TIME-DEPENDENT BEHAVIOR OF HARD-ROCK IN DEEP-LEVEL GOLD-MINES

In deep level hard rock mining the rheological behaviour of the rock i s important as time-dependent aseismic processes can lead to a stable dissipation of energy thus reducing violent outbursts. Underground clo sure measurements, observations of time-dependent fracture formation a nd siesmic data indicate that the rock in deep South African gold mine s shows significant time-dependent behaviour. The objective of this wo rk is to quantify the time-dependent rock behaviour for laboratory spe cimens and underground conditions. Laboratory experiments indicated th at lava and quartzite specimens undergo measurable creep strain at str ess magnitudes below the failure stress. Quartzite is however prone to more significant creep at lower stress levels. A viscoelastic analysi s showed that: laboratory creep results can not fully explain the beha viour observed underground. The in situ behaviour is governed mainly b y the rheological behaviour of the fracture zone and the resulting tim e-dependent formation of new fractures. A viscoplastic discontinuity m odel implemented in a boundary element program is used to simulate thi s behaviour.