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Burden is commonly set at 25-35 times the blasthole diameter, and spacing at 1.0-1.3 times the burden for a staggered pattern. Sub-drilling of about 0.3 times the burden below grade prevents toe problems and hard digging. Volume per hole is burden x spacing x bench height.
Pattern geometry
Burden = burden ratio x hole diameter; spacing = burden x spacing ratio
Volume
Volume per hole = burden x spacing x bench height; sub-drill = 0.3 x burden
Blast design is a safety-critical activity governed by explosives regulation. Patterns must be designed by competent persons with flyrock, vibration and airblast assessment.
A staggered layout distributes explosive energy more evenly than a square pattern, improving fragmentation for the same powder factor. Spacing-to-burden ratios above 1 suit staggered patterns.
The charge cannot break to the free face, giving coarse fragmentation, back-break, excessive ground vibration and a risk of the hole venting through the collar with flyrock.