MSHA - Code of Federal Regulations - 30 CFR 57.4131
30 CFR § 57.4131
Surface fan installations and mine openings.
(a) On the surface, no more than one day's supply of combustible materials
shall be stored within 100 feet of mine openings or within 100 feet of fan
installations used for underground ventilation.
(b) the one-day supply shall be kept at least 25 feet away from any mine
opening except during transit into the mine.
(c) Dry vegetation shall not be permitted within 25 feet of mine