Experimental Investigation of Blast-Induced Fractures in Rock Cylinders

Fractures in rock cylinders with a central blasthole have been investigated, focusing on the borehole expansion, the crushed zone around the borehole, and the effects of the boundary conditions. Nine small-scale blasting tests were carried out on granite cylinders with diameters of either 228 or 240 mm and lengths of 300 mm, with fully coupled or decoupled explosive charges. The granite cylinders were confined by steel tubes with an inside diameter of 268 mm, where the gap (20 or 14 mm) between was left either empty, filled with gravel or filled with cement grout to simulate various lateral boundary conditions. The fractures around the blasthole were examined on cross-sections of the cylinders. The extent of the crushed zone and the expansion of the borehole were found to depend largely on the decoupling ratio of the charge, both decreasing with an increasing ratio. For small charges, the cylinders confined by gravel fill developed more and longer radial cracks than cylinders confined by the more rigid cement fill. For large charges, a cylinder with an empty gap fragmented into very small pieces, while a cylinder with cement fill broke into large fragments. Hoop strains measured on the steel tubes were smaller for a specimen with gravel fill than for similar specimens with cement fill. For the cylinder with an empty gap, fragment collisions with the steel tube caused significant hoop strains in the confining steel tube. The experimental findings of this investigation may contribute to a better understanding of rock blast fracturing, particularly in the region near the blasthole.