BLASTING PARALLEL HOLE CUTS WITH FRACTURE PLANE CONTROL.

An experimental study of cylinder cuts was made using dynamic photo-elastic recordings to make both the stress waves and dynamic fracture patterns visible. The experiments clearly showed that the cracks which are necessary to remove the region between the charged hole and the dummy hole are difficult to control. Experimental findings are introduced which show the zone of permissible crack control is only plus-minus 21 degrees from the centerline of the two holes. A new parallel hole cut is proposed which is based on fracture plane control. Three holes at equiangular spacing are notched and then fired with a highly cushioned charge to produce six controlled fracture planes which isolate a hexagonal region. A fourth hole, heavily charged, located in the center of the hex-region is fired to fracture this region and eject the fragments from the face.