A conceptual model of radial crack formation around an explosively loaded hole in mine bench blasting has been developed. Both stress waves and quasi-static gas pressure can initiate multiple microcracks around the hole but the originally long microfractures will ultimately become the radial cracks. A multicrack model of bench blasting has been developed using the MSC/NASTRAN finite element code. Results from this model show that zones of high tensile stress form near the radial crack tips. When discrete fractures are modeled in these areas with increased radial crack length, two zones of tensile stress concentration are observed. One zone forms near the new radial crack tips indicating formation of new tension cracks. A second zone forms near the previously formed tension fractures. These fractures will grow in sliding along the crack surfaces and form second and lower order fractures. The fracturing process can continue for a long time if the explosive gases are allowed to pressurize the radial crack surfaces. Observation of continued fracturing of the burden in the numerical model fills a gap in the conventional gas pressure theory in explaining the fragmentation resulting from bench blasting.
Relative importance of stress waves and gas pressure in fragmenting the rock in bench blasting has been debated for a long time. It has been generally accepted that stress waves generate microfractures around the hole and also condition the rock in the burden region by creating microfractures. Radial cracks grow from the hole by the quasi-static gas pressure and break up the burden. Absence of any wedge-shaped fragment in the blasted muck pile shows that the gas pressure theory does not explain all the breakage observed in a bench blast. This study, using numerical simulations, provides a possible explanation of continued breakage of the burden, especially in between the radial cracks, which is not explained by the gas pressure theory.