Extract coal by the method the cover allows: open-cut digging from the surface where the rock above the seam is less than about 2.6 times the seam’s thickness, and a drift into the hillside where it is deeper. Open-cut work is the safer choice by a large margin, and a drift should only be attempted shallow, timbered, ventilated and never alone.
Part of Coal Surface Mining
Choosing between surface digging and hillside entry for a coal seam, based on cover thickness, tilt of the seam and the danger each method carries.
The Ratio Of Cover To Coal Makes The Decision
Every tonne of coal under a thick roof costs more tonnes of rock to uncover. The parent article’s rule is that open-cut work stops paying when you must move about 5 tonnes of rock for each tonne of coal. Because coal is lighter than rock, that corresponds to a depth, not a mass: with coal at 1.3 t/m³ and rock at 2.5 t/m³, the limit is cover of 5 × 1.3 ÷ 2.5 = 2.6 times the seam thickness (derived in the open-cut article). A 1 m seam can be uncovered by hand under up to about 2.6 m of rock; a 0.5 m seam only under 1.3 m.
Decision Table
| Situation | Choose | Why |
|---|---|---|
| Cover less than about 2.6 times thickness | Open-cut | Cheapest and safest; the roof is air |
| Seam dips into a hill, cover grows with distance | Open-cut to the limit, then a short drift | Follow the coal only as far as ventilation and timber allow |
| Seam at base of a cliff or stream bank | Drift or open-cut from the bank | Low cover but a steep face above: watch the highwall |
| Seam gassy or old workings nearby | Neither | Leave it; see the safety articles |
What You Need
- Strip tools for open-cut: spades, mattocks, picks, barrows
- Timber and saws for a drift: props, caps and lagging (Tools & Equipment)
- Drainage for both: a cut-off ditch above the work so rain does not run in
- A second person for any underground work
Common Mistakes
- Choosing a drift because it looks less work. A drift moves less rock but replaces it with the risk of roof fall, gas and no second exit.
- Following the seam down-dip for months. Cover and water both increase with every metre.
- Skipping the surface preparation. Stripping topsoil separately keeps it for restoring the ground.
Safety
Underground coal work kills more than surface work
Roof falls, methane and carbon monoxide kill underground, while surface working mainly threatens through highwall collapse. Pick the surface method wherever the cover allows, and read the roof, gas and dust articles before any entry.
What’s Next
- Learn next: Open-Cut Mining — the surface method in detail
- Combine with: Coal Mine Safety — the four killers across both methods