Goaf / Gob

A worked-out area of a coal mine, left behind once coal has been extracted and usually partly filled with waste rock ('gob') as the roof above it settles or is deliberately packed.

Historical usage

Goaf (and its synonym gob) is coal-mining terminology going back to the earliest days of British deep coal mining, describing the inevitable void left behind wherever a coal seam had been worked out — a permanent, unavoidable feature of any coal mine's underground geography.

How it worked

As coal was extracted along a working face, the space left behind was allowed to collapse in a controlled way, or was packed with waste rock, forming the goaf; overlying strata settle and compact over it in stages, sometimes hanging unsupported for a time before collapsing.

Why it was used

Managing the goaf, rather than leaving it entirely open, let a mine control roof collapse in a predictable way and reduced the risk of sudden, uncontrolled falls reaching still-active workings.

Risks & limitations

A goaf is a genuine, ongoing hazard rather than simply dead ground: loose broken rock allows fast air leakage, and old goaf areas are prone to spontaneous combustion, since residual coal left behind can self-heat in a long-enclosed, humid space — a fire hazard that can persist underground long after the coal around it was worked out.

Sources

1. (secondary) Definition of goaf — Mindat
2. (secondary) Dynamic distribution and prevention of spontaneous combustion of coal in gob-side entry retaining goaf — PMC

Record created: 22 August 2026 · Last researched: 22 August 2026