Sinkholes and subsidence in areas underlain by dolomite rock continue to pose considerable risks to infrastructure, requiring that developers carefully assess geological stability before designing any building or civil engineering works.
According to John Stiff, partner and principal engineering geologist at SRK Consulting (South Africa), dolomite-related risks have been particularly high in areas of South Africa where mining has taken place – due to the extensive dewatering that typically accompanies mining activities.
“Groundwater fills the cavities within dolomite rock, exerting hydrostatic pressure that helps stabilise these voids,” said Stiff. “When groundwater is removed during dewatering, this pressure is reduced, making the ground more prone to instability.”
Equally, however, dolomite-related risks to infrastructure occur in non-mining areas due to the gradual dissolution of dolomite rock through geological time by slightly acidic rainwater.
“In Gauteng province, for instance, parts of Soweto, West Rand, East Rand and Centurion are underlain by dolomite,” he explained. “Development in these areas has been plagued by sinkholes and subsidence, leading to significant damage. In mining areas, this risk can be magnified where tailings dams are operated of these ground conditions.”
History of sinkholes
The country has in fact lost many lives in accidents caused by sinkholes. Among the well-known cases was a tragedy in December 1962 at West Driefontein Mine on Johannesburg’s West Rand, when a three-storey crusher plant fell into a sinkhole – killing all 29 people in the building. Two years later, a family of five along with their live-in domestic worker were killed when a 100 m wide sinkhole swallowed their home in Carletonville.
“In response to these disasters, South Africa introduced stringent regulations and standards to manage the risks associated with infrastructure and development on dolomitic ground,” he said. “These include the current South African National Standards (SANS) 1936, which provide guidelines for geotechnical investigations, risk assessment and mitigation measures.”
The risks associated with dolomite are not confined to South Africa. As SRK’s experts have observed, regions such as the Katanga Copperbelt in the Democratic Republic of Congo (DRC) and parts of the Northern Cape also face similar challenges due to the widespread presence of dolomite rock. Rapid infrastructure development in mining countries like the DRC underscores the importance of effective risk management strategies.
Early identification
Ensuring the safety of human life and preventing damage to infrastructure starts with thorough geotechnical investigations of the area where developments are planned, according to Basetsana Mmileng, senior engineering geologist at SRK Consulting (South Africa).
“Preventative measures include a dolomite risk management strategy, conducted in accordance with the standards and regulations,” said Mmileng. “This is a proactive way to mitigate the risk of these incidents occurring, and ensuring that your land use and infrastructure design is suitable for these conditions.”
Techniques such as gravity surveys are used to identify anomalies in the subsurface, indicating voids or weak zones. Hennie Booyens, principal geotechnical engineer at SRK Consulting (South Africa), highlighted the importance of integrating findings from geophysical surveys with detailed drilling programmes. Percussion or rotary core drilling is employed to investigate the site in more detail, and to gain a comprehensive understanding of site conditions. This approach allows the investigation team to identify potential hazards early, said Booyens, and to guide the development of tailored solutions.
Tailings storage facilities (TSFs) represent mining infrastructure which is particularly vulnerable in the context of dolomitic ground, said Stiff, because they contain such large volumes of water.
Instability, contamination
“If you have concentrations of water – either in the pipes delivering tailings product or even in the separation of water in the TSF itself – this can cause problems by leaking into the substrate,” he said. “Sinkholes can form directly underneath the tailings dam, leading to structural instability, groundwater contamination or, in severe cases, dam wall failure.”
For these reasons, Booyens emphasised that the basin of a tailings dam is a critical area for investigation prior to construction.
“The liner under a tailings dam, while effective to a degree, has limitations and cannot fully mitigate the risks posed by unstable dolomitic ground,” he explained. “If you have sinkhole formation in the basin of the facility, there’s only so much strain that the liner can accommodate before it ruptures and water and tailings starts seeping into the substrate and groundwater.”
These factors further demonstrate why careful site characterisation and risk assessment are vital to ensure the long-term stability of these facilities. Mmileng added that risks extend beyond the TSF itself.
“It is also crucial to understand what this failure would mean to surrounding infrastructure,” she said. “For example, if there’s a ventilation shaft system nearby, subsidence in the TSF could cause damage to these facilities as well.”
Key skill sets
SRK Consulting’s long history of involvement in dolomite investigations, and the depth of expertise it has developed in this field, are demonstrated by having certified Level 4 dolomite specialists in its ranks.
“Recognised for their expertise and contributions to the field, a Level 4 dolomite specialist has over 10 years of experience in dolomite investigations and risk management,” said Mmileng. “They can assess and manage risks, characterise sites and ensure compliance with the standards.”
Investigations are most effectively conducted by multidisciplinary teams, with a key collaboration between engineering geologists and geotechnical engineers.
“Engineering geologists characterise the karst conditions on the site, while the engineers design the solutions and mitigating measures that help prevent failures and safeguard the structures,” said Stiff.
The company also boasts its own in-house geophysicists, which sets the company apart. This facilitates the ground characterisation phase with gravity or resistivity surveys, ensuring a detailed understanding of subsurface conditions.
“The analysis of geophysical data is only as good as the interpretation, which is handled by our skilled in-house team,” he said. “This streamlines the accurate identification of anomalies during the early stages of a project – which in turn helps with the design of targeted interventions, saving time and costs in the long run.”
He added that expertise in groundwater management is also vital to the team.
“One of the critical factors is the groundwater – not only because it is at risk of pollution, but because groundwater is a key factor in the stability of the ground,” he explained. “If the underground dolomitic cavities are water-filled, the risk of subsidence is mitigated.”