30 CFR § 57.9360
Shelter holes.
(a) Shelter holes shall be
(1) Provided at intervals adequate to assure the safety of
persons along underground haulageways where continuous clearance of at least
30 inches cannot be maintained from the farthest projection of moving
equipment on at least one side of the haulageway; and
(2) At least four feet wide, marked conspicuously, and
provide a minimum 40-inch clearance from the farthest projection of moving
equipment.
(b) Shelter holes shall not be used for storage unless a 40-inch
clearance is maintained.