What powder factor means, and what it does not

Powder factor is the mass of explosive used to break a quantity of rock. In Australia it is usually written in kilograms per bank cubic metre (kg/m³) or kilograms per tonne (kg/t). The two are related by the rock density: a powder factor of 0.80 kg/m³ in rock of 2.65 t/m³ is about 0.30 kg/t.

For one hole, it is the charge in the hole divided by the rock that hole is responsible for: burden × spacing × bench height. For a whole blast it is the total charge divided by the total bank volume. The two agree only when every hole carries the same charge over the same ground.

It is one number, it is easy to calculate from the drill log and the charge sheet, and it tracks cost. Over many blasts in the same ground with the same product, a change in powder factor usually shows up as a change in fragmentation and in dig rate. That is why it is on every blast report.

Distribution. Two patterns with the same powder factor can put the energy in very different places. A tight pattern with a smaller charge spreads it; a wide pattern with a large charge concentrates it near the hole and leaves the middle of the burden under-broken.

Confinement and timing. Powder factor says nothing about stemming, burden relief or delay timing. The same kilograms, fired with no relief, make vibration and flyrock instead of fragmentation.

Where the explosive actually went. The design powder factor assumes the hole was drilled where it was planned, to the depth planned, and charged as planned. Over-drilling, wet holes, voids and collars that were not stemmed all change the real figure without changing the number on the report.

Compare powder factor only within the same ground, product and hole size. Read it beside the fragmentation result, the dig rate, the vibration record and the drill log. When the number moves and the result does not, or the result moves and the number does not, that is the interesting blast.

A 102 mm hole with a 1.15 g/cm³ product carries about 9.4 kg per metre. An 8 m charge is about 75 kg. On a 3.2 m by 3.7 m pattern with a 10 m bench, each hole breaks about 118 m³ of bank rock. That is about 0.64 kg/m³, or about 0.24 kg/t at 2.65 t/m³. Change the pattern to 3.5 m by 4.0 m and the same charge gives about 0.54 kg/m³: an 18 percent drop in explosive per cubic metre, which the fragmentation will notice.