%goaf%

n.; pl. Goafs r 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 . [ 1913 Webster ]

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. [ 1913 Webster ]