A numerical study of the effects of accurate timing on rock fragmentation

Abstract

Some effects of accurate delay time on rock fragmentation have been examined numerically using a newly developed continuum damage model. Stress wave collision created by simultaneously initiating a continuous explosive column from opposite ends is simulated in both an axisymmetric environment and in bench blasting. It is shown that such a collision results in poor distribution of the explosive energy in the rock burden to be fractured and does not benefit rock fragmentation. When two small charges in adjacent boreholes are fired simultaneously, the stress waves generated by each borehole superimpose resulting in a larger volume of fragments. This volume of fragments obtained is affected by the scatter of the firing times of the detonators. It is pointed out that the increase in the volume of fragments by stress wave superposition is always accompanied by less satisfactory fragmentation. The simulation results of a small scale bench shot with three boreholes tend to agree with the documented actual blasting results within the time range studied. The implications of the numerical findings to practical applications are discussed. It is concluded that for rock blasting purposes, delay detonators having microsecond accuracy do not seem to benefit, instead, millisecond accuracy that does not depend on the length of the delay used is sufficient to achieve optimized rock fragmentation. The study modelled two rock-explosive combinations, granite-emulsion and dolomite-dynamite.