Comparison of sinkhole size versus borehole hazard classification

Sinkholes are a common occurrence on dolomitic land in South Africa. Different methods have been used in the past to classify the risk of sinkhole occurrence. The inherent hazard classifications (IHC) of boreholes are compared in this article to actual sinkhole occurrences. The national sinkhole database and the ENGEODE database of boreholes, both belonging to the Council for Geoscience (CGS), were used for this comparison between the IHC of the borehole and the size of the sinkhole that occurred within a 50 m radius. Thus, the sinkholes in the database were used for this comparison that had a borehole located within 50 m from the sinkhole. Four regions were identified and described separately, namely the Far West Rand, the Tshwane and Ekurhuleni Municipalities as separate areas, and the fourth group as the Johannesburg and Sedibeng Municipalities and the Mpumalanga, North West and Northern Cape Provinces. Although a very high percentage has been underestimated (ranging from 42% to 59% for the different regions), there is a relatively good IHC correlation between the hazard classification of the boreholes and the sinkholes (between 56% to 67%). Another problem is that an IHC of 1 to 4 (low to medium risk) was assigned to the boreholes in 17% to 29% of the cases for the different areas, yet sinkholes still formed. These areas are more problematic, since the risk from the drilling has been underestimated. The current methodology to classify boreholes has not proved to be one hundred percent accurate to predict the size of the sinkhole for the area, and several factors have been highlighted that should be taken into account when boreholes are classified. Further research needs be carried out around the discrepancies between the IHC of boreholes compared to actual sinkholes.