Sinkholes and Subsidence in South Africa

Home , Atteridgeville, Carletonville, Chuniespoort, Council for Geoscience, Gauteng, Hopetown

Western Cape Unit

P.O. Box 572 Bellville 7535 SOUTH AFRICA c/o Oos and Reed Streets Bellville Cape Town

Reception: +27 (0) 21 946 6700 Fax: +27 (0) 21 946 4190

Sinkholes and subsidence in South Africa

A.C Oosthuizen and S. Richardson

Council for Geoscience Report number: 2011-0010

1 Contents

Contents...... 2 Figures...... 2

Tables...... 3 1 Introduction to sinkholes...... 1

2 Mechanism of sinkhole formation...... 1

2.1 Weathering of Dolomite...... 1 2.2 Sinkhole Formation...... 6

2.2.1 Sinkholes formed under the Ingress Scenario ...... 6

2.2.2 Sinkholes formed under the Dewatering Scenario (lowering of the groundwater table) 8

2.3 Subsidence Formation...... 10

2.3.1 Surface Saturation-type subsidence ...... 11 2.3.2 Dewatering-type subsidence ...... 12

2.3.3 Partly developed sinkholes ...... 13

3 Affected areas in South Africa ...... 15

3.1 Towns on Dolomite...... 20

4 SINKHOLE AND SUBSIDENCE INCIDENCE IN SOUTH AFRICA...... 21 4.1 Dewatered areas ...... 22

4.2 Non-dewatered areas...... 22

5 CONSEQUENCES OF SINKHOLE AND SUBSIDENCE FORMATION ...... 24 6 Summary...... 29

7 References ...... 30

Figures Figure 1. Occurrence of dolomite across South Africa ...... 2

Figure 2: Conceptual diagram of typical karst landscape in South Africa (after Waltham and Fooks; 2003) ...... 4

Figure 3: Dissolution of dolomite bedrock (Lyttelton Quarry, Centurion)...... 5 Figure 4: A sinkhole that has formed as a result of a leaking service pipe (Waterkloof, Pretoria) ...... 5

2 Figure 5: Example of sinkhole that bottleneck, i.e. narrow opening at surface (Atteridgeville, Pretoria) ...... 7

Figure 6: Large sinkhole (± 15 m diameter) triggered by ingress of water (Centurion, Gauteng)...... 8

Figure 7: Large sinkhole (> 50 m diameter) caused by lowering of the groundwater level (Bapsfontein, Gauteng)...... 9

Figure 8: Sinkhole formation process in both ingress and dewatering scenario’s ...... 10

Figure 9: Example of a surface saturation-type subsidence, < 5 m in diameter (Centurion, Gauteng)12

Figure 10: Example of a dewatering-type subsidence (Babsfontein, Gauteng) ...... 13 Figure 11: Example of a partly developed sinkhole (Centurion, Gauteng) ...... 13

Figure 12: Subsidence formation in both an ingress and dewatering scenario ...... 14

Figure 13: Distribution of Dolomite in the Gauteng Province ...... 16 Figure 14: Distribution of Dolomite in the Limpopo and Mpumalanga Provinces...... 17

Figure 15: Distribution of Dolomite in the North West Province ...... 18 Figure 16: Distribution of Dolomite in the Northern Cape Province ...... 19

Figure 17: Sinkhole and subsidence occurrence in the Far West Rand...... 22

Figure 18: Sinkhole and subsidence occurrence in the East Rand...... 23 Figure 19: Sinkhole and subsidence occurrences in the area south of Pretoria...... 24

Figure 20: The ‘Sinkhole farm’ in the Wonderfontein spruit valley, Venterspost Compartment ...... 25 Figure 21: The 55 m diameter sinkhole that swallowed the West Driefontein mine crusher...... 25

Figure 22: A sinkhole swallowed a house with a family of 5 in the Blyvooruitzig village...... 26

Figure 23: A sinkhole killed one spectator at Venterspost recreational club, October 1970...... 26 Figure 24: Sinkhole in Laudium during 1970’s (Pretoria, Gauteng)...... 27

Figure 25: Sinkhole in Waterkloof during 1980’s (Pretoria, Gauteng)...... 27

Figure 26: Sinkhole damaging a house in Thaba Tshwane (Pretoria, Gauteng) ...... 28

