Underground coal roofs fall, and timber props cannot carry the full weight of the ground above: 25 m of rock exerts about 0.61 MPa, around 140 tonnes-force on a single 1.5 m by 1.5 m prop area. Props hold up only the loosened slab directly overhead, so the defences are to sound the roof with a bar, set timber closely and tight, hold loose pieces with lagging, and leave at the first creak, pop or fall of dust.
Part of Coal Surface Mining
Controlling the roof of a small hand-worked coal drift: how roofs fail, how to read the warning signs, and how to place timber support.
Why Timber Support Works At All
The parent article calls roof collapse the primary killer in coal mining and lays out the basics: timber every underground working, use hardwood props (oak, ash) at a maximum spacing of 1.5 m, cap each pair with a horizontal beam, listen for creaking, popping or falling dust, never work alone, and leave pillars of coal where the seam is thicker than the tunnel.
The reason for the rules is the arithmetic of load. Taking cover of 25 m, a rock density of 2,500 kg/m³ and g = 9.81 m/s²:
- Pressure = 2,500 × 9.81 × 25 = 613,125 Pa, about 0.61 MPa.
- A prop set at 1.5 m centres in both directions has 1.5 × 1.5 = 2.25 m² of roof.
- Total weight above that patch = 0.613 MPa × 2.25 m² = 1.38 MN, which is 1.38 × 10⁶ ÷ 9.81 ≈ 140,000 kg, about 140 tonnes-force.
No timber prop carries that. Rock above a tunnel forms an arch that sends most of the load to the solid ground at the sides, and props hold only the loosened slab below the arch. The job of the timber is to keep that slab in place and stop it from starting to move, which is why they must be tight against the roof: a prop with a gap does nothing until the roof has already dropped.
Method: A Timbered Set
Step 1: Sound the roof. Tap it with a long bar after each advance. Sound rock rings. A dull or drumming sound means a loose slab. Bring it down with the bar, from a position out of its fall zone, or support it.
Step 2: Set the legs on firm floor. Two legs standing on a flat firm footing, not on loose coal or mud; a plank under each leg spreads load on a soft floor.
Step 3: Cap and wedge tight. Lay a horizontal cap across the two legs and drive wedges between the cap and the roof until the timber takes load and the roof can not move.
Step 4: Lag between sets. Lay planks or poles across the caps to hold small loose pieces up. Loose rock falls between sets as readily as it falls where there is no timber.
Step 5: Inspect every shift. Look for split caps, kicked-out legs, new cracks in the roof and floor heave. Replace damaged timber before working beneath it.
Failure Modes And What To Do
| Warning | Meaning | Response |
|---|---|---|
| Creaking or popping from timber | Roof is loading it | Leave, then reassess |
| Fine dust or grit falling | Roof is shifting | Leave |
| Cracks running along the roof | A slab is separating | Support or retreat |
| Roof damp or dripping, shale flaking | Water is softening the roof | Extra lagging, more timber, or stop |
| Smooth polished surfaces in the roof | A slip plane | Support before working below |
| Rounded circular outlines in the roof | A fossil tree trunk that can drop out whole | Support or avoid |
| Props bending | Overloaded | Add support, do not remove |
Common Mistakes
- Working beyond the last timber. The unsupported span is where falls occur.
- Pulling timber to reuse it while people are still working inward of it.
- Cutting the roof coal to gain height and leaving a thin, weak roof.
Safety
A roof fall gives little or no warning
Hand-timbering reduces the risk and does not remove it. Do not enter any working that has not been sounded and timbered up to the face, never work beneath a bulging or dull-sounding roof. If you hear a fall behind you, you are cut off: that is why you need a second exit.
What’s Next
- Learn next: Gas Hazards — the invisible killer in the same tunnel
- Combine with: Drift Mining — where this support is applied