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Geotechnical Engineering: Ways to minimise sinkholes

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 keeping an accurate set of municipal records, and by acting promptly when signs are detected.

Past experience in designing remedial solutions for sinkholes in urban areas has enabled SRK Consulting to continually hone it expertise in this area. A past example is the work carried out for the City of Tshwane in November 2012 when a large sinkhole occurred on a suburban street in Centurion.

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. For this reason, SRK Consulting highlights the need for local authorities to devise contingency plans and to set aside the necessary funds to deal with these incidences, and where possible, put in place measures to prevent them.

“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,” says 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.”

Tshwane sinkhole

 “The 2012 sinkhole was one of the largest to have occurred in the Centurion area over the last 25 years,” comments 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,” Stiff continues. “It also disrupted municipal water, sewer and stormwater services to the area.”

Repairing the damage

SRK’s solution began with a services audit to establish the nature and location of municipal services in the area. These services could then be diverted to allow SRK’s investigation and repair work to continue.

“We conducted rotary percussion drilling to a depth of about 30 metres, as well as dynamic probe super heavy tests to establish the presence of near surface voids and weak zones in soil horizons blanketing the bedrock,” he explains.

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

“It’s important to conduct an adequate investigation to guide the appropriate rehabilitation design,” Stiff stresses. “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 and providing technical supervision and direction, particularly during excavation. Further responsibilities included quality control supervision 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 SANS 2001 BE3. This was done by placing enough coarse rock fill to block the throat, followed by layers of progressively finer material.

The advantage of this method is that it limits infiltration of surface and groundwater 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.

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