Figure 27: Sinkhole damaging a house in Lyttelton Manor, 2008 (Centurion, Gauteng)...... 28

Figure 28: Townhouse in Valhalla damaged due to a sinkhole, 2010 (Centurion, Gauteng) ...... 29

Tables Table 1: Suggested classification of sinkholes in terms of size (after Buttrick & Van Schalkwyk, 1995) 6 Table 2: Towns on dolomite...... 20

3 1 Introduction to sinkholes Certain parts of the ground surface of South Africa are prone to sudden, catastrophic collapse which may lead to death, injury or structural damage. Such features are known as sinkholes and in South Africa occur in areas underlain by dolomite rock. Approximately 25% of Gauteng Province, as well as parts of Mpumulanga, Limpopo, North West and Northern Cape Provinces, are underlain by dolomite (Figure 1). This poses a potential risk to the safety of many people and the structures in which they work and live.

Sinkholes are generally circular, up to 125 m in diameter, steep sided and deep (up to 50m). They can occur with little warning; however, cracks in walls or settlement of the ground are often the early warning signs of impending sinkhole formation.

At least 38 people are known to have died over the last 50 years in South Africa due to sinkhole formation. An estimated cost of the damage caused by sinkholes to date is in excess of R1 billion (Buttrick et al., 2001).

2 Mechanism of sinkhole formation 2.1 Weathering of Dolomite Although karst weathering commonly occurred during Karoo to recent times (approx < 250 Ma), there were several much older karst events in the preserved Transvaal basin carbonates (Eriksson and Altermann, 1998). A major karst event, for instance, took place during the time interval represented by the unconformity (c < 2.436 Ga - ≥ c. 2.35 Ga) that separates the Chuniespoort and Pretoria Groups (Martini et al., 1995). Large cavities are not only associated with this contact but also occur at several hundred meters below this level.

The weathering process is well summarised in the Guideline for engineering geological characterisation and development of dolomite land (2003):

Rain water (H 2O) takes up carbon dioxide (CO 2) in the atmosphere and soil (where the concentration of this gas may be up to 90 times greater than in the atmosphere) to form a weak carbonic acid (H 2CO 3). The weakly-acidic groundwater circulating along tension fractures, faults and joints in the dolomitic succession causes leaching of the carbonate

Figure 1. Occurrence of dolomite across South Africa

2 minerals. The solubility of dolomite is high in comparison to other rocks, but significant solution cannot be observed over short periods (months and years).

This process may be represented as follows:

CaMg(CO 3)2 + 2 H 2CO 3 → Ca(HCO 3)2 + Mg(HCO 3)2

The process of dissolution progresses slowly in the slightly acidic groundwater (above and at the groundwater level). The resultant bicarbonate-rich water emerges at springs and is carried away.

The dissolution process results in a vertically zoned succession of residual products, which in turn are generally overlain by younger formations or soils (Figure 2). Hard, unweathered dolomitic bedrock is overlain by slightly weathered jointed bedrock and thereafter, a sudden upwards transition to totally weathered and low strength, insoluble residual material consisting of mainly manganese oxides (wad), chert and iron oxides, that reflect the original insoluble matrix structure. Depending upon the local subsurface structure, this very low strength, porous and permeable horizon may in certain locations be up to several tens of meters thick but is generally less than 10 meters thick.

The passage of geological time concurrent with the downward progression of the intense weathering of the dolomitic bedrock, results in compression of the mass of residual materials overlying bedrock, so that progressive densification of the low strength materials occur. Consequently, the vertical succession of the residual products of weathering reflects an upward increase in strength and a decrease in porosity and permeability.

Due to the described weathering process, overburden consistency reduces with depth. Consequently, higher penetration rates are often noted with increasing depth in drilling investigations, (i.e. higher penetration rates when approaching the dolomitic bedrock). Infiltration of water from leaking services or surface accumulations acts on the low-density material resulting in a loss of support through slumping or subsurface erosion.

Figure 2: Conceptual diagram of typical karst landscape in South Africa (after Waltham and Fooks; 2003)

An example of the result of dissolution of dolomite is shown in Figure 3.

