The Collaborative International Dictionary of English v.0.48:

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 .
   [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]