor through the early stages of the rehabilitation works, providing technical supervision and direction particularly during excavation and ensuring quality control on the backfill and compaction of the excavation.

To prevent the continued migration of material down the throat of the sinkhole, an inverted filter method was applied; in line with the national standard – SANS 2001 BE3 – this was done by placing rock fill coarse enough to block the throat, followed by layers of progressively finer material. The advantage of this method is that it limits infiltration of surface and ground water into the impaired area.

Bulk earthworks were followed by dynamic compaction of underground layers, and the reinstatement of services to the satisfaction of the client and the residents.

“The investigation and repair took about nine months, and cost the municipality about R12 million.” said Sudu.

Centurion Sinkhole in Lyttleton Manor

John Stiff, associate partner, principal engineering geologist | SRK Consulting

SRK Africa