Given sufficient time and the correct triggering mechanisms, instability may occur naturally, but is expedited many orders of magnitude by man’s activities. The primary triggering mechanisms in such instances include:

• The ingress of water from leaking water-bearing services (Figure 4);

• Poorly managed surface water drainage and

• Groundwater level drawdown.

Instability can occur in the form of sinkholes and subsidences.

Figure 3: Dissolution of dolomite bedrock (Lyttelton Quarry, Centurion)

Figure 4: A sinkhole that has formed as a result of a leaking service pipe (Waterkloof, Pretoria)

5 2.2 Sinkhole Formation

A sinkhole is a feature that occurs suddenly and manifests itself as a hole in the ground, sometimes with catastrophic consequences. Buttrick and Van Schalkwyk (1995) proposed a classification of sinkholes in terms of size, as shown in Table 1.

Table 1: Suggested classification of sinkholes in terms of size (after Buttrick & Van Schalkwyk, 1995)

Maximum diameter of surface

manifestation (dimension: meters) Terminology

< 2 Small sinkhole

2 – 5 Medium-size sinkhole

5 – 15 Large sinkhole

> 15 Very large sinkhole

Jennings et al ., (1965) and Brink (1979) described the mechanism of sinkhole formation in detail. They argue that sinkholes can form due to concentrated ingress of water or dewatering. The different processes are shown in Figure 8.

2.2.1 Sinkholes formed under the Ingress Scenario

The mechanism of sinkhole formation in terms of an ingress scenario is briefly summarized as follows:

• Cavities exist within bedrock or the overburden, which may be in a state of equilibrium.

• Active subsurface erosion caused by concentrated ingress water will result in downwards transportation (mobilisation) of materials into the nearest cavity (receptacle).

• Headward erosion leads to successive arch collapse. The last arch may be stable for a considerable length of time and is sometimes supported by a near-surface layer of hardpan ferricrete.

• A triggering mechanism leads to the breaching of the last arch. Particularly in the case of small sinkholes, the cross-section resembles a bottleneck (narrow opening at surface), a shape that may be maintained for some time (Figure 5).

Figure 5: Example of sinkhole that bottleneck, i.e. narrow opening at surface (Atteridgeville, Pretoria)

Jennings et al., (1965) prescribed five concurrent conditions that must exist for sinkhole formation;

1. There has to be adjacent rigid material to form abutments for an arched roof. These abutments are provided by dolomite pinnacles or the sides of a steep sided subsurface canyon. The span has to be appropriate to the strength of the bridging material, since with a span which is too large or with a material which is too weak, the arch cannot form.

2. A condition of arching has to develop in the residuum, i.e. the vertically acting force due to self-weight has to be carried by arching thrusts to the abutments.

3. A void must develop in the residuum below the arch. This void can be quite small.

4. A receptacle has to exist below the arch to accept the material removed in the enlarging void. Some means of transportation, such as flowing water is also essential.

5. When a void of appropriate size has been established in the residuum, some disturbing agency has to arise to cause the roof to collapse. The void will move progressively upwards towards the surface.

Topography and drainage, the natural thickness, engineering properties and origin of the transported soils and residuum, the nature and topography of the underlying strata, the depth and expected fluctuations of the groundwater level, and the presence of structural features such as faults, fractures and dykes are all factors which influence the risk of subsidence taking place.

Figure 6: Large sinkhole (± 15 m diameter) triggered by ingress of water (Centurion, Gauteng)

2.2.2 Sinkholes formed under the Dewatering Scenario (lowering of the groundwater table)

Sinkholes are also triggered by lowering of the water table (Figure 8). The mechanism of this type of sinkhole formation is briefly summarized as follows:

• Cavities exist within bedrock or the overburden which may be in a state of equilibrium. The cavities are occupied by groundwater.

• Lowering of the water table disturbs the equilibrium and active subsurface erosion may be accelerated. Buoyant support within the overburden is also removed, leading to compression of the wad which may lead to collapse.

• Headward erosion finally results in collapse at surface level.

