Observational studies of the rock mass response to mining in highly stressed gold mines in South Africa

Abstract

A multidisciplinary study of the response of the rock mass to mining and mining-induced earthquakes has been conducted in six gold mines in South Africa at sites deeper than 3 km, or with equivalent stress. More than 80 holes (with a total length of more than 2.8 km) were drilled in earthquake-prone areas to locate faults and install instruments. Microfracturing activity associated with an ML2.1 event at 3.3 km depth in Mponeng Mine and a highly stressed rock mass at 1 km depth in Ezulwini Mine were finely delineated and analysed. A hole drilled through the hypocentre of the ML2.1 event at Mponeng Mine allowed the stress and strength in the seismogenic area to be constrained. Previously published in situ stress measurements had been limited to depths or stresses smaller than 2.7 km or 100 MPa, respectively. We successfully measured stress at depths and stresses up to 3.4 km and 146 MPa, respectively, at four mines. These in situ measured stresses were used to calibrate elastic stress modelling and yielded better estimates of stress and strength on the rupture planes of seven earthquakes (ML2.1 to 4). Comparison of the elastically modelled stress with the constrained stress in a hole drilled across the ML2.1 fault and the strain change monitored in situ at a close distance showed that the elastically modelled stress was smaller but correctable. The South African National Seismograph Network was enhanced by installing 10 surface strong-motion seismometer stations in the Far West Rand mining district. Two other such dense surface networks are https://papers.acg.uwa.edu.au/p/1410_06_Ogasawara/ Observational studies of the rock mass response to mining in highly stressed H Ogasawara et al. gold mines in South Africa 124 Deep Mining 2014, Sudbury, Canada currently operated in mining areas by the Council for Geoscience: 25 stations in the Klerksdorp region, supported by the Mine Health and Safety Council; and 17 stations in the Central Rand to monitor fluid-induced seismicity. Research on the routine processing of spectral parameters was carried out using seismicity recorded by these 52 new surface stations.

Has research and development contributed to improvements in safety and profitability of deep South African mines?

“…To our knowledge it is the most ambitious observational mine-seismology research project ever undertaken in terms of the number of sites and sensors and the scope of research. The project is only briefly described here, as it is the subject of another paper in this volume (Ogasawara et al 2014).…”

2019 status report: Drilling into seismogenic zones of M2.0–M5.5 earthquakes in South African gold mines (DSeis project)

“…Built on the outcomes of the Japanese -South African seismological collaboration since 1992, Japanese and South African researchers and six deep South African gold mines collaborated during 2010-2015 in a multidisciplinary study of the response of the rock mass to mining and mining-induced earthquakes particularly the 2014 M5.5 Orkney earthquake below the mining horizons at the Moab Khotsong mine. See Figure 1, Ogasawara et al, 2014a andDurrheim, 2015. Hickman et al(2007).…”

Analysis of the Gutenberg-Richter b-values of overlapping seismic clusters with application to Cooke 4 gold mine

“…Hudyma and Potvin (2004) developed a seismic hazard scale specifically for underground hard rock mines, and the b-value analysis can play an important role in this seismic hazard assessment. More recently, a Japanese-South African collaborative research project (Ogasawara et al, 2014), carried out high-sensitivity monitoring of seismicity produced during the extraction of a shaft pillar in a deep Witwatersrand gold mine, and, among many others topics, studied the b-value . This will be discussed in more detail in the Results section of this paper.…”