Non-linear analysis techniques are used to simulate a case history involving the yielding of a large crown pillar, and slip on a fault at INCO's Creighton mine. The results indicate that although fault slip is predicted by the model, thrust faulting is not a likely failure mechanism. The timing, distribution and magnitudes of the plastic deformations occurring during yielding of the pillar are in agreement with observations made in situ. The plastic work done is large enough in magnitude (5 MJ) to indicate that quantities of energy, sufficient to cause sizable rockbursts, are being liberated.
In deep mining, it is common for stresses to achieve magnitudes large enough to cause yielding of the rock surrounding excavations. As mining progresses to increasing depths, attempts to predict stress concentrations, induced by mining excavations, with elastic analysis techniques become progressively less reliable. The load transfer associated with pillar and abutment yielding begin to dominate the behaviour of the rock mass in response to mining. Elastic analysis results not only become more inaccurate, they cease to be useful indicators of in situ behaviour. Since ground control problems are always associated with non-elastic behaviour of the rock mass, it follows that any attempt to predict ground conditions with elastic Continuum analysis techniques must be limited to precursory indicators only. The amount of damage occurring to the rock during failure, and hence the difficulty in supporting it, can only be predicted by methods which accommodate the inelastic behaviour of the rock, whether it be a result of fault slip or yielding en masse. In the mining case history presented in this paper, successive lifts of overhand, mechanized cut and fill mining are analysed to demonstrate the progressive yielding of a large crown pillar. A two-dimensional plane strain analysis of a cross-section taken perpendicular to strike is considered. The results from elastic and elasto-visco-plastic analyses with fault slip are compared to illustrate the stress redistribution associated with inelastic behaviour of the rock mass. Both the distribution and magnitude of the plastic strains are used to describe the ground conditions predicted by the model. These results are compared with underground observations.
Both two-dimensional (BEM2D) and three dimensional (BEM3D) elastic boundary element models have been developed by the author (Wiles and Curran, 1982 and Wiles, 1986) over the past decade. These models have been applied to a wide range of mining problems, and have proven to be both easy and economic to use (Wiles, 1988b and Wiles and MacDonald, 1988). Because of the many successful applications, this technique has recently been extended by the author, to accommodate zones of elasto-visco-plastic material as well as discrete faults (Wiles, 1987, 1988a). This development has taken place at INCO's Ontario Division, Mines Research Department. Funding has been jointly provided by INCO, the province of Ontario, and the government of Canada through the Canada Ontario Mineral Development Agreement. The inelastic behaviour is provided by superimposing initial stresses and body forces over finite zones of integration.