The natural process of sinkhole formation is drastically accelerated by groundwater level drawdown or ‘dewatering’. In the Bapsfontein area for example, which had, prior to 2003, largely been unknown for sinkhole formation, the recorded rapid lowering of the water table during that time led to some 28 sinkholes; one of the largest of these is pictured in Figure 7. On the Far West Rand sinkholes triggered by dewatering are known to have reached diameters of 125 m and depths of 50 meters (Brink, 1979), while sinkholes triggered by ingress are generally smaller.

Figure 7: Large sinkhole (> 50 m diameter) caused by lowering of the groundwater level (Bapsfontein, Gauteng)

Figure 8: Sinkhole formation process in both ingress and dewatering scenario’s

2.3 Subsidence Formation A subsidence is an enclosed depression, which forms as a result of the compression at depth of low- density dolomite residuum. Two main types of subsidence’s can be identified based on the

10 mechanism of formation, namely surface saturation-type and surface dewatering-type subsidence (Figure 12). A third type, which can be referred to as an incompletely developed sinkhole, has a similar surface appearance as the former two types but is caused by the erosion of subsurface materials.

2.3.1 Surface Saturation-type subsidence These subsidences are typically relatively small (i.e. less than 5 m in diameter). The mechanism of subsidence formation in this instance is as follows:

• An area is underlain by compressible dolomitic material at relatively shallow depth with the groundwater level within or below the compressible material. The movement of the groundwater level does not play a role in ground surface movement.

• The surface materials are saturated due to poor water management i.e. poor drainage or a leaking wet-service.

• The wetting front penetrates the surface material and reaches the low-density material.

• A surface depression occurs gradually due to the increased load of the near-surface materials on the deeper lower-density materials, which settles into a denser state because of saturation.

• The movement will generally decrease rapidly when the cause of wetting is stopped.

• The size of the features depends on the profile underlying the saturated area i.e. the thickness, nature and depth of the near surface and deeper lower-density materials, the configuration and depth of the bedrock dolomite and the extent of the saturation (e.g. the extent of the area covered by water, the volume of the water and the length of the period during which saturation occurs).

Figure 9: Example of a surface saturation-type subsidence, < 5 m in diameter (Centurion, Gauteng)

2.3.2 Dewatering-type subsidence

A dewatering-type subsidence occurs gradually and typically manifests itself as a large enclosed depression. The mechanism of this type of subsidence formation is briefly summarized as follows:

• A deeply weathered zone within the dolomite rock is filled with potentially highly compressible material (referred to locally as wad), part of which is submerged below the groundwater level.

• Rapid drawdown of the groundwater level results in exposure of the previously submerged and unconsolidated debris.

• Compression may be excessive and the rate of surface settlement is rapid if a thick succession of wad is exposed by this drawdown.

• The settlement manifests as a depression at surface.

• Surface tension cracks occur in the peripheral areas of differential movement.

Figure 10: Example of a dewatering-type subsidence (Babsfontein, Gauteng)

2.3.3 Partly developed sinkholes The premature termination of subsurface erosion by ingress water may also result in a settlement feature at surface, which appears to be similar to a subsidence.

Figure 11: Example of a partly developed sinkhole (Centurion, Gauteng)

Figure 12: Subsidence formation in both an ingress and dewatering scenario

14 3 Affected areas in South Africa In Gauteng Province, the carbonate formations comprise the Malmani Subgroup of the Chuniespoort Group (Transvaal Supergroup), which is ~ 2600–2400 Ma. The Subgroup is subdivided into various formations of which some are chert-poor and some are chert-rich. The dolomitic formations are, in places, overlain by younger rocks, of the Pretoria Group (2350–2100 Ma), Transvaal Supergroup, and/or the Karoo Supergroup (300–200 Ma), and/or mantled by unconsolidated material of Cenozoic age (≤ 65 Ma).

In Mpumalanga Province, the carbonate formations comprise the Malmani Subgroup (Chuniespoort Group, Transvaal Supergroup). Alteration of dolomite to limestone (de-dolomitization) has occurred in many places due to the intrusion of the Bushveld Complex.

In the North West and Limpopo Provinces, the carbonate formations comprise the Malmani Subgroup of the Chuniespoort Group of the Transvaal Supergroup. Alteration of dolomite to limestone due to the intrusion of the Bushveld Complex is particularly evident north-east of Mokopane (formerly known as Potgietersrus).

