1 INTRODUCTION
Gold mining in South Africa is currently being carried out at depths of up to 3 600 m below surface. During 1983, a total of just under a hundred million tonnes (1011 kg) of ore was extracted from an average depth of about 2 000 m yielding some 660 tonnes of gold (Chamber of Mines S.A. 1985). Virtually all of the }} operating mines which produce this gold design their mining layouts on the basis of numerical models which assume elastic behavior of the rockmass. Even the simplest calculation of stress around an excavation such as a tunnel, at these depths, shows that the strength of the rockmass is easily exceeded. Unfortunately it is not yet generally possible to model the inelastic rockmass behavior, nor has it been possible to predict the extent and type of fractures which form. Since the average mining depth increases every year, and since safety and the possibility of large scale mechanization are increasing in importance, it has become imperative to improve the understanding of the rockmass behaviour, particularly in the region of the stope face.
2 FRACTURING AROUND STOPE FACES
An intensive investigation of the fracture patterns which form around advancing stope faces has revealed the common features shown in Figure 1. Two types of fracture are evident; a series of fairly regularly spaced shear fractures (bold lines) exhibiting slip in the sense shown, and a more pervasive family of extension fractures (fine lines) oriented in the direction of maximum principal stress. The regular spacing of the shear fractures is almost certainly responsible for the cyclic occurrence of fractures as observed in boreholes reported previously by Adams et al (1981). Another feature commonly observed is shear displacement on the ubiquitous parting planes which separate the sedimentary layers of the host rock. Shear displacements of up to 200 mm have been observed on shear fractures and on parting planes (in the sense shown> though displacements of about 30 mm are more common.
Figure I Fracture pattern and displacements near typical stope face(available in full paper)
NUMERICAL MODEL OF FRACTURED ROCK
The regular systematic shape of the rock blocks or wedges formed by the shear fractures, together with the observed displacements on both fractures and parting planes, led to the idea that the fractured rock could be idealized as shown in Figure 2. Rock remote from the excavation could be regarded as elastic, and analyzed for convenience by boundary element methods, while the fractured rock could be regarded as a set of regular, rigid wedges which are displaced towards the excavation by the elastic rock. The hanging wall (roof) and footwall (floor) strata are believed to behave as passive, loose layers of rock. This is substantiated by the fact that the floor strata are commonly observed to move horizontally into an opening such as the one shown in Figure 1 since the crushed rock ahead of the face "squeezes" out into the mined area. Movements of this nature of up to 600 mm have been observed and documented. This movement results in considerable difficulties being experienced with the installed support.
