Effect of width-to-height ratio on the strength and behaviour of chromitite rock

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The South African Bushveld Complex, Upper Group 2 (UG2) Reef pillar design has been based on a strength formula originally determined for quartzite. This formula was modified to cater for the weaker UG2 chromitite material. However, there is evidence that the pillars are stronger than predicted by the modified formula. In this study, UG2 chromitite samples were tested at a wide range of width-to-height ratios to determine the material behaviour and the relationship between strength and width-to-height ratio. Such test results on this material have not been published in the past. All testing was performed in a servo-controlled stiff testing machine (MTS 815) at a constant deformation rate of 0.08 mm/min. Specially prepared load spreader end-pieces were used to minimise any bending effects of the loading platens on the sample behaviour. The analysis showed a non-linear strengthening with increasing width-to-height ratio, best described using a power formula. A change in failure mode from pure shear to extension was observed at a width-to-height ratio of one, which has been documented previously for other materials. The change in fracture mode did not appear to affect the strength of the material. Further tests were conducted to determine the critical strain at which extension fractures initiate in chromitite. It was found that extension fracturing initiated at a critical strain of between − 2.2 mε and 3.0 mε, which was about 10 times the strain calculated by existing formulae. This new information has not been previously published for chromitite. An additional useful product of the work is an equation that can be utilised to evaluate the true uniaxial strength from samples prepared with a w/h ratio of one. Strain softening was observed on all samples up to a w/h ratio of eight. Samples with w/h ratios of three and higher were able to sustain high stresses for large strains, but almost all samples finally failed with complete loss of load. The final sudden loss of load is believed to be associated with radial cracks observed in these samples after failure.