(a) Roof drilling: Drilling shall be done in friable strata, similar
to the roof in the Bureau's Experimental Mine, which tends to produce large
(b) Horizontal drilling: Drilling shall be done in strata comparable
in hardness to that of coal-mine draw slate. Holes shall be started near the
roof of the test space under conditions simulating the drilling of draw
slate in coal mining.
(c) Down drilling: Drilling shall be done in typical mine floor
strata with a pneumatic percussion-type drill. Five holes shall be drilled
vertically and five holes shall be drilled at an angle.
(d) At MSHA's discretion drilling in "on site" strata may be
acceptable in lieu of strata requirements in paragraphs (a), (b), and (c) of
this section. (See §33.20(a).)