goaf

A goaf is the excavated void or cavity remaining after coal, ore, or mineral has been extracted from a seam or deposit. In underground mining, it is the space where productive work has ended, distinguished from active working areas and from unmined rock still containing recoverable material. The goaf can range from a small chamber to an enormous cavity depending on the seam thickness, extraction method, and extent of working.

In coal mining, goafs are particularly significant because they are typically allowed to collapse or are deliberately managed as the working face advances. Longwall mining, the dominant method in deep coal seams, leaves a planned goaf behind the powered roof supports as the face moves forward. This contrasts with room-and-pillar mining, where goafs are the worked rooms between pillars of unmined coal left to support the roof. Pillar extraction, or robbing, involves recovering these support pillars late in mine life, transforming pillars into goafs and accepting the risk of collapse.

Hazards and management

Active goafs present serious dangers. Spontaneous combustion of residual coal can occur, particularly in sealed goafs where air circulation is restricted and temperatures rise. Methane accumulation in unmined goafs creates explosive risk if the sealed atmosphere is breached. Subsidence may result if goafs collapse unevenly or if pillars fail under load, causing ground movement that damages surface buildings and infrastructure. Water infiltration and mining-induced seismicity from strata collapse are additional concerns.

The term comes from Yorkshire dialect, entering mining vocabulary during the expansion of English coal production. A goaf that remains open is actively managed through roof support systems or is permitted to fail in a controlled sequence. A sealed or abandoned goaf is isolated from working areas by stoppings, barricades of masonry or seal materials, to prevent gas mixing or spontaneous combustion spread. Surveyors track goaf geometries to predict subsidence zones and monitor ground movement at the surface.

Understanding goaf behavior is central to mine closure and abandonment planning. Coal Authority records in Britain document goaf locations and extents in closed collieries to inform land-use decisions on historic mining sites. The presence and condition of unmapped or poorly recorded goafs beneath industrial land or housing creates ongoing ground stability risk and is a factor in environmental assessments of former mining regions.