CONTROLLED BLASTING-PART-III
MUFFLE BLASTING
Fly-rock is another important adverse impact of blasting operations, specially, when conducted in the vicinity of dense human habitation/congested areas.
Muffling or covering of blast holes properly before blasting, is the common solution to prevent fly-rock from damaging human habitants and structures. structures.
Generally, mat or mesh (40 mm x 40 mm size) made of preferably of locally available steel ropes (5 to 6 mm) are used for muffling purpose.
Sand bags weighing 40 to 50 kg are kept over the mesh at an interval of 3 m.
Efficiency of arresting of fly-rock depends mainly on the quality of muffling system implemented.
AIR DECKING
The air deck technique involves the loading of a small charge of explosive at bottom and near the top, hole is sealed with a plug (inflatable device) and then stemmed.
When explosive is fired, the peak bore hole pressure is lowered due to expansion of gases in empty space, the crushing and cracking is reduced around the hole.
Air-decking technique is used to create air-gap in blast hole by separating the column charge into a number of decks.
This technique is used for the purposes of better fragmentation quality by extending the induced fracturing, control of back break reducing the fracturing and reducing the generation of fine.
The air decking technique is noted to work particularly well in highly jointed sedimentary rock masses because the amount of shock energy required to generate additional cracks is minimal. This may result in incomplete fragmentation of the rock mass and leave loose fragments on the excavation line.
The main objective of the air decking technique is to improve the explosive utilization within blastholes to improve the rock fragmentation while reducing the ill effects of blasting such as ground vibration, air blast, fly rocks, etc.
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