A New Blasting Method for Crack Control by Using Charge Holders

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Fracture control is very important in rock excavation and partial demolition of concrete structures with blasting. In this research, a new method for achieving high degree of fracture control by using charge holders with two wedge-shaped air cavities, based on the dynamic effects caused by the shock wave interactions in a borehole, are presented. The effectiveness of the charge holders for blast crack control is studied exprimentally. The charge holder produces shock converging effects in a borehole and the shock wave pressure acts effectively on the cavity walls. The tensile stress fields are produced on the borehole wall at the locations of the apexes of the cavities and controlled cracks are initiated. The following results were obtained from model experiments using a electric detonator as a charge, PMMA plate for optical visualizations of stress waves, mortar specimens and lead ones for examing blasting effcts. The photographs taken by the laser-shadowgraphy show that the stress waves propagate outward from the apexes of two wedge-shaped air cavities along the crack control directions. The blasting effects caused by the actions of the shock waves and gases in the charge holders initiate the cracks forming a control fracture plane. The charge holders are effective in eliminating the damage to the borehole wall and in driving the controlled cracks to greater distances.