If the blasting is not off-the-solid, the position, direction, depth and number of holes at a face are governed by the following factors in an underground coal mine.
1. Position and depth of cut: A hole should be at least 150 mm shorter than the depth of the cut, made by coal cutting machine.
2. Type of roof: If immediate roof is shale the hole should terminate 0.3 m below the roof; if the roof is sand stone, the hole should terminate 150 mm below the roof.
3. Type of bands and their position: the hole should be drilled near the band and if it is of hard rock, it should be drilled towards it, slightly below or above so that the inclined hole terminates near the band where the explosive force is maximum. The holes should be drilled below the band if it is near the roof, especially when the roof is weak or friable. Fig. 8.5 shows the position of holes at an undercut face with a stone band.
4. Cleavage in the coal: Shot holes should cross the cleavage planes as near to right angles as possible.
5. Strength of explosives and hardness of coal: Harder coal requires more density of holes or explosives. If output of lump coal from the mine is important it is advisable to use a low density explosive and this influences the number and position of holes.
6. Dimensions of the gallery: A gallery of large cross section gives better yield of coal per kg of explosive (Gallery with more height, say between 1.5 m and 2.4 m) requires two rows of holes but only one row suffices for low height.
No comments:
Post a Comment