Time-dependent behaviour of deep level tabular excavations in hard rock

Abstract

Although hard rock is not usually associated with large creep deformation, significant time-dependent behaviour is observed in the tabular excavations of the South African gold mines. Time-dependent closure data was collected in stopes of the Ventersdorp Contact Reef and Vaal Reef. This data typically consists of a primary closure phase after blasting, followed by a steady-state closure phase. This closure behaviour is the result of the rheology of the fracture zone around these excavations and the time-dependent extension of this zone following a mining increment. An elasto-viscoplastic approach was developed to simulate the time-dependent nature of the fracture zone. This model proved successful in simulating the experimental closure profiles. It appears that the closure data may provide useful diagnostic information of the stress conditions in the fracture zone ahead of the face. This may possibly be used to identify hazardous conditions such as areas prone to face bursting. The effect of preconditioning on the time-dependent closure behaviour is also illustrated. This paper investigates the time-dependent behaviour of the tabular excavations in the deep South African gold mines. Intuitively, one would not expect significant time-dependent rock behaviour in these mines as the creep rate of the intact hard rock is low. Contrary to this belief, appreciable time-dependent movements have been observed underground as illustrated in the next section. This study is an attempt to increase the understanding of the time-dependent behaviour of hard rock and to simulate it numerically.