When a roof fall takes place, the air in the goaf area is displaced as the latter is filled by the broken roof rocks. The displacement is so quick that the air is pushed out at the high speed resulting in air blast if the air has limited outlets to escape. The intensity of the air blast depends upon the volume of the air that is displaced.
If the immediate roof is massive sandstone or has a sill, the local fall does not occur for a long period even after withdrawal of supports. The volume of air that is displaced in such case when a roof fall takes place is large. Similarly the volume of air displaced in a thick seam is also large.
The air blast in these cases is strong and dislodges roof supports and ventilation doors, damages isolation stoppings and injures workers in the path of the air blast. The dislodged roof supports may initiate some additional roof collapse.
Precautions against Air Blast:
1. Steps should be taken to see that an extensive area of uncollapsed goaf does not exist at a time. Where possible, steps should be taken to bring down the roof at regular intervals.
2. In any depillaring district, all the workers should be withdrawn to a safe place when the usual indications of imminent roof fall are observed.
3. Apart from the entries for ventilation and haulage, additional roadways should be kept open in a thick seam.
4. After every blast there is usually a thick cloud of coal dust in suspension in the air. The electric switches should be put off before such anticipated air blast and put on only after coal dust has settled down.
5. Construction of a few isolation stoppings with an easily breakable zone (by air blast) should receive attention.
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