Bench blasting in granite with holes with axial notches and radial bottom slots

摘要

Five benching rows with 48, 2-3 m deep, 64 mm diameter holes have been blasted in a granite quarry to develop a technique for smooth blasting with slotted holes that decreases the blast damage (and consequently maintenance costs) in road cuttings. Most holes had radial bottom slots which were cut with high pressure abrasive water jets. The rounds were surveyed and filmed, the bench faces and rock walls mapped and the cracking behind the half-casts detected. The work gives a description of the breakage and the cracks in the remaining rock. The technique developed uses no subdrilling and a burden x spacing of 1,0·0,8 m. The bottom slots are 75 mm deep and decoupled 22 mm diameter column charges are used, with a 50 g primer instead of a bottom charge. EPD caps are used to achieve a simultaneous firing of the blast-holes. The breakage works well, leaving the excavation surface very flat, with undamaged half-casts and shallow radial cracks. Along the toe line a continuous fracture is formed which helps the build-up of an undamaged nose below. It also shields the nose from cracks which start from the columns of the blast-holes. A firing scatter of +-6 ms is enough to seriously offset many of the advantages of the technique and axial notches cannot replace the simultaneous initiation. The technique is technically very promising. Limitations and requirements associated with it are discussed. New, production oriented trials are already under way.