The Collaborative International Dictionary of English v.0.48:

Astel \As"tel\ ([a^]s"t[~e]l), n. [OE. astelle piece of wood,
   OF. astele splinter, shaving, F. attelle, astelle: cf. L.
   astula, dim. of assis board.] (Mining)
   An arch, or ceiling, of boards, placed over the men's heads
   in a mine.
   [1913 Webster]