TECHNICAL PAPER Sinkhole and subsidence JOURNAL OF THE SOUTH AFRICAN INSTITUTION OF CIVIL ENGINEERING size distribution across ISSN 1021-2019 Vol 60 No 2, June 2018, Pages 2–8, Paper 1373 dolomitic land in Gauteng SAMANTHA CONSTANTINOU (PrSciNat) worked for the Council for Geoscience for S Constantinou, J L van Rooy nine years after graduating from the University of KwaZulu-Natal with a BSc Honours in Engineering and Environmental Geology. She spent the last eight of these Karst-related sinkholes and subsidences occur on areas underlain by Chuniespoort Group years predominantly working on dolomite dolomite bedrock in the Gauteng Province, South Africa. Although dolomite land is found stability report reviews for housing and infrastructure development, and on dolomite research, having received across several South African provinces, sinkhole and subsidence formation in Gauteng, the her Master’s in 2013 from the University of Pretoria. She is registered with economic hub of South Africa, has been considerably more severe than in other provinces. SACNASP (South African Council for Natural Scientific Professions) and is a Thousands of sinkhole and subsidence events have occurred in the past 50 years. Previously, member of SAIEG (South African Institute for Engineering and data on sinkhole and subsidence occurrence was amassed separately by various consultants, Environmental Geologists). She also sat on committees for the SANS 1936: 2017 document revisions. companies, research institutions and state authorities. There is no legal requirement for sinkhole and subsidence events to be reported to a central authority, yet the data is crucial for future Contact details: T: +27 31 266 2479 assessment of sinkhole hazards and decision-making. Information regarding the sinkhole record E: [email protected] for Gauteng was collated to develop an inventory of the study area. Compiling a comprehensive database presented many challenges; most importantly the large volume of incomplete data PROF LOUIS VAN ROOY (PrSciNat) started that could not be retrieved. Sinkholes, and subsidence and crack events prior to the end of 2011 lecturing in the Department of Geology, were considered for Gauteng. Data was organised into multi-wave frequency tables, and various University of Pretoria, in 1985 and obtained his PhD in Engineering Geology in 1992. He aspects were then analysed. This paper deals with available dimension data (i.e. diameter and lectures to pre- and postgraduate mining depth) across the Gauteng Province. and civil engineering, hydrogeology and engineering geology students. His research interests include durability of basic igneous rocks for use in civil engineering construction, problem soils, safe INTRODUCTION In the past, data on sinkhole and subsid- development on dolomite land, and rock engineering. He has supervised According to Ford and Williams (1992), ence occurrences was amassed separately in or co-supervised more than 25 MSc and PhD students, and is author or sinkholes are the most diagnostic surface papers, research theses and databases held co-author of more than 30 papers in journals and conference proceedings. He serves on the councils of both SAIEG and IAEG (International expression of karst landscapes and can by various consultants, companies, research Association for Engineering Geology) and is the IAEG’s Vice-President be found extensively throughout the institutions and state authorities. There is for Africa. world (approximately 7–10% of the earth currently no legal requirement for sinkhole Contact details: land surface has been classified as karst and subsidence events to be reported to Department of Geology terrain). Karst-related sinkholes and sub- a central authority (Heath & Oosthuizen University of Pretoria Private Bag X20 sidence events occur on areas underlain 2008). Sinkhole statistics have not been Hatfield 0028 by dolomite ground in South Africa. available since the work by Wolmarans South Africa Dolomite land occurs across several South (1984) and Schőning (1990), although Heath T: +27 12 420 2023 African provinces, including Gauteng, and Oosthuizen (2008) indicated in excess E: [email protected] Mpumalanga, Limpopo, North West and of 2 400 instability events in a preliminary the Northern Cape. However, sinkhole overview of the sinkhole record for South formation in Gauteng Province poses a Africa. More recent research (Richardson greater risk to infrastructure than in any of 2013) shows numbers are in excess of 3 000 the other provinces to date. (sinkhole/subsidence/ground crack events). Thousands of sinkhole, subsidence and Sinkhole and subsidence data is crucial for crack events have occurred in the past future assessment of sinkhole hazards and 50 years within the Gauteng