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In the laboratory, core obtained from certain platinum mines in the Bushveld Complex in South Africa showed a strong nonlinear relationship between applied load and induced deformation. This nonlinear behaviour invalidates assumptions that are typically used for numerical modelling and stress measurements. Consequently, the results from such numerical models and stress measurements may be unrealistic. It was found that specimens that exhibited this nonlinear behaviour often had a relatively low compressive strength. Petrographic study of thin sections showed no connection between mineral composition and this nonlinear behaviour. Also, no correlation was found between the density of micro-fracturing and this nonlinearity. Under the scanning electron microscope (SEM), however, a specimen exhibiting this nonlinear behaviour was found to contain open micro-cracks; no such cracks were observed in a similar specimen, which exhibited a more conventional linear stress–strain relationship. Upon further investigation, it was found that mainly samples from depths in excess of about 1000 m demonstrate nonlinear behaviour. Preliminary results also suggest that this behaviour is affected not only by the in situ stress, but also by the direction of drilling. In these samples, complete crack closure appears to occur only under applied stresses far in excess of the in situ virgin stresses. In addition, it was found that crack sliding also contributes to the observed nonlinear behaviour. This paper analyses and quantifies the nonlinear behaviour and discusses practical implications of such nonlinear behaviour.
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