In the Northern Cape Province, carbonate rocks mainly comprise the Campbell Rand Subgroup (Ghaap Group, Transvaal Supergroup), which is ~ 2600–2400 Ma. Carbonates can also be found in various Groups of the Gariep Supergroup, which is ~ 700 Ma, near Eksteenfontein, as well as in the Schwarzrand Formation of the Nama Group, which is ~ 550 Ma, near Vioolsdrif.

Figure 13 to Figure 16 shows detailed maps of the each of the areas discussed above.

Figure 13: Distribution of Dolomite in the Gauteng Province

Figure 14: Distribution of Dolomite in the Limpopo and Mpumalanga Provinces 17

Figure 15: Distribution of Dolomite in the North West Province 18

Figure 16: Distribution of Dolomite in the Northern Cape Province

19 3.1 Towns on Dolomite Table 2 below shows all the towns in South Africa that are either completely or partially underlain by dolomite.

Table 2: Towns on dolomite

COMMON NAME NEW NAME

Barberton Umjindi Bo -Karoo Bo -Karoo Burgersfort/ Ohrigstad/Eastern Tubatse Greater Tubatse Municipality

Bushbuckridge Bushbuckridge Municipality Carltonville Merafong City Local Muni cipality Dani ëlskuil Dan -Lime Municipality Delmas Delmas Diamondfields Diamondfields East Rand Greater East Rand Metro Ellisras Lephalale Elukwatini / Carolina Albert Luthuli Ganyesa Kagisano Local Municipality Griekwastad Siyancuma Municipality Groblersdal Greater Groblersdal Municipality Hoedspruit Drankensberg Municipality Hopetown Oranje -Karoo Municipality Johannesburg City of Johannesburg Kathu Gammagara Municipality Kgalagadi Segonyana Municipality Klerksdorp Klerksdorp Local Municipal ity Krugersdorp Mogale City Local Municipality Kuruman Ga - seganyana Local Municipality Lebowakgomo Lepelle -Nkumpi Lichtenbu rg Lichtenburg Local Municipality Mafikeng Mafikeng Local Municipality Marble Hall Greater Marble Hall Municipality Meyerton Midvaal Local Municipality Mogwase Mankwe -Madikwe Local Municipality Nylstroom Modimolle Parys Ngwathe Local Municipality Pietersburg Polokwane Pomfret Molopo Local Municipality Postmasburg Re A Ipela Municipality

20 Potchefstroom Potchefstroom Local Municipality Potgietersrus Mogalakwena Pretoria Tshwane Metroplitan Municipality

Prieska Priemanday Municipality Randfontein Randfontein Local Municipality Reivilo Greater Taung Local Municipality Sabie Thaba Chweu Sasolburg Metsimaholo Local Munici pality Sterkfontein Sterkfontein Thabazimbi Thabazimbi Tzaneen Greater Tzaneen Municipality Ventersdorp Ventersdorp Local Municipality Vereeniging Emfuleni Local Municipality Vryburg Naledi Local Municipality Warmbaths Bela Bela Westonaria Westonar ia Local Municipality Zeerust Zeerust Local Municipality

Quite frequently developments take place on the edge of the dolomite ground, which raises the question as to whether a dolomite stability investigation is required. Dolomite stability investigations are, by definition, required where dolomite bedrock is present up to 60 m below ground surface in a non dewatering scenario and 100 m below ground surface in a dewatering scenario.

4 SINKHOLE AND SUBSIDENCE INCIDENCE IN SOUTH AFRICA

Sinkhole and subsidence occurrence over the past 60 years have been recorded by various, departments, municipalities, consults and companies. Not all occurrences are reported when they occur on private or even public land and therefore only an estimation of the number of events can be given.

Some 2500 sinkhole and subsidence events have been recorded in South Africa. A significant (~98%) majority have occurred in Gauteng. Gauteng province has three prominent areas of sinkhole and subsidence development; the Far West Rand, the area south of Pretoria, and the East Rand.

