Rock formation at shallow depths is generally associated with complex
geology consisting of many joints, fractures, bedding planes, etc.
Fragmentation results from primary blasts in such strata are therefore
significantly influenced by structural discontinuities. To evaluate the role
of discontinuities, which is of great use in the design of blasts in such
conditions, a two-stage investigation program was taken up. In the first
stage, model scale tests were conducted with single and double holes
with the strike of discontinuities parallel and perpendicular to the face.
Later, field studies were conducted in a limestone mine and a coal mine.
Blast patterns were modified based on the results obtained from model
scale blasts. Secondary blasting was taken as a measure to assess blast
performance in the field, whereas mean fragment size, fine and coarse
fragmentation indices were considered as fragmentation measures in
model scale tests. Model tests indicated confinement of explosive energy by the
discontinuities surrounding blasthole. Crater shape and size were
significantly influenced by the orientation of joints with respect to the
face. Modified blast designs with decreased burden x spacing patterns
resulted in improved fragmentation in the field. A reduction of about 12
per cent in explosive consumption was recorded with modified blast
designs. Boulder formation was observed to decrease by about 79 per
cent as well.