Sinkhole and subsidence record in the Chuniespoort group dolomite, Gauteng, South Africa

Abstract

Karst related sinkholes and subsidence‟s occur on areas underlain by Chuniespoort Group dolomite bedrock in the Gauteng Province. Dolomite land occurs across several South African provinces; however sinkhole and subsidence formation in Gauteng is more well-known than in of the other provinces. Thousands of sinkhole and subsidence events have occurred in the past 60 years. In the past, data on sinkhole and subsidence occurrence has been amassed separately by various consultants, companies and state authorities. There is currently no legal requirement for sinkhole and subsidence events to be reported to a central authority, yet this data is crucial for future assessment of sinkhole hazards and decision making. This study focuses on the dolomitic land areas within four Gauteng municipalities, vulnerable to karst related sinkhole and subsidence formation. Historical as well as current information regarding the sinkhole record for Gauteng was compiled from various sources to develop as comprehensive an inventory of the study area as possible. The importance of sinkhole inventories are reviewed as well as the expediency and efficiency of Geographic Information Systems in data capturing and viewing. Data originates from numerous sources and compiling a comprehensive database presented many challenges, most importantly the large percentage of missing data that could not be retrieved and that the format and quality varies. Only karst related sinkhole, subsidences and crack events prior to 31 December 2011 were considered in this study. The data compiled is only an estimation of the number of events that has occurred in Gauteng. Once all the available sinkhole and subsidence data was collected and compiled, it was organized into multiwave frequency tables and various aspects were analyzed. The data collected, although limited in some cases, is used in statistical analysis to investigate the relationship between the formation of sinkholes and subsidences and underlying geology, size distributions, frequency of events and external influences. Results indicate that to date just over 3000 events (sinkholes, subsidences and ground cracks) are recorded within the study areas, and:  Sinkholes and subsidences are still regularly occurring in areas underlain by dolomite in Gauteng, however based on available data, events in the West Rand and Tshwane appear to show decreasing trend over the last decade.  More events occur in high rainfall months or years (due to increased ingress water entering the ground profile).  The most dominant type of event recorded is sinkholes.  Overall the largest percentage of events has occurred on the chert-rich Monte Christo Formation and Eccles Formation.  Triggering mechanisms were considered for the different areas: on the West Rand most occurrences before 1984 were due to dewatering while after 1984, most are attributed to ingress. The largest percentage of events on the West Rand has occurred in the Oberholzer Groundwater Compartment. In Tshwane almost all occurrences can be attributed to ingress, while in Ekurhuleni, just under a quarter of events were identified as due to dewatering.  When considering sinkhole and subsidence size and depth distributions; the largest percentage of sinkholes in the West Rand (>60%) are large to very large (i.e. from greater than 5m to greater than 15m diameter), the largest percentage of sinkholes in Tshwane (>60%) are medium to large (i.e. from greater than 2m to less than or equal to 15m diameter) and the largest percentage of events in Ekurhuleni (>70%) are small to medium (i.e. less than or equal to 5m diameter).  When considering size distribution on the different formations, it was not possible to determine if certain sizes were more prevalent for specific formations.