Orientation of Radial Fractures in the Vicinity of Effective Face-Perpendicular Preconditioning Holes: Proceedings of the First International Conference on Advances in Rock Mechanics, an International Society for Rock Mechanics (ISRM) Specialized Conference (TuniRock 2018). 29 – 31 March 2018, Hammamet, Tunisia

Abstract

When explosives confined in a preconditioning holes and detonated, a high-speed compressive radial strain wave is generated, which then travels outwards from the hole in the rock. Under the influence of the wave, rock particles are moved radially outwards resulting in a tensile stress in the tangential direction. Where this tensile stress is greater than the tensile strength of the rock, radial stress fractures develop. As the stress wave propagates outwards through the rock, it is followed by high-pressure gas associated with the reacting explosive. The gas expands and penetrates the cracks caused by the stress wave. The cracks then extend until the gas pressure drops below the critical level required for propagating the fractures. The objectives of the study were to understand the orientation of radial fractures in the vicinity of the effective preconditioned holes in deep level mechanized gold mines South Africa. In order to achieve the objects of the study, the Examine 2D model was used to simulate the stress distribution in four conditions of the mining faces, mining face with a homogenous rock and mining faces with bedding planes to determine the growths of fractures looking at low and high clamping forces. Owing to that the simulated sigma 1 and clamping forces were compared with the underground observations. The results of the study have indicated that numerical modeling results have shown some degree of similarity with the field observations. Longer radial fractures were clearly noted in the direction of maximum principal stress when the mining face consist of homogenous rock. On the other hand, where the mining faces consist of bedding planes, the orientation of the long radial fractures in the vicinity of preconditioning holes were orientated parallel to the dipping of the bedding planes. The work presented in this paper is part of a Master of Science in Mining Engineering by dissertation study at the University of the Witwatersrand, Johannesburg, School of Mining Engineering, conducted by the first author.