Shot Design, Charging and Stemming
Shot design links the geological and geometrical inputs to the explosive charge. Understand burden, spacing, bench height, sub-drill, stemming length, powder factor, and hole diameter. Too small a burden for the charge leads to flyrock and overbreak; too large a burden causes poor fragmentation, toe problems and misfires. The powder factor (mass of explosive per unit volume or tonne of rock) is a design indicator, not a guarantee.
Charging procedure: inspect holes for depth, water and obstructions; prime correctly (primer position and orientation); charge to design; and stem with suitable inert material to the required length. Never charge a hole that has been over-drilled into a void or that is blocked. Tamp carefully to avoid damaging the initiation system. Stemming confines the gases and improves energy use; inadequate stemming increases airblast and flyrock.
Common traps: confusing burden (distance to nearest free face) with spacing (distance between holes); assuming more explosive always means better breakage; and forgetting that water in holes affects explosive selection and sleep time.
Rule of thumb: stemming length of roughly the burden (or per site design) is a useful check; if you cannot achieve it, reconsider the design.
Worked example: a hole holds 60 kg over a 10 m bench with 3 m burden and 3.5 m spacing. Volume per hole = 10 × 3 × 3.5 = 105 m³; powder factor = 60/105 ≈ 0.57 kg/m³.