The effect of rock bridges on blasting fragmentation in sedimentary rock

Blasting is a major expense and important part of many mining operations. Coal mines, metal mines, and quarries all use explosives as a tool to fragment rock. Since one blast can cost tens or even hundreds of thousands of dollars, it is imperative that the mine optimize its blast design to create the desired fragmentation. In that spirit, this project is an evaluation of the effect of rock bridges on fragmentation. The project evaluated three blasts through an experimental approach. Parameters of blasts (burden, spacing, hole diameter, etc.) were obtained from mine operators. Three dimensional photoanalyses of highwalls were performed to determine in-situ properties of rocks. Photoanalyses of muckpiles were performed to determine blast fragmentation. In order to assess the effect of bridges on fragmentation, two separate approaches were taken. One approach assessed the effect of bridges on comminution factor. The other approach entailed the comparison of results to a well-known empirical fragmentation model. The only clear correlation of rock bridges and fragmentation was the ratio of bridge to joint length. A higher ratio of bridge to joint length resulted in finer fragmentation for each of these field experiments. No correlation between fragmentation and total length of bridges or length density of bridges was detected. It was therefore concluded that bridges only correlate to fragmentation when considered alongside length of joints.