'True bottom hole initiation9 and use of precision electronic delay detonators e-DET and e-DET ft: tools for optimum bench blasting

摘要

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 in-hole 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.