Abstract
Five single perimeter rows, each with 8–13, 2-3 m deep Ø 64 mm holes have been blasted in a granite quarry to develop a technique for perimeter blasting with slotted holes that decreases the blast damage primarily in road cuts and consequently the maintenance costs. 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 were mapped and the cracking behind the half-casts was detected. The results give a description of the breakage and the cracks in the remaining rock.
The technique developed uses no subdrilling and a burden × spacing of 1.0 × 0.8 m. The bottom slots are about 75 mm deep and decoupled 0 22 mm column charges with a 50 g primer are used instead of a bottom charge. Electronic Programmable Delay (EPD) caps are used to achieve a simultaneous firing of the blast-holes.
The breakage was fine and the excavation surfaces were very flat with undamaged half-casts and shallow radial cracks behind. A continuous fracture is formed along the toe line, which helps with the build-up of an undamaged nose below the holes. The continuous fracture also shields the nose from cracks that start from the column parts of the blast-holes above. A firing scatter of ×6 ms is enough to seriously offset many of the advantages of the technique and axial notches can not replace simultaneous initiation.
The technique is technically very promising. Limitations and requirements associated with it are discussed. New, production oriented trials have subsequently been made.