Powder Factor

Powder factor is the mass of explosive spent per unit volume of rock broken, and it is the number a blast is judged and costed by. It is also the number most often quoted without its units, which is a problem, because there are four in common use: kilograms per cubic metre, kilograms per tonne, pounds per cubic yard, and pounds per short ton. The conversions between the volume-based and the mass-based versions run through the ROCK DENSITY, which varies from one orebody to the next, so a figure carried between reports without its unit is worth nothing at all.

Typical values give a feel for the scale. Soft or heavily jointed rock in a surface bench might take 0.2 to 0.35 kg/m³; ordinary hard-rock production blasting sits around 0.4 to 0.8; hard massive rock where fine fragmentation is being bought deliberately runs above 1.0. Underground development rounds are far heavier — 1.5 to 3 kg/m³ and beyond — because a tunnel face has only one small free surface and the rock has almost nowhere to go. Confinement, not hardness, is what drives that difference.

What the powder factor tells you is cost and roughly how fine the muck will be. What it is entirely silent about is DISTRIBUTION. The same mass of explosive spread over many small-diameter holes breaks rock quite differently from the same mass concentrated in a few large ones: more holes give finer, more even fragmentation, less oversize and less overbreak, and they cost more to drill. That trade — drilling money against explosive money against crushing and grinding money downstream — is the real blast design problem, and the powder factor is only one of its three terms. Two blasts with identical powder factors can produce muck piles a shovel operator would not recognise as related.

The commonest arithmetic error is measuring the wrong volume. The volume in the denominator is IN-SITU volume, the solid rock as it stood before the shot. Broken rock swells by something like 30 to 60 %, so a powder factor worked out against a truck count or a loose-measure survey comes out roughly a third too low, and it does so silently, with a plausible number. Measure the solid: burden times spacing times bench height per hole, or a survey of the face before and after.

Finally, there is a cost the powder factor never shows. Explosive energy that does not break rock has to go somewhere, and it goes into ground vibration, airblast, flyrock, and damage to the rock you are keeping. A heavy powder factor in a wall or a pillar you intend to leave standing translates directly into a disturbance factor in the Hoek-Brown criterion, and from there into a rock mass strength that is genuinely lower than it was before you shot. The cheapest fragmentation is not always the cheapest blast.