The Role of Stress Waves and Gases in the Development of Fragmentation

Abstract

There are two components to a blast which create damage in a rock or grout material: the initial stress wave, followed by the crack penetrating gases. In order to understand the development of fragmentation in grout as a result of the individual and combined action of these components, three types of experiments were conducted. Medium scale grout cylinders were used to measure pressures in the material away from a detonating charge, located in a borehole having the same axis. Some of the cylinders were lined with copper tubes to minimize gas penetration from the detonation of the charge. It was determined that there are statistically significant differences in the slope of the attenuation of pressure for unlined spherical and lined cylindrical samples. Small scale grout cylinders loaded with various charges and coupling configurations were blasted with reduced loads in order to crack the cylinders without destroying them. This process allowed for measurement of the crack patterns produced. A third of the samples were lined with copper tubes, another third were stemmed without tubes. The remaining samples were not stemmed nor lined. Post blast analysis revealed that individual cracks were significantly propagated by gases in unlined samples. However, when measuring overall damage, there were no significant differences between copper lined, unlined and stemmed samples. These results suggested that stress waves were mainly responsible for the crack patterns produced. Computer modelling using Particle Flow Code (PFC) was performed to confirm experimental crack patterns of small scale grout cylinders. PFC produced similar cracks as the small scale cylinders, in number and length, without modelling gas penetration. A secondary study into delay times showed evidence of short delay times being the most effective in producing smaller fragments. This confirmed the role of stress waves as a major contributor towards developing fragmentation.