The other four provinces (Mpumalanga, Limpopo, Northwest and Northern Cape) have comparatively a very low record of occurrence of these events even though they have large surface areas underlain by dolomitic rocks. These karst areas do however have well developed cave systems, indicating the presence of subsurface voids. It is significant to note that these Provinces are not as highly urbanized or as extensively built up (especially on dolomitic land) as Gauteng Province. At least 5 sinkhole events are known between Postmasburg and Kuruman in the Northern Cape and at least 7 sinkholes and subsidences have been recorded in Mpumalanga.

21 4.1 Dewatered areas Several of the groundwater compartments (Venterspost, Oberholzer, Bank and Gemsbok-west) in the dolomite formations on the Far West Rand near Carletonville and Westonaria were dewatered in the 1960’s to allow mining operations on the gold bearing reefs which underlie the dolomite on the Far West Rand, to continue safely (Bezuidenhout & Enslin, 1969). Dewatering lead to accelerated sinkhole and subsidence occurrence on the Far West Rand and experienced approximately instability 1200 events have occurred to date (Heath & Oosthuizen, 2008) (Figure 17).

Figure 17: Sinkhole and subsidence occurrence in the Far West Rand

Dewatering of the Bapsfontein compartment on the East Rand has occurred more recently (2000’s) due to agricultural abstraction (Wagener, 2008), leading to 27 instability events (Figure 18).

4.2 Non-dewatered areas The East Rand (Ekurhuleni District Municipality) has experienced approximately 160 sinkhole and subsidence events (Figure 18) and the area south of Pretoria has experienced approximately 1100 events (Figure 19).

Figure 18: Sinkhole and subsidence occurrence in the East Rand

Figure 19: Sinkhole and subsidence occurrences in the area south of Pretoria

5 CONSEQUENCES OF SINKHOLE AND SUBSIDENCE FORMATION

Signs of damaged structure and infrastructure began in the 1940’s just south of Pretoria (now known as Centurion), mainly in the Military areas.

In the 1950’s due to dewatering of the Venterspost Compartment on the Far West rand accelerated sinkhole and subsidence formation began. Several sinkholes occurred in the Wonderfontein spruit valley which became later known as the ‘Sinkhole farm’ (Figure 20). The Oberholtzer Compartment was the first to experience a catastrophic sinkhole (55 m in diameter and 50 m deep) on 12 December 1962; the West Driefontein mine crusher was swallowed resulting in 29 deaths (Figure 21). Two years later on 3 August 1964 a slightly larger sinkhole swallowed a house with 5 people in the Blyvooruitzig village (Figure 22). The dewatering of the Bank Compartment and the Gemsbok West Compartment lead to more sinkholes and subsidence’s in the 1970’s – 1990’s and the deaths of a further 2 people (one of which occurred as shown in Figure 23).

Sinkholes and subsidence’s also continued to form in the area south of Pretoria (Figure 24 to Figure 28) and 3 people were killed in Centurion in 1970 (Brink, 1996).

Figure 20: The ‘Sinkhole farm’ in the Wonderfontein spruit valley, Venterspost Compartment

Figure 21: The 55 m diameter sinkhole that swallowed the West Driefontein mine crusher

Figure 22: A sinkhole swallowed a house with a family of 5 in the Blyvooruitzig village

Figure 23: A sinkhole killed one spectator at Venterspost recreational club, October 1970

Figure 24: Sinkhole in Laudium during 1970’s (Pretoria, Gauteng)

Figure 25: Sinkhole in Waterkloof during 1980’s (Pretoria, Gauteng)

Figure 26: Sinkhole damaging a house in Thaba Tshwane (Pretoria, Gauteng)

Figure 27: Sinkhole damaging a house in Lyttelton Manor, 2008 (Centurion, Gauteng)

Figure 28: Townhouse in Valhalla damaged due to a sinkhole, 2010 (Centurion, Gauteng)

Not only do sinkholes and subsidence’s cause damage to developments and infrastructure, it is also quite costly to remediate. To date the damage due to sinkholes is estimated in excess of R1 billion in South Africa (Buttrick et al, 2001). The current cost to remediate a sinkhole is in the order of R200 000 to more than R1 000 000, depending on the size of sinkhole.

6 Summary Certain parts of the ground surface of South Africa are prone to sudden, catastrophic collapse which may lead to death, injury or structural damage. Such features are known as sinkholes and in South Africa occur in areas underlain by dolomite rock. This poses a potential risk to the safety of many people and the structures in which they work and live.

