Joburg built on a honeycomb

PROBLEM: MINING HISTORY HOLLOWED OUT GROUND

- Hein Kaiser

Johannesburg is literally built on a hollow honeycomb, with its mining history playing a significant role. Adding to the instability, dolomite rock beneath much of the city and Ekurhuleni is inherently unstable.

Research by the University of Johannesburg showed how flooded mine voids increase hydrostatic pressure on dolomitic rock. A study by the University of Pretoria documented over 3,000 cases of subsidence and sinkholes across Gauteng, showing a clear link between disturbed dolomitic land and ground collapse.

Dolomite dissolves when exposed to water, forming cavities that can collapse at any time. Abandoned gold mines made the problem worse by creating voids that intersect water-bearing rock layers. When mine pumping stopped, many of these voids flooded, raising the groundwater table.

When water rises to within 6-10m of the surface, the risk of sinkholes escalates dramatically. Each blast or dig weakens the underground framework a little more.

Blasting closer to the surface, because of the rising water table is causing even more structural damage.