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The Collaborative International Dictionary
To work the goaf

Goaf \Goaf\; n.; pl. Goafsor Goaves. [Cf. 1st Gob.] (Mining) That part of a mine from which the mineral has been partially or wholly removed; the waste left in old workings; -- called also gob .

To work the goaf or To work the gob, to remove the pillars of mineral matter previously left to support the roof, and replace them with props.
--Ure.