1. Approximately 25% of Gauteng Province, as well as parts of Mpumulanga, Limpopo, North West and Northern Province, are underlain by dolomite.

2. At least 39 people are known to have died over the last 50 years because of sinkholes in South Africa. An estimated cost of the damage caused by sinkholes to date is in excess of R1, 3 billion.

3. Given sufficient time and the correct triggering mechanisms, instability (sinkholes and subsidences) may occur naturally but is expedited many orders of magnitude by man’s activities. The primary triggering mechanisms in such instances include:

• The ingress of water from leaking water-bearing services;

• Poorly managed surface water drainage and 29 • Groundwater level drawdown.

4. Some 2500 sinkhole and subsidence events have been recorded in South Africa to date. A significant (~98%) majority have occurred in Gauteng Province.

5. Gauteng province has three prominent areas of sinkhole and subsidence development; the Far West Rand, the area south of Pretoria, and the East Rand, which have experienced approximately 1200, 1100 and 160 instability events respectively.

6. The other four provinces (Mpumalanga, Limpopo, Northwest and Northern Cape) have comparatively a very low record of occurrence of these events even though they have large surface areas underlain by dolomitic rocks. At least 5 sinkhole events are known between Postmasburg and Kuruman in the Northern Cape and at least 7 sinkholes and subsidences have been recorded in Mpumalanga.

7. It is significant to note that these Provinces are not as highly urbanized or as extensively built up (especially on dolomitic land) as Gauteng Province.

7 References

1. Altermann, W. and Nelson, D.R. 1998. Basin analysis, sedimentation rates and regional correlations as implied by precise U-Pb zircon ages from volcanic sediments of the Neoarchean Campbellrand Subgroup of the Kaapvaal Craton. Sedimentary Geology. Vol 120/1-4, pp. 225-256.

2. Bezuidenhout, C.A. and Enslin, J. F., 1969. Surface subsidence and sinkholes in the dolomitic area of the Far West Rand Transvaal, Republic of South Africa. International Symposium on land subsidence, Tokyo, September 1969.

3. Brink, A.B.A., 1979. Engineering Geology of South Africa , Vol.1, Building Publications, Pretoria.

4. Buttrick, D.B. and Van Schalkwyk, A., 1995. The method of scenario supposition for stability evaluation of sites on dolomitic land in South Africa. Journal of South Africa Institution of Civil Engineers, 37(4), pp. 4-14, 1995.

5. Buttrick, D.B., Van Schalkwyk, A., Kleywegt, R., and Watermeyer, R.B. 2001. Proposed method for dolomite land and hazard and risk assessment in South Africa. Journal of South Africa Institution of Civil Engineers, 43(2), pp. 27-36, 2001.

6. De Bruyn, I.A., & Bell, F.G., 2001. The Occurrence of Sinkholes and Subsidence Depressions in the Far West Rand and Gauteng Province, South Africa, and Their Engineering Implications. Environmental & Engineering Geoscience , Vol, VII, No.3, August 2001, pp. 281 – 295

7. Council for Geoscience/ South African Institute of Engineering and Environmental Geologists, 2003. Guideline for engineering geological characterization and development of dolomitic land.

8. Heath, G.J and Oosthuizen, A.C., 2008. A Preliminary overview of the sinkhole record of South Africa. S.A.I.C.E Conference, Problems Soils in South Africa. Midrand, 3-4 November 2008.

30 9. Jennings, J.E., Brink, A.B.A., Louw, A., Gowan, G.D 1965. Sinkholes and subsidences in the Transvaal dolomite of South Arica. In; Proc 6 th International conference of soil Mech. And found. Eng., Montreal, pp 51-54, 1965.

10. Wagener, F von M., 2008. Sinkholes in the Bapsfontein dolomite water compartment caused by dewatering. S.A.I.C.E Conference, Problems Soils in South Africa. Midrand, 3-4 November 2008.

11. Waltham, A.C. and Fooks, P.G. 2003. Engineering Classification of karst ground conditions. Quarterly Journal of Engineering Geology and Hydrogeology. Volume 36 2003 pp.101-118.