In general, as the uniaxial compressive strength of rock samples is tested, the uniaxial strength of the rock decreases with increasing sample size until a strength is reached beyond which no further decrease in strength is observed for further increases in size. The size at which this occurs was termed the critical size by Bieniawski (1968) and the corresponding strength the critical strength. Once these values are obtained no significant changes in strength may be expected as a result of further volume changes. For the purposes of pillar design, this strength should be adjusted to account for other factors that affect pillar strength, the main factors being the width to height ratio (w/h) effect, jointing and contact conditions. Further test work on Merensky Reef was required to clarify the: 1. Values of it’s critical size and strength 2. Effect of the w/h on it’s strength 3. Effect of the frictional contacts between the reef and the surrounding rock on the reefs uniaxial strength. These results could then be integrated into a holistic pillar design methodology to improve current pillar designing practices. These effects were examined through the laboratory testing of samples originating from Amandelbult Platinum mine. A critical strength of approximately 110 MPa was obtained for samples with diameters, 130 - 250 mm (w/h =1). Increasing the frictional contacts between sample and loading platens was found to increase the sample's strength. A marked difference was found between the insitu and laboratory contact friction angles for Merensky Reef. The insitu contact friction angle was found to be approximately 2.5 times larger then the laboratory contact friction angle. The uniaxial strength increased linearly with increasing w/h ratios up to a w/h ratio of 6. For w/h ratios greater then 6 the strength continued to increased with increasing w/h ratios, but no curve could be acceptably fitted to the data to describe this trend. The results of this study can be applied to mine pillar design in the Bushveld Igneous complex.
Dissertation (MEng (Mining Engineering))--University of Pretoria, 2007.