\Goaf\; n.; pl. {Goafs}or {Goaves}. [Cf. lst {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 {gob}, to remove the pillars of mineral
   matter previously left to support the roof, and replace
   them with props. --Ure.