karst subsidence and associated risk€¦ · into these caves or voids resulting in catastrophic...
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2014/02/03
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U R B A N H Y D R O L O G Y : WAT E R I N T H E C I T Y O F T S H WA N E H i g h P e r f o r m a n c e C e n t r e , U n i v e r s i t y o f P r e t o r i a
2 3 – 2 4 J a n u a r y 2 0 1 4
T H A R I N A O O S T H U I Z E N C O U N C I L F O R G E O S C I E N C E
KARST SUBSIDENCE AND ASSOCIATED RISK
WHAT IS DOLOMITE
• A carbonate rock type(2,3 billion years old), which dissolves (very slowly) to form cavities which leads to sinkhole formation
• Highly variable over short distances
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• Dolomite is soluble, i.e. dissolves in water.
• The dissolution of the dolomite has given rise to cave systems and voids in the rock. Soils covering the rock can collapse into these caves or voids resulting in catastrophic ground movement on the surface such as sinkholes or subsidences.
• Results in wad: a highly compressible and erodible material immediately above bedrock surface
DISSOLUTION OF DOLOMITE
IMPLICATIONS OF DOLOMITE
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Pretorius Avenue - 2013 The Pines (Jean Avenue) - 2013
Lyttelton Manor - 2008
Pretorius Avenue - 2012 Jean Avenue - 2011
Alto Complex (Basden Street) - 2008
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DISTRIBUTION OF DOLOMITE IN SOUTH AFRICA
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DOLOMITE IN TSHWANE
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SINKHOLE FORMATION
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Karst landscape
2 Main Triggering Mechanisms: • Ingress water (leaking services, poor stormwater management, etc.) • Lowering of the groundwater level
HOW DO THEY FORM?
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SINKHOLE
SUBSIDENCE
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SINKHOLES IN TSHWANE
~1400 sinkholes /
subsidences to date
98% related to
man’s influence
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SINKHOLES ARE:
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INCREASED SINKHOLES ARE RELATED TO INCREASED DEVELOPMENT
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Smallholdings -1950’s
Multi storey - >2010’s? Townhouses-1990’s
SINKHOLE STATISTICS
• 3 people have died as a result of sinkholes (during remediation thereof)
• >60% of the sinkholes are sized between 2 m and 15 m in diameter
• >60% of the sinkholes are less than 5 m deep • Average of 7,58 events per km2 • 0,13 sinkholes / km2 / year • 8 residential dwellings demolished in central Centurion area • No sinkholes have formed as a result of dewatering
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HOW DO WE INVESTIGATE?
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Hard Rock
Soil Void
CLASSIFICATION OF DOLOMITIC LAND
• Using the Method of Scenario Supposition (Buttrick et al., 2001) • Based on the properties of the
overburden • General assumption – the deeper the
dolomite bedrock, the larger the expected sinkhole
• Classify a site for both ingress and a dewatering scenario
• 8 Inherent Hazard Classes (IHC)
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IHC TABLE
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CLASSIFICATION
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Site specific Regional
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APPROPRIATE LAND USE
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• Based on the SANS 1936-1(2012) documents
• Dolomite Area Designation
(D-designation) assigned to each site, which specifies the required precautionary measures
Residential development in Centurion
PRECAUTIONARY MEASURES
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• Although there is always a risk in dolomitic areas, the risk can be managed by the implementation of appropriate precautionary measures.
Raft Foundations
Storm water management
Concrete aprons
HDPE Pipes
Gutters & Downpipes
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CONCLUSION
• In Tshwane, all sinkholes are caused by water ingress
• Generally a low hazard in terms of a dewatering
scenario • Groundwater relatively deep, and mostly within dolomite
bedrock • Only limited areas where a high hazard in a dewatering
scenario has been identified
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THANK YOU
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Tharina Oosthuizen Council for Geoscience
toosthuizen@geoscience.org.za
012 841 1160
www.geoscience.org.za
Never R.I.P…..
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