[S02-2] World overview of anthropogenic seismicity II

[S02-2-03] Integrated Petrographic, Geomechanical and Seismological studies of rockmass behaviour during the final phase of ore extraction at Cooke 4 shaft in South Africa

^○Siyanda Mngadi^{1, 6}, Raymond Durrheim^{1, 6}, Halil Yilmaz^1,6, Musa Manzi¹, Thabang Kgarume^2,6, Jan Kuijpers², Tony Ward^3,6, Dave Roberts^1,3,6, Makoto Naoi^4,6, Hiroshi Snr Ogasawara^5,6, Akimasa Ishida⁵, SATREPS⁶ (1.The University of the Witwatersrand, Johannesburg, South Africa, 2.CSIR, Johannesburg, South Africa, 3.Sibanye Gold, Johannesburg, South Africa, 4.Kyoto University, Kyoto, Japan, 5.Ritsumeikan University, Kyoto, Japan, 6.JST-JICA Science and Technology Research Partnership for Sustainable Development (SATREPS), Pretoria, South Africa)

invited

Seismic events, rockbursts and fault instabilities are prime concerns during the final phase of ore extraction at the deep (>1km) Cooke 4 shaft in the West Rand gold mining district, South Africa. This research aims to understand the seismic and mechanical behavior of the rockmass during the shaft pillar extraction operations by studying petrography, geomechanics and stress model, and to establish correlations with monitored microseismicity and underground mapping. The composition of the shaft pillar, observed through underground mapping and core sample analysis, was found to be quartzite, pebbly quartzite, argillaceous quartzite and conglomerate. Forming the roof of the hangingwall is the Ventersdorp Contact Reef (VCR) and Ventersdorp lavas, soft lavas. This affects the ore excavation operations and the stability of the pillar. Rock mechanical laboratory tests were restricted to rocks forming the shaft pillar, and the tests showed that quartzite has the strongest uniaxial compressive strength (UCS), followed by pebbly quartzite, argillaceous quartzite and lastly, meta-conglomerate, making quartzite the preferred rock type in pillars in terms of strength. Different lithologies exhibited specific mining-induced fracturing patterns in the hangingwall, mainly influenced by weak Ventersdorp lavas. Majority of microseismic events were found to be associated with the mining fronts. These seismic clusters delineated Ortlepp shears forming ahead of the stope owing to the excavation-induced stress field. This interpretation is supported by underground damage observations, stress model and ubiquitous discing in core samples. The acoustic emissions (AEs) were also concentrated in the vicinity of service excavations (tunnels). Some AE clusters were located away from excavations, likely delineating faults. In conclusion, microseismicity is largely influenced by structural discontinuities, ore extraction operations, and rock geomechanical properties in underground conditions.