Firing Explosive Charges with Millisecond Delay In Surface Mining – Historical Outline

Abstract

Blasting works conducted in surface mines with large explosive charges are associated with the problem of unfavourable influence of vibrations, induced by detonating the charges, on structures in the vicinity of the mines. Applying explosives to mine deposits also influences effectiveness of blasting works, associated with fragmentation of rocks. Since the beginning of the 1950s, explosive charges are most often fired with millisecond delays. The article pre-sents the historical outline of research into millisecond firing with electric, non-electric and electronic systems, conducted in surface mines in Poland and around the world. As a result of the works, it was concluded that the interval and precision of set millisecond delays signifi-cantly influence intensity of vibrations in - duced by detonating explosive charges, and fragmen-tation of rocks. In electrical systems the actual firing times of detonators may significantly differ from their nominal times, hence there is a risk of overlapping delay times and, as a re-sult, a risk of increasing intensity of vibrations. Researchers indicate that maintaining specified time interval between detonations of consecutive explosive charges may successfully limit the seismic effect. It was the reason behind introducing “8 millisecond criterion” into the practice of blasting works in 1960s, as the minimal delay time between consecutively fired charges. With technical progress in initiation systems, precision of set delays significantly improved, as electronic initiation systems show it. The research conducted with the system clearly shows that the commonly assumed minimal 8 ms time does not have to be a binding rule any more. Precision of state-of-art electronic detonators successfully enables designing multiple row fir-ing patterns, with minimal delay time shorter than 8 ms between consecutively fired charges.