The solution to Gauteng's illegal mining threat could be sitting on the province's doorstep - using sand from mine dumps to fill up disused shafts.
Mining engineers and researchers said backfilling, which Sibanye Stillwater is experimenting with, involves mixing the sand with water into a slurry and pumping it down disused shafts and tunnels.
But mining companies warn that the process is expensive and unproven, and that there may not be enough sand to fill all the abandoned tunnels.
Warnings from seismologists and geophysicists have been sounded as the department of mineral resources (DMR) this week announced it would conduct a ground stability study in the Johannesburg area.
The study, through the Council of Geoscience, follows last week's report in the Sunday Times on growing alarm over blasting and tunnelling by illegal miners near gas pipelines.
Johannesburg mayor Herman Mashaba described the situation as a crisis, saying the city was on the brink of a disaster caused by ground collapses and explosions.
The city approached the department after Transnet and Sasol raised the alarm over how dangerously close illegal miners were to breaching gas and fuel pipelines and damaging critical infrastructure, including bridges, roads and the FNB Stadium and precinct.
The study will use sensors to monitor for explosive signals from potential illegal mining; use satellites to map ground subsidence around areas of suspected illegal mining, and use special techniques to map tunnel systems throughout the Main Reef.
The Mineral and Petroleum Resources Development Act, the National Environmental Management Act, the Mine Health and Safety Act and the National Water Act make closing down and rehabilitating mines the responsibility of mining companies.
DMR spokesperson Ayanda Shezi said there were several laws governing mines from "cradle to grave", including rehabilitation to restore the land to its natural, pre-mined state.
The DMR oversees mine rehabilitation through monitoring legislative compliance and reviewing closure plans before it issues a closure certificate, Shezi said.
Professor Bryan Watson of the Wits University School of Mining Engineering said though backfilling shafts was relatively easy, filling the stopes, which are large pocket-like areas where the gold ore is mined, was more complicated, although "it would stabilise stopes and reduce the risks of surface collapses".
Watson said stopes were what illegal miners targeted. "They run parallel to each other and are separated by rock support pillars which keep the roof up. Illegal miners mine gold ore from these pillars, using explosives, which risk causing collapses."
He said if the collapsing rock had several faults in it, it could cause the surface to drop by as much as 1m, "which is catastrophic to buildings and structures".
"Even a 10cm drop will have severe effects on the integrity of structures," he said.
Professor Ray Durrheim, a mining seismology researcher at Wits's School of Geosciences, said illegal miners targeted shallow tunnels and would not go below 100m because of the water table. They tended to mine near the surface.
"In general, threats by illegal miners to seismic activity are small, but if they mine under structures there will definitely be problems," said Durrheim. There might not be sudden infrastructure collapses, but there could be threats to the integrity of walls of buildings and homes.
"Other infrastructure which could be affected are railway systems. This could impact on a city's functioning and governance."
Geophysicist and seismotectonics researcher Dr Chris Hartnady said the Council of Geoscience and DMR needed to get ahead of the problem and use seismology equipment to pinpoint illegal mining activities so the authorities could home in on them.
He said solutions to illegal mining lay in part with backfilling.
Sibanye-Stillwater spokesperson James Wellsted said the mining house had been conducting backfilling tests, but it was more to address acid mine drainage...