ABSTRACT:

Seismic events near the stope face of six longwalls at Western Deep Levels Limited are discussed in terms of their relation to blasting time. First, the time period is established during which seismic events can be regarded as directly blasting-induced. Later, the different seismicity levels of the six longwalls are discussed. Finally, the ratio of the number of seismic events, which occurred during the blast and after the blast is evaluated for several magnitude categories. Longwall geometry, geological discontinuities of large and small scales did however result in anomalies such as abnormal concentrations of rockbursts during and outside blasting time. It is also shown that seismicity levels of longwalls, which advanced at an oblique angle to geological features, were strongly reduced when compared with other longwalls. The potential of production blasts to trigger impending seismic events under certain conditions becomes apparent.

INTRODUCTION

The gold mine Western Deep Levels Limited (WDL) is situated in the Witwatersrand Basin approximately 75 km south-west of Johannesburg. The lease area totals nearly 45 km2, extending 11km on strike and about 4km on dip. Mining operations began in 1957 with shaft sinking operations. The first gold pour took place in 1962. Two economic gold-bearing reefs, the Ventersdorp Contact Reef (VCR) and the Carbon Leader Reef (CLR), are extracted (Fig. 1). The VCR is worked between 1500m and 2300m below surface, dipping on average 21 degrees South-east and sub-outcrops in the north-west. About 900 m below the VCR lies the CLR horizon which is continuous over the whole lease area. The VCR consists of a conglomerate with a great variation in pebble sizes and channel width Which exceeds 2,5 m in places ("reef roll"). The CLR is formed by a narrow, carbon-rich conglomerate with a channel width of a few centimetres.

For the exploitation of the VCR both, No.2 and No.3, shafts were sunk in the northern part of the lease area to 1930 m below surface. To gain access to the CLR sub-vertical shafts were sunk down to 2975 m below surface. Further development in the form of tertiary vertical shafts was necessary to enable mining in the lower sections of the CLR-horizon.

(Figure in full paper)

The mine adopted a longwall mining system in which approximately 75 % of alliongwalls are protected by systematic stabilizing pillars. Mini-longwalls consisting of six panels, with a total length of about 200m on dip, are separated by 40 m wide strike stabilizing pillars. Stabilizing pillars were introduced in 1980 to address the eminent rockburst problem of the late 70's. Since 1987 backfill in the form of classified tailings is added in some areas to improve the regional support.

The reef is extracted conventionally by drilling and blasting. The broken rock is scraped to boxholes which lead to haulages in the footwall. These haulages ("follow-behinds") are developed some distance behind the actual face-position to avoid high field stresses.

Both horizons are intersected by a number of dykes which are mainly north east - south west orientated.

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