gob

A gob is the space in a coal mine where the coal seam has been extracted and the working is left to collapse or is deliberately filled with waste rock and mining debris. The term comes from old English and appears in mining records dating back centuries, though the practice itself is far older. In modern usage, gob refers both to the physical void itself and to the material that occupies it, creating persistent confusion in technical documentation.

In longwall mining, the gob is the area behind the advancing face where the roof is allowed to fall naturally into the extracted space. This controlled collapse serves several purposes: it allows subsidence to occur gradually rather than catastrophically at the surface, it reduces pressure on the active working face, and it prevents spontaneous combustion by limiting air circulation in sealed-off areas. The gob in longwall operations typically extends hundreds of meters behind the active extraction line and can be hundreds of meters wide.

In older pillar and stall workings, particularly in anthracite and bituminous coal regions, the gob accumulated differently. Miners would leave coal pillars standing to support the roof while extracting coal from the surrounding area, and the void spaces would fill with slack (small coal dust and fragments) and rock fall. When pillars were later extracted in a second mining pass, called pillar robbing, the gob would suddenly collapse, sometimes violently, causing surface subsidence that damaged buildings and fractured water supplies.

Hazards and contemporary problems

Old gobs present ongoing risks to surface communities and to active mining operations. Water percolating through the gob can emerge as acidic mine drainage, contaminating streams and groundwater. Spontaneous combustion occurs when coal left in the gob is exposed to oxygen through cracks and fissures, creating underground fires that can burn undetected for decades. Gob fires in anthracite regions of Pennsylvania and West Virginia have been documented burning continuously since the 1960s.

Subsidence above large gobs can be sudden and severe, particularly where old shallow workings lie beneath urban or agricultural land. Modern mining engineers must account for historical gobs when planning new operations, often conducting surface surveys and borehole investigations to locate them. The term gaff or gob house sometimes refers to the surface infrastructure built over gob areas to monitor and manage these zones.