Slope or drift openings required to be sealed under § 75.1711 shall be sealed with solid, substantial, incombustible material, such as concrete blocks, bricks or tile, or shall be completely filled with incombustible material for a distance of at least 25 feet into such openings.
source: 35 FR 17890, Nov. 20, 1970, unless otherwise noted.
cite as: 30 CFR 75.1711-2