Limitations on carriage of explosives., Preparation of charge,

Limitations on carriage of explosives.

Explosives and detonators should not be carried in the same box. Not only this, one person should not carry explosives and detonators together though they may be in different boxes. An explosive carrier can carry only 1 case containing 5 kg of explosive but Director of Mines Safety may give relaxation. Explosive case should be numbered. Jeep or trailer is allowed to carry more explosives for opencast mines for deep holes.

Preparation of charge

The charge for blasting in a hole may consist of one or more cartridges. It is desirable to have the least number of separate cartridges as far as possible, commensurate with the work to be done.

One of the cartridges should have a detonator inserted into it. Such cartridge, equipped with a detonator, is called a "primer cartridge".

WEDGE CUT

Number of holes	15
Charge
Holes in bottom row	600 to 550 g.
Rest of the holes	350 to 400 g.
Total charge	6 to 7 kg.
Blasting Ratio	2.6 to 2.8t/kg.
Yield/detonator	1.1 to 1.2 t.
Pull	1.30 to 1.52 m

FAN CUT

Number of holes	14
Charge
Holes in bottom row	500 to 550 g.
Rest of the holes	350 to 400 g.
Total charge	6.0 to 6.5 kg.
Blasting Ratio	2.5 to 3t/k«.
Yield/detonator	1.2 to 1.4 t
Pull	1.4 to 1.52 m

 

Fig. 8.14. Scraper and break detector.

If an electrical detonator is used to prepare a primer cartridge, open the cartridge at one end, make hole with a pricker of brass or wood, insert the detonator until it is completely buried in the explosive, put back the flap of the cartridge and hitch the leading Wires around the cartridge to prevent the detonator being withdrawn accidentally during charging. (See fig. 8.15). Permitted slurry explosive can be primed from the sides by pricking a hole into it.

Charging a shot hole

A cartridge of non-combustible stemming material is first pushed in the hole. The charge is then placed in the hole and the primer cartridge pushed last of all, so that the "business end" of the detonator points towards the main body of the charge. This position of primer cartridge is termed "direct initiation." (Fig. 8.17). With this position of the detonator, the strongest wave is directed towards the back of the hole and the chances of all the cartridges being properly exploded are a maximum. Direct initiation is best to prevent ignition of fire damp, reduces risk of blown-out shots, and gives maximum yield of coal having a free face.

When the detonator is at the back of the charge and the "business end" points towards the front of the hole, it is called "inverse initiation". This is not practised on coal faces, but is adopted.

(a) in "sumping" or "cut" shots in shafts and tunnels, and

(b) When using delay action detonators to fire a round of shots.

 

After the charge is placed in the hole, the shot-hole is stemmed with stemming material, keeping ends of the leading wires of detonator out of the hole. The stemming material should be compact but plastic, consisting of sand and clay in 3 : 1 proportion, and dried at the surface in the form of cartridges 150 mm to 200 mm long and 25—30 mm dia. The first 2 or 3 stemming cartridges near the charge should be tamped lightly by the stemming rod and the rest should be tamped hard.