The identification and mitigation of geohazards using shallow airborne engineering geophysics and land-based geophysics for brown- and greenfield road investigations

South Africa is a mineral rich country with a diverse geology and a long history of mining. Historical mining records have not been accurately recorded, or lost with time. This has resulted in significant geohazard risk during infrastructure development, especially in and around historical mining towns, e.g. Johannesburg and Ermelo. These geohazard risks require careful appraisal and quantification prior to any infrastructure design or construction. This case study aims to set out the development aspects of the Multi-Faceted Geophysical Modelling Systems approach used by the South African National Roads Agency SOC Ltd (SANRAL), in an investigation of undermined ground for the historical mining town of Ermelo for the N11/N2 ring road. The systems approach used a combination of airborne geophysics (Versatile Time Domain Electromagnetic System (VTEMTM and magnetics), generally used in mining exploration, land-based and borehole geophysics, borehole water testing, and ground-truthing. The systems approach developed used a continuous, iterative approach, building on the data at hand, thus reducing unnecessary investigations while eliminating the possibility of anomalies being missed, as in the case of the conventional discrete drilling. The investigation ensured that 100% of the route was comprehensively investigated with a high confidence in the geological and geophysical data, and concomitant mitigation of infrastructure risk. The Multi-Faceted Geophysical Modelling Systems approach was successfully used to identify a previously unknown 1m x 1m mining stope cavity at 90m depth and a 3m x 5m access tunnel at 24m depth in a timely and cost-effective manner. Seven RC percussion boreholes carried out confirmed the structural integrity of these underground cavities, as well as the structural geology along the centreline. Based on the great success achieved in identifying shallow anomalies, this Multi-Faceted Geophysical Modelling Systems approach is now being considered for field trails on the Dolomitic formations and the Wild Coast greenfields road project where there are large historical slumps and many fault lines. South Africa's rich history of mining activities includes the extraction of coal from the Ecca Group Sediments of the Karoo Supergroup (250 Ma), gold and uranium from the Witwatersrand Supergroup (2900 Ma), as well as platinum, uranium, tin and lead from the layered Bushveld Igneous Complex (BIC)(2150 Ma). The extraction of gold, copper, tin, lead and rare earth minerals also took place in the Archaean Rocks of Swazium Age (3000-3500 Ma). In this case study, SANRAL planned a ring road around the coal-mining town of Ermelo (South Africa), to ensure mobility between major routes, whilst maintaining town access.