ABSTRACT:

A programme was set up to determine the response of a backfilled stope to seismic activity in a South African gold mine. Ground motion was measured at two triaxial geophones which were attached to the hangingwall of the stope in dip gullies. One of the dip gully geophone sites had backfilling on both sides of the gully and the other had backfilling on only one side. Twenty-two events, each with a magnitude, a location and recorded waveforms were selected from the data base for analysis. The findings were:

  1. In general ground motion is damped in the presence of backfill.

  2. Backfilled stopes experience higher dominant frequencies than unfilled stopes.

  3. With insufficient backfill to generate high stresses in the rock the seismic waveforms are out-of -phase.

  4. The zone of low and tensile stresses above the gully is restricted by the placement of backfill.

  5. The layout of backfill can cause local amplification of the waveforms.

INTRODUCTION

An understanding of the dynamic behaviour of the rockmass surrounding a stope during seismic events is important for designing support to control rockburst damage. As rock burst damage is caused by seismically induced ground motion which destroys the stability of the excavation, analysis of the response of mining excavations to non-damaging seismic events can provide invaluable insights into damage mechanisms.

Observations of rockburst damage in stopes with filled and unfilled panels suggest that backfilling has a positive effect on maintaining the integrity of the stope (Castelyn 1988). Documentation of rockburst damage observations in backfilled stopes by Gay et al (1988), indicates that the backfill maintains the h.angingwall of the stope and contains damage in the stope faces and gullies. However, observations underground are subjective and limited, since not every part of a stope is visible, especially after severe seismic damage. It is also not always clear what damage is due purely to the rockburst and what is due to lack of cleaning after a blast. In addition, the variation in geology, fracturing and support and the error in event location relative to the stope may make comparisons of rockburst damage difficult.

Spottiswoode and Churcher (1988) observed that ground motion in a deep-level conventionally supported stope during seismic events was profoundly different to ground motion measured at points in the rockmass remote from the stope. They predicted that backfill would have the following effect on stopes during seismic events:

  1. Reduce the loosely supported spans in the stope.

  2. Increase the resonance frequency of the hangingwall of the stope.

  3. Reduce absolute vibration levels by increasing the mass of the rock immediately adjacent to the stope.

  4. Reduce differential movements between the hanging wall and the footwall and thus reduce deformation of support units.

The first three predictions formed the basis for an evaluation of seismic data collected at a backfill site established on a deep South African gold mine. The last prediction was not assessed because geophones were not positioned on the footwall of the stope.

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