Site characterization of the area surrounding a sinkhole, a case study

In 2014, a sinkhole occurred below a fuel pipeline at Tarlton International Speedway, near Johannesburg in South Africa. Following the sinkhole collapse, visual inspections were conducted to determine the direction of the throat and the probable causes of the sinkhole. Emergency backfilling of the sinkhole was undertaken, followed by site characterisation of the area comprising geophysical gravity surveys and percussion borehole drilling. The boreholes were drilled on the gravity highs and lows as per the results of the gravity surveys. The gravity survey was able to establish the shallow and deep weathering profile of the dolomite rock head, but was unable to define the cavity or subsurface receptacle. Additional boreholes were drilled in the perceived visual direction of the sinkhole throat observed from the initial inspection. This drilling defined a cavity that extended beyond the surface opening of the sinkhole. The investigation confirmed the importance of using more than just one method of ground investigation to compile an accurate geotechnical model of the site. The true extent of the subsurface cavity could only be discovered through exploratory drilling based on observations of the throat itself soon after the sinkhole was discovered. Had the cavity, that extends beyond the surface opening of the sinkhole, not been identified, it would have resulted in an underestimate of the extent and cost of the remedial works.