"True bottom hole initiation" and use of precision electronic delay detonators e-DET and e-DET ft: Tools for optimum bench blasting

The technique of initiating the explosive charged in drill holes in bench blasting has seen significant changes during the last five decades. The evolution has completed a full circle starting with in-hole pyrotechnic based electric delay detonators, and now the fully field programmable electronic delay detonators. Both varieties use insulated leadwires inhole to transmit the energy to the detonator for firing. In between, saw the large scale use of detonating cord as down lines and surface trunk lines followed by low energy detonating cord (LEDC) and subsequently the shock tube (signal tube) based non-electric pyrotechnic delay detonators. In India, LEDC based detonators have been used in underground metal mines for ring blasting. Except detonating cord downlines which initiate the drill hole at the top and detonation travels downwards, all the other systems effect 'true bottom hole initiation' of the explosive in the drill hole. Bottom priming of drill holes viz initiating the explosive truly at the grade level and retention of stemming in the drill hole assists in maximum utilization of explosive energy resulting in improved fragmentation and heave (loose muckpile). Bottom hole initiation also eliminates toe formation and reduces flyrock generation. Detonators based on pyrotechnic delays have a disadvantage of timing scatter resulting in variations in the intended/desired firing times and firing sequence of drill holes. This is known to cause poor fragmentation, tight muck piles, over break and side break in the bench being blasted and also generation of ground vibrations, airblast noise and flyrock. Use of detonating' cord as down line causes undesirable effects such as stemming ejection, partial de-sensitization of booster-sensitive explosive in the drill hole, detonation to travel across the drill hole diameter (not along the column), while surface trunk lines cause noise generation. Applied research in the field of blasting has revealed many interesting aspects of the blasting process. Accuracy of firing times of delay initiators used in blasts is of utmost significance with regard to the resulting fragmentation, muckpile looseness, flyrock generation, ground vibrations, airblast overpressure, over break (back break and sidetear) and pit-wall stability. Electronic detonators provide very precise drill hole delay firing times, as also true bottom hole initiation. The implications of drill holes being initiated by various methods as also the advantages of accurate firing times of drill holes and improvements/advantages observed in the Indian scenario are discussed in detail in this paper.