Effects of bench height over-estimation on fragmentation

Abstract

Auchi-Igarra Mineral District and Okene-Lokoja Mineral District are two mineral districts in Nigeria with a lot of quarries for the exploitation of granite and marble for road construction, buildings, paint and chemical industries. However, most quarries in these districts have been having very high percentage boulders (80% - 90%) after blasting, as against 5% maximum for effectively blasted rounds. Our observations show that the boulders range in sizes from 1-2m. In this paper, we have deciphered the reasons for this boulder formation: mainly high benches when compared to hole diameters. Samples were collected for strength tests and maximum bench height for each hole diameter was recommended.The recommendations were tried in the field and have yielded effective fragmentation with fragment sizes from 10cm to 25cm.

Keywords: Bench height, Boulders, Fragmentation, Secondary blasting, Hole diameters