AdvisorDaeman, Jaak J.K.
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PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractPresent study concentrates primarily on radial crack propagation by the explosive gas pressure and the associated fracture formation, and on estimating the effects of natural discontinuities on rock fragmentation in bench blasting. Boundary element and finite element methods coupled with fracture mechanics theories are used to study the crack propagation. Fractal geometry principles are used to study the effect of natural discontinuities. Predicted stresses from conventional bench blasting model without any radial crack differ little from those around a pressurized hole in an infinite medium. Contrary to field observations, the radially symmetric stress field predicts a thin failed zone concentric with the hole. A loading rate dependent model, developed considering microflaws, suggest only long micro fractures become the radial cracks. The biaxial compression zone at the side of a radial crack suppresses smaller cracks. Other cracks can grow beyond this zone. Some die as the cracks become pressurized. Tensile σ₃ outside this zone peaks near the crack tip. A biaxial tension zone forms ahead of the crack tip. A multicrack model of bench blasting is developed. Biaxial compression zone forms near the hole. σ₃ is tensile outside this zone and peaks near the crack tips. Numerous tensile fractures form in these regions. Tensile fractures continue to form with radial crack growth and existing fractures grow in sliding. Stress redistribution around the fractures produces second and lower order fractures. These fractures break rock between the radial cracks. Pressurization of radial cracks is essential to propagate them for longer distances and to form associated fractures for further breakage of burden. The beam bending model produces unrealistically large burden displacement. The equivalent cavity hypothesis correctly estimates the stresses beyond the radial cracks but ignores the radial crack propagation and the associated breakage. It predicts a failed region concentric with the hole. The effect of natural discontinuities on fragmentation is determined by comparing the Schuhmann size distribution curves of the blasted fragments and the in-situ blocks. In-situ block and after blast fragments sizes, measured from photographs, are fractal which is analogous to Schuhmann Distribution. Exploiting the fractal characteristics eliminates the problems associated with size determination. Automated data reduction processes can make this method very powerful for routine monitoring and design optimization of blasts. Discontinuity pattern, fracture density, block density, fault structure, and microcracks in laboratory specimens are also fractal. Fractal behavior at microscale (10⁻⁶ m) to megascale (10⁵ m) implies Self-similar rock fracture formation. The fractal dimension may be related to the applied stress field.
Degree ProgramMining and Geological Engineering