Blast design to improve fragmentation in a mismatched combination of drill diameter and depth in bedded rock

Blasting experiments were conducted in a constrained condition in an opencast coal mine where the hole diameter was 250 mm and bench height was 7.0 and 14.0 m. Strata blasted contained prominent thick bedding planes and the shooting direction was always along the strike of the bedding plane. The blasting was to be carried out without muffling 50 m from the office. Detailed mapping of the attitude of discontinuities was carried out in order to confine the explosive, as far as possible, within the two bedding planes. A parameter termed “release time” has been introduced in this study to compare the degree of fragmentation of different blasts.