Authors: Ali, Z; Karakus, M

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Ali, Z & Karakus, M 2022, 'Experimental investigation of hydraulic fracturing in granite under hydrostatic stress conditions', in Y Potvin (ed.), Caving 2022: Proceedings of the Fifth International Conference on Block and Sublevel Caving, Australian Centre for Geomechanics, Perth, pp. 1241-1252,

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Cave propagation and rock mass fragmentation are the major challenges in block cave mining. In deep underground mines where the rock mass is stronger and stresses are higher, an artificial weakening is required. Hydraulic fracturing (HF) has evolved as one of the most preferred methods of preconditioning in block cave mining as it provides better control of fracture geometry and orientation and helps in the dissipation of the stored excess strain energy. The breakdown pressure, fracture propagation and orientation are some of the important parameters which govern the success of HF operations. In order to understand these parameters and to collect reliable data for numerical investigations, laboratoryscale HF experiments were performed on cylindrical samples of Adelaide black granite under various hydrostatic stress conditions. A borehole of 8 mm in diameter and 63.5 mm in length was pressurised using dyed water until the specimen failed under four different confining pressures. The failure process was monitored using an acoustic emission monitoring system with several piezoelectric sensors in order to characterise the fracturing process during the experiments. A positive linear relationship was observed between the breakdown pressure and the hydrostatic stresses which is in line with the theoretical results. However, it was observed that the rock tensile strengths found using Brazilian disc tests are slightly lower than the breakdown strengths.

Keywords: block caving, preconditioning, de-stressing, hydraulic fracturing, acoustic emission

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