Sinkhole formation over hard rock mining areas and its risk implications

Discontinuous mining subsidence over hard rock orebodies may pose geotechni cal and safety hazards through the formation of sinkholes. Most sinkholes f orm following the collapse of a surface crown pillar above mined-out stopes . Nine case studies of sinkhole occurrences in Western Australia and the No rthern Territory of Australia were analysed to provide data for the risk ma nagement of sinkhole formation. The conditions in which sinkholes formed in cluded where an orebody dipped steeply, where underground openings had a la rge open span and where the surface crown pillars comprised oxidized rock m aterial. All the collapsed surface crown pillars were in oxidized zones and they had thicknesses of up to 60 m. Sinkholes formed between 10 and 60 yea rs after mining of the stopes. A relationship is shown to exist between the length of the unsupported, open spans of underground excavations and the d iameters of a surface projection of the sinkholes.