Stemming, Subdrilling and Spacing from the Burden

Once the burden is fixed, the rest of a bench pattern follows from it as simple multiples. Konya's starting points in FHWA-HI-92-001 are stemming at about 0.7 B, subdrilling at about 0.3 B, and spacing between roughly 1.15 and 1.4 B for holes fired on a delay — about 2 B for holes fired instantaneously together. Enter the burden and the ratio and this page returns the dimension. What the page cannot do is choose the ratio for you, and each of the three deserves a paragraph.

STEMMING is the uncharged length at the collar, and its job is to keep the gases in the hole long enough to break rock rather than letting them escape into the air. Too little and the shot vents at the collar: airblast, flyrock, noise complaints, and the top of the bench left coarse. Too much and the top of the bench is simply not charged, and it comes away in boulders. The MATERIAL matters as much as the length — angular crushed stone at about a tenth of the hole diameter locks in the hole and holds several times better than the drill cuttings that came out of it, which are rounded, fine, and blow out like sand from a tube. It is the cheapest single improvement available to most blasts and it is skipped constantly because the cuttings are already lying beside the hole.

SUBDRILLING is the depth drilled below the intended floor, and it exists because a blast hole does not break rock cleanly at its own bottom. Without it you get a hard high bottom that the loader climbs over and the next bench has to fight. With too much of it you are paying to drill and charge rock below the grade you want, shattering the floor you are about to drive on, and — the part people forget — putting extra confined explosive exactly where ground vibration is generated most efficiently. Where a horizontal parting or a bedding plane sits at grade, take the subdrilling to nothing: the rock will break to the parting on its own, and drilling past it wastes explosive into the rock below.

SPACING is measured along the row and is always larger than the burden. Konya's ratio depends on the initiation: holes fired instantaneously act more like a single long charge and want a wide spacing near 2 B, while delayed holes each break their own burden and want something near 1.15 to 1.4 B. The spacing-to-burden ratio also shows up directly in Cunningham's correlation for the Rosin-Rammler uniformity index, which is another way of saying that a pattern near the recommended ratio produces more UNIFORM muck, not merely finer muck. That uniformity is what a crusher actually cares about.

Two closing cautions. These ratios are starting points from ordinary bench blasting in reasonably competent rock, and every one of them moves with the geology and with what you are trying to achieve — the honest way to set them is to shoot a pattern, look hard at what came out, and adjust one variable at a time with the results written down. And because every dimension here is a multiple of the burden, an error in the burden propagates into all of them at once and in the same direction. Auditing an old pattern by dividing its stemming by 0.7 and comparing against the surveyed burden is a quick and revealing check: patterns drift over the years, usually towards longer stemming, because a hole is easier to fill than to charge.