To work the goaf

from The Collaborative International Dictionary of English v.0.48

Goaf \Goaf\; n.; pl. {Goafs}or {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]