CONTROLLED BLASTING TECHNIQUES (PART 3)�

8. 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.

8. MUFFLE BLASTING

9. 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.

9. AIR DECKING

Figure of Air decking

Some MCQs on Controlled blasting

1. Which of the following is a method to minimize ground vibrations in controlled blasting?
1. Using a smaller blasthole diameter c) Increasing the distance between blastholes
2. Using a higher explosive energy d) Initiating the blast simultaneously

Answer: c) Increasing the distance between blastholes

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2. Which of the following is a technique to mitigate the air blast effects of controlled blasting?

a) Increasing the stemming length b) Using a smaller blasthole diameter

c) Employing blast mats or curtains d) Decreasing the drilling accuracy

Answer: c) Employing blast mats or curtains

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3. What is the purpose of the air decking technique in controlled blasting?

a) To reduce the fragmentation of rock b) To improve the efficiency of blasting

c) To minimize the ground vibration d) To increase the safety of blasting operations

Answer: c) To minimize the ground vibration

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4. Air decking in controlled blasting involves:

a) Using a higher explosive charge b) Placing an air-filled chamber between explosive charges

c) Using a lower explosive charge d) Using a water-filled chamber between explosive charges

Answer: b) Placing an air-filled chamber between explosive charges

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5. How does muffle blasting reduce ground vibration?

a) By using a lower explosive charge b) By using a higher explosive charge

c) By placing an absorbent material around the explosive charge d) By increasing the distance between blastholes

Answer: c) By placing an absorbent material around the explosive charge

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