The Influence of Footwall Rock Materials on the Rock Mass Response to Seismicity at Mponeng Gold Mine

Abstract

Mponeng Mine is currently the deepest mine in the world, mining at a depth of 3500 m below the surface. The active mining stopes are in a very high-stress environment, and the mine is highly seismically active. Seismicity has been the main driver of the fall of ground incidents recorded at the Mponeng Mine over the years. The seismically induced falls of ground have contributed to multiple fatal accidents and loss of production. Between 2013 and 2023, there were 315 seismic related accidents, of which 13 resulted in fatalities. Of the 13 fatalities, 12 were in the stopes and one was in an off-reef development excavation. There are two footwall zones at the mine: quartzite on the west and shale to the east. Of the 12 fatalities in the stopes, 8 were in the quartzite footwall zone and 4 in the shale footwall zone. Some conclusions were drawn following laboratory strength analyses of the two different footwall rock types at Mponeng Mine. The mechanical properties of the lava hanging wall are almost constant throughout the mine. However, the footwall rock types are different. The mechanical properties vary significantly between the quartzite and shale footwalls. In particular, the quartzite footwall is significantly more brittle than the shale. The different footwall properties play a role in the stress distribution around the stopes, which influences the ground conditions of the hangingwall and footwall. The hangingwall above in the quartzite footwall was significantly more fractured than in the areas with a shale footwall. The quartzite footwall was also highly fractured and undulating. At the current depth of mining, the hangingwall was expected to be highly fractured, except where the face was de-stressed by pre-conditioning or over-stoping. Preconditioning in both footwall zones was done properly, but the deeper fracture distance in the shale footwall contributed to a better ground condition. The quartzite footwall zone generated a high number of small magnitude events ahead of the face with higher damaged volume, while the shale footwall zone generated a high number of larger magnitude events with far less damage. However, Map3D elastic numerical modelling software showed a higher energy release rate in the shale area. The softer Booysen’s shale appears to absorb more strain energy during static and dynamic loading than the quartzite zones. The shale is also more prone to creep, allowing closure and reducing fracturing in the hangingwall. The quartzite material fractures more easily, generating smaller magnitude events. The highly fractured rock in the quartzite zone suffers greater shakedown damage during the vibrations caused by a seismic event. Based on the findings of this research, the support resistance in Booysen’s shale footwall stopes may be lower and the extraction ratio higher than in areas with a quartzite footwall.