WITH subsidence-prone dolomite underlying 25% of Gauteng and with economic growth and development requiring more and more land, the expertise of the Engineering Unit of the Council for Geoscience (CGS) is becoming critical for non-mining sectors of the economy.
“Dolomite land has a potential to threaten life and property,” highlights CGS senior engineering geologist Greg Heath.
“Over the past 50 years, we have had about 1 300 sinkholes in the greater Gauteng area – that is, Gauteng and adjacent areas of the neighbouring provinces. But no longer can dolomite land be left unused, as development encroaches on to these areas.”
“We have little choice but to use this land, and manage the risk. For example, the Gautrain has to cross dolomite areas to be able to join Johannesburg and Pretoria, including some high risk land,” he cites. “The builders have to put in extraordinary measures, including piles going down 50 m to the dolomite bedrock.”
Dolomite areas are far from uniform, and the risk posed varies considerably over very short distances, which makes planning very difficult. While different types of development– residential, commercial, industrial, infrastructural – can tolerate different levels of risk, managing this risk has to start at the planning stage of a development.
Thus, the first step is to identify the worst areas within the dolomite regions and to eliminate them from consideration for development. The CGS implements a risk classification system which has a scale of eight grades of risk, with one being the lowest, and eight the highest, risk.
This risk is revealed through a dolomite stability investigation, involving the drilling of a number of boreholes on the site. Residential development is not allowed in areas classified as having an inherent risk class of six or higher as these are considered to be high risk areas where the potential size of a sinkhole could threaten the safety of the home and/or homeowners. “This often comes as a shock to property owners to find out that the subsurface conditions are such that it is unsafe for development,” he points out.
“We act as a watchdog body to ensure safe and sustainable development although we have no legal duty to stop a development that we perceive to be unsafe,” explains Heath. “We make recommendations to both the National Homebuilders Registration Council, which provides a warranty scheme for residential developments, and to local authorities, who tend not to give a go-ahead for developments on dolomite without our support. Our concern is to ensure that by avoiding the high risk areas, and managing the risk on the rest, that the dolomite areas will be sustainable for centuries to come.”
The CGS also provides assistance with other forms of risk management, such as making recommendations on foundations. Thus, for areas in which there is only a low to moderate chance of small to medium size (up to 5 m in diameter) sinkholes forming, raft foundations – which can span such sinkholes – should be used, which should prevent a house falling into any sinkholes that do form, or from experiencing significant stress such as cracking.
Heath also points out that all infrastructure that involves water (the water supply, drainage of rainwater, and sewage) must be monitored and maintained, to ensure that there are no leaks, as a leak is the major ‘trigger’ for sinkhole formation. All houses in dolomite regions must have gutters and paving around the walls so that rain water is carried away from the structure. “Body corporates of townhouse complexes have a major responsibility to ensure that no leakages of servces occur as a sinkhole on such a site could affect a number of units,” he observes.
“For industrial and commercial developments, the rules are a little different. They can be placed in slightly higher risk areas, because they use stronger, better, properly engineered structures,” he states. “This is not usually the case with normal houses – even upmarket houses.” In all cases, however, maintenance of the risk mitigation measures can never be neglected.
Engineering Geosciences is a unit of the CGS with the mandate from the government to develop an understanding of the geological conditions which could affect the construction of infrastructure, and industrial, commercial and residential developments, waste disposal sites, and even cemetaries.
“We seek to understand soil and ground conditions across the country, to define where problem areas occur,” he reports. “We have a programme to quantify these problem soils and to map them, and make this information available. This process involves the taking and testing of soil samples. We seek to produce ‘development potential’ maps to ensure that development does not take place on unsuitable ground.”
To date, the unit has completed some 20 sheets of 1: 50 000 scale maps, covering lareg areas Cape Town, Durban, and much of Gauteng. ”Our intention is to eventually make this information available on our web portal, so that it can be easily accessed by local governments. developers, and even individual home owners,” he reveals.
“Developers and individuals don’t want to find at a late stage that significant portions of their property are unuseable or require expensive foundation measures. That’s why we want to make this information available, so that no-one is exposed to undue risk or late stage surprises. These maps are supported by a national collection of geotechnical reports (a databank) should anyone want to know of any geotechnical investigations conducted on their properties.”