Open Pit Mining

How an open pit works

In an open pit, rock is mined in horizontal slices called benches. The pit deepens as benches are mined one after another, and several benches are usually worked at the same time. Upper benches are mostly waste rock that must be removed to uncover the ore body below.

Does drilling happen at the top of the pit?

No. Drilling and blasting happen on every working bench, wherever the mine is currently breaking rock, from the upper waste benches down to the ore benches near the floor. A blasthole rig stands on the surface of the bench it is drilling, and the holes are drilled down through the bench height plus a little subdrill below floor level. After the shot, the broken rock falls to the foot of the face as a muck pile, and an excavator digs it from there.

The cycle

1. Drill a pattern of blastholes (typically 150–311 mm in diameter in large pits).
2. Blast: load the holes with explosives, stem them and fire them in a timed sequence towards the free face.
3. Load: an excavator or shovel digs the muck pile and fills a haul truck in a few passes.
4. Haul: trucks climb the ramp at roughly 8–10% grade to the crusher (waste goes to a dump instead).
5. Crush and convey: a primary crusher reduces the lumps, and a conveyor carries ore to the processing plant.

Typical numbers (site dependent)

• Bench height about 10–15 m in large pits; ramps about 8–10% grade.
• Burden about one-third to one-half of bench height; spacing about 1.0–1.5× burden; stemming about 0.7–1.0× burden.
• Haul trucks carry roughly 100–400 tonnes. Road width is about 3–3.5 truck widths for two-way straights.

Click any equipment in the animation, or a name above, to open a detailed animation of it.