In the case of medium-steep and steep dip coal deposits of loose structure, supported air vent holes and/or delivery holes cannot be formed at all with the aid of hitherto employed methods and apparatus, because the bore holes tend to constrict or cave-in promptly. The hills up the dip however can be formed exclusively by advance from above downwards, while carrying the coal masses upwards; this operation requires intensified precautions, and in the degree of advancing or deepening the entry, is increasingly difficult; it demands increasing amounts of live labor, apparatus and power.
USPTO Backgrounds
USPTO Backgrounds