What is BCM in mining?

Why the ground is measured, not the truck

Rock that has been broken takes up more room than it did in the ground, because of the gaps between the pieces. A truck therefore carries loose material, but the mine plan, the survey and usually the contract are written in solid ground. BCM is that solid measure. The “bank” is the face being dug.

BCM and LCM

A loose cubic metre, LCM, is the same material after digging. The growth from bank to loose is called swell. Blasted rock commonly swells by 30 to 50 percent, and softer material by less. Your own figure comes from your survey and your material, not from a table.

LCM = BCM x (1 + swell)

At a swell of 30 percent, 100 BCM in the ground becomes 130 LCM on the trucks.

BCM to tonnes

Multiply by the bank density, the weight of one cubic metre of the material in the ground. Overburden such as sandstone and shale is often between 2.2 and 2.6 tonnes per BCM. Coal in the seam is much lighter, often 1.3 to 1.5.

tonnes = BCM x bank density

At 2.4 tonnes per BCM, 1,000 BCM of overburden weighs 2,400 tonnes.

BCM per load

Nobody surveys the pit every hour, so production during a shift is counted in loads. Each load is given a BCM figure, sometimes called the truck factor, for that truck and that material.

BCM per load = body volume x fill / (1 + swell)

A truck with a 24 cubic metre body, filled to 90 percent, carrying material that swells 30 percent:

24 x 0.90 / 1.30 = 16.6 BCM per load

Check the weight too. At 2.4 tonnes per BCM that load is about 40 tonnes, which is the limit of a 40 tonne truck. A truck is full by volume or by weight, whichever comes first, so heavy material fills it by weight and light material such as coal fills it by volume.

BCM per hour and per shift

Count the loads and multiply.

12 loads in the hour x 16.6 = 199 BCM

A shift target works the other way, from the rate the site expects of each loading unit. With two excavators at 250 BCM an hour each, and 10.5 productive hours in a 12 hour shift:

250 x 10.5 x 2 = 5,250 BCM

Counted BCM is an estimate. The surveyor’s measurement at month end is the truth, and the two are reconciled then. A count that drifts far from survey usually means the BCM per load needs resetting.

Where the counting goes wrong

On paper, the loads are tallied in the pit, added up at the end of the shift and typed into a spreadsheet the next morning. A tally is misread, a truck moves to another excavator halfway through the hour, the material changes and the old figure is still used. By the time the total is known the shift is over.

CPL is the app we built for this. The clerk taps once per load, on a phone or a PC, with or without signal. Each load counts for the figure the site has set for that machine and material, and the office sees the hour against plan while it can still be changed.

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