--> An Integrated Approach to Understanding Different Geotechnical Zones Using High Resolution Microseismicity in Deep Underground Mines, South Africa

AAPG ACE 2018

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An Integrated Approach to Understanding Different Geotechnical Zones Using High Resolution Microseismicity in Deep Underground Mines, South Africa

Abstract

Mining-induced seismicity and rockbursts pose a risk to mine infrastructure, ore extraction operations and workers in deep and highly stressed mines such as the Cooke 4 shaft in South Africa. The shaft pillar is prone to large seismic events and rockbursts, which may cause fatalities and loss of production. Seismicity and rockbursts result from high levels of stresses, which exceed rock strength and lead to rock failure, which is often violent and emit elastic waves.

This makes the rock composition very important when taking decisions of the preferred pillar composition in underground mines. 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 soft lavas.

All rock samples (excluding lavas) that were tested failed in a brittle manner, especially the quartzite and pebbly quartzite. This suggests that seismic activities could be highly anticipated. The laboratory tests showed that quartzite has the strongest uniaxial compressive strength (UCS), followed by pebbly quartzite, argillaceous quartzite and lastly conglomerate.

Different lithologies exhibited specific rockmass mechanical behaviour, and this was expressed by rock-specific mining-induced fracturing patterns in the hangingwall, mainly due to the presence of the interbedded weak Ventersdorp lavas. Using high-resolution acoustic emission sensors deployed underground, rock-specific mining-induced fracturing patterns in the hangingwall were delineated, with moment magnitudes down to Mw -5. These fracture patterns correlate positively with fracture models proposed by Roberts and Schweitzer (1999) for different geotechnical zones defined by footwall/hangingwall rock assemblages.

Majority of these acoustic emissions were found to be associated with the mining stope faces. These acoustic emission clusters delineated Ortlepp shears forming ahead of the stope caused by the excavation-induced stress field. This interpretation is supported by underground damage observations, core sample analysis which showed ubiquitous discing, and local stress measurements of 127 MPa made by Ogasawara et al. (2014).