Risk Assessment from Historical Coal Undermining to the Ermelo Ring Road using Helicopter TDEM and Ground Geophysics


Mr Jean Legault, Mr Thomas Wade

The South African National Roads Agency Ltd (SANRAL) appointed AECOM SA (PTY) Ltd (AECOM) in August 2014 to investigate the viability of constructing the new proposed ring road around the town of Ermelo, in the Mpumalanga Province, South Africa. A portion of the new proposed route was affected by previously undermined areas around the north-eastern part of the town. A decision was taken to evaluate the planned route across the undermined area for old mine voids (tunnels and cavities) using airborne and ground geophysics, followed by drilling. Experience with old coal undermining areas indicated that voids can potentially be filled with Acid Mine Water (AMW). AMW’s low pH makes it conductive. The air-layer above it is resistive. VTEM helicopter TDEM and aeromagnetics were employed to survey the large planned area swiftly. This was followed by ground TDEM, electrical resistivity tomography (ERT) and gravity. The results were used to guide drilling to verify undermining targets, which was successful. The airborne TDEM geophysical investigation showed that the data across a major section of the proposed route shows no anomalies. However, five areas of interest were identified (A1 to A4) and were surveyed in detail with the ground geophysical methods and subsequent borehole drilling. Modelling showed that large portions of the surveyed area are not anomalous and are therefore inferred to be unaffected by shallow undermining. This was confirmed by borehole investigations. In contrast, detailed investigative work resulted in the detection of undermining, specifically in borehole BH05, where a predicted void of 5m was intersected at 26m. Not all the anomalies detected were due to undermining, but are instead related to geological structure, such as dykes and faults. All of this information needed to be evaluated to determine whether the proposed route was safe from undermining. This culminated in a risk assessment of the probability undermining on the proposed route.