Province. decision-making. According to Buttrick et al (2011), four to five million people currently work or reside on dolomite land, and these instability events DOLOMITE AND SINKHOLE have resulted in loss of life and/or damage FORMATION to property when they coincide with human In Gauteng Province the Malmani development. Damage to buildings and other Subgroup (Chuniespoort Group, Transvaal infrastructure has been more severe on dolo- Supergroup) dolomites occupy a surface mite than on any other rock type in South area of approximately 2 576 km2 (14% of Africa (Brink 1979; Wagener 1985), and thus Gauteng’s surface area) and form two broad far 39 people have lost their lives (Buttrick & arches (northern and southern) around the Keywords: dolomite, sinkhole, subsidence, Chuniespoort, Gauteng Roux 1993). Halfway House Granite. However, the area Constantinou S, Van Rooy JL. Sinkhole and subsidence size distribution across dolomitic land in Gauteng. 2 J. S. Afr. Inst. Civ. Eng. 2018:60(2), Art. #1373, 7 pages. http://dx.doi.org/10.17159/2309-8775/2018/v60n2a1 strength and compressibility with depth. Voids are also sometimes present in the GAUTENG PROVINCE wad (De Bruyn & Bell 2001). The mechanism of sinkhole and subsid- ence formation is described in detail by Jennings et al (1965) and Buttrick (1992). They describe ground subsidence in dolo- mitic formations as taking place in one of City of Tshwane two ways: as a gradual or caving subsidence or a rapid and catastrophic sinkhole. These Laudium events are most often caused by ingress Centurion water or lowering of the groundwater table. Thembisa Bapsfontein Oaktree SINKHOLES AND SUBSIDENCES City of Etwatwa Johannesburg Daveyton IN THE GAUTENG PROVINCE Randfontein Ekurhuleni Numerous papers and reports have been Springs Venterspost Soweto Kwathemba published, and an abundance of research Westonaria Khutsong Lenasia Katlehong Vosloorus exists on the subject of sinkholes and Carletonville investigation techniques on dolomite land in South Africa, particularly on Gauteng in West Rand Meyerton which the bulk of the incidents have been Sedibeng reported. Historically the Far West Rand Vereeniging (the area from Westonaria to Carletonville) was the focus of many studies (i.e. Brink & Partridge 1965; Brink 1979; Kleywegt & Pike 1982; Wolmarans 1984) due to the frequent occurrence of sinkholes and subsidences in 0 20 40 80 120 160 Kilometres the 1960s and 1970s resulting from dewater- ing of several of the groundwater compart- Legend ments by mining companies. The area south Gauteng Province Municipalities Towns of Pretoria has also seen scores of sinkholes Study area: Dolomitic land Instability events Chuniespoort Group N in recent history (Roux 1984; Schőning1990; De Bruyn & Trollip 2000; Heath & Figure 1 Distribution of instability events and dolomitic land across Gauteng in the different Oosthuizen 2008; Buttrick et al 2011), while District and Metropolitan Municipalities relatively few events have been reported in the municipalities of Ekurhuleni (De Bruyn considered as “dolomitic land” (Bosch 2003), valleys are corroded within the shear zones & Trollip 2000; Heath & Oosthuizen 2008), including areas covered by younger non- (of faults), with dolomite rock standing as Sedibeng and the City of Johannesburg, all dolomitic formations but still underlain by pinnacles between the corroded grykes/val- within the Gauteng Province. dolomite at depth (within 60–100 m), covers leys. The hard rock dolomite is usually cov- an area of approximately 4 005 km2, i.e. 24% ered by an upward succession of residual of Gauteng’s surface area (see Figure 1). products (weathered dolomite, wad1, chert DEVELOPMENT OF The dolomite rock which occurs in the and residual chert) that are often overlain SIZE CATEGORIES Transvaal Supergroup comprises a series by younger formations or are intruded by While Wolmarans (1984) indicates numer- of alternating bands of insoluble chert dykes or sills (Brink 1979). ous sinkhole events prior to 1984 on the Far and soluble dolomite. Small amounts of The residual mantle can be extremely West Rand, dimension data was not given iron and manganese carbonates are also irregular (Martini in Johnson et al 2006). and size categories were not yet developed. commonly present (Brink & Partridge A residual product such as wad has low Research by Schőning (1990) analysed 1965). Dolomite rock possesses a system of strength in most cases, and is highly com- surface diameters and depths of over discontinuities (fractures, joints and faults) pressible and may be tens of metres thick. 200 sinkhole events in the area south of which act as preferential solution passages The vertical succession of these residual Pretoria (Tables 1 and 2 refer). Schőning’s for water ingress. Although dolomite rock