Open-cut coal mining strips the soil and rock off the seam in a long advancing cut, slopes every face deeper than 1.2 m back at 1.5 horizontal to 1 vertical, takes the coal from the top down, and backfills behind. Never dig the coal out from under overhanging cover: an undercut highwall falls onto whoever is standing at its foot.
Part of Coal Surface Mining
The surface method for a coal seam: stripping cover, working the seam from above, and managing water, slopes and spoil.
A Long Narrow Cut Beats A Square Pit
The reason is arithmetic, not taste. With cover that is deeper than 1.2 m, the corpus rule is to slope the faces (1.5:1, so 3 m of run for 2 m of depth) or shore them (Excavation). The slope eats ground.
Where The Depth Limit Comes From
The parent article sets the practical limit at about 5 t of rock per tonne of coal. Over one square metre of plan, rock mass is cover depth d × 2.5 t/m³ and coal mass is seam thickness t × 1.3 t/m³. Setting d × 2.5 ÷ (t × 1.3) = 5 gives d = 5 × 1.3 ÷ 2.5 × t = 2.6 t. A 1 m seam therefore tolerates about 2.6 m of rock cover before the ratio fails. Sloped faces and soil cover change the figure, which is why the next example counts them.
Worked Example: The Same Seam, Two Layouts
Inputs: seam 1 m thick; cover 2 m of rock; coal 1.3 t/m³; rock 2.5 t/m³ (the figures the parent uses). Target: expose a 5 m wide seam.
Square pit, 5 m × 5 m of seam exposed, all four sides sloped.
- Top of the pit = 5 + 2 × 3 = 11 m each way.
- Volume of rock = (2 ÷ 3) × (25 + 121 + √(25 × 121)) = (2 ÷ 3) × (25 + 121 + 55) = 134 m³.
- Rock mass = 134 × 2.5 = 335 t; coal = 25 m³ × 1.3 = 32.5 t.
- Ratio = 335 ÷ 32.5 = 10.3 t of rock per tonne of coal, twice the 5:1 limit.
Advancing strip, 5 m wide, one face sloped, the other side the backfilled previous cut.
- Cross-section of rock removed per metre of advance = 5 × 2 + ½ × 3 × 2 = 13 m².
- Rock = 13 × 2.5 = 32.5 t per metre; coal = 5 × 1 × 1.3 = 6.5 t per metre.
- Ratio = 32.5 ÷ 6.5 = 5.0, right at the limit.
The strip is twice as efficient because each cut’s spoil refills the one behind it and only one slope is open at a time.
Method
Step 1: Divert water. Dig a ditch above the cut so surface water does not run in (Drainage Ditches).
Step 2: Strip topsoil, then rock, in separate heaps. Keep topsoil for restoration. Put spoil at least 0.6 m back from the edge, the distance the excavation guide gives.
Step 3: Slope or bench. Beyond 1.2 m cut the face back at 1.5:1 or bench it.
Step 4: Work coal from the top. Break it with pick, mattock and wedges, starting at the exposed surface and working down; do not cut a hollow beneath the cover.
Step 5: Backfill. Put the next cut’s spoil into the pit you have just emptied, and leave the topsoil on top.
Common Mistakes
- Undercutting for easy coal. The coal is softer, the cover above hangs, and it falls in one piece.
- Leaving the pit unfenced. An open water-filled pit is a drowning hazard.
Safety
The highwall kills
A highwall is the vertical face left standing on the uphill side of the cut. Keep it sloped, inspect it every morning and after rain, stay out of its fall zone, and stop work at the first crack at its top, trickle of muddy water or new overhang. Coal exposed to air can also begin to heat; see Storage & Weather Protection.
What’s Next
- Learn next: Tools & Equipment — what the work needs in hand
- Combine with: Sorting & Grading — what to do with the coal as it comes out