The effect of geotechnical conditions on the time-dependent behaviour of hard rock in deep mines

Measurements in the deep tabular excavations of the South African gold mines indicate significant time-dependent deformation. Although the host rock consists of hard brittle material, creep-like movements of the order of 0.6 mm/h have been observed in certain areas. This paper describes measurements of time-dependent deformation in three different geotechnical environments. The time-dependent behaviour is the result of the rheology of the mining-induced fracture zone surrounding the excavations at these depths, while pre-existing discontinuities such as bedding planes also play a prominent role. It appears that detailed monitoring of stope closure profiles can provide a useful signature of the geotechnical response of the rock mass to the mining process. The closure profiles are simulated using a displacement discontinuity framework that allows for the time-dependent failure processes. Preliminary studies indicated that the initial jump in stope closure after blasting is correlated with the stope face stress.