The Ubzean Sfyrh

...

Sequence of detonation

For both fragmentation and throw, blasting efficiency depends on the delay sequence of blast hole detonation. Delayed detonation improves load ability of the entire cut, contributes to a better strata control and reduction of blast-induced vibrations.

It is often desirable or necessary to have a series of changes firing at intervals. This can be achieved by either using delay detonators. If fired electrically or plastic igniter chord is fired with safely fuse. The choice between the long delay and short delay detonators depend upon the profile of the rock pile desired. Where shovel loaders are used long delay firing is preferable since the rock pile lies close to the face, whereas short delay firing which gives a flatter rock pile profile is preferred if loading is done by a scrapper loader. Generally special gelatins are best suited for this work, Consumption of special gelatins varies between 0.7 to 2.5 kg/m3 of rock broken, depending on the nature of the rock and size of the excavation.

If charges are fired electrically, great care must be taken in wiring the circuit. Since more than 100 detonators can be involved in each blast, they are connected either in parallel or in series-parallel.

It is therefore, important to ensure that the resistance of the circuit is properly balanced and that no charged hole is omitted from the circuit.

Due to higher risk of such errors in the generally unfavourable shaft sinking environment, Nonel type detonators are increasingly preferred for initiation.

Drilling pattern used for blasting in coal under-ground mine.

When a coal seam is blasted using cut to provide a free face before blasting or a cut face. The depth of the hole should be 15 cm less than the depth of the cut. If however the shot holes are too short compared to the cut, it will be found that the coal at the back is not properly broken and secondary blasting may be necessary.

A typical round of hole is used in blasting and undercut face is shown in the figure. The depth of holes equal to 1.5 m and no. of holes 8 and charge per hole equals 200 gm.

Total yield of coal = 14 tonnes.

Yield per kg = 8.75 tonnes.

---------------------------------------------------------------

[pic 1]

FIG.1:- WEDGE CUT

[pic 2]

FIG.2:-PYRAMID CUT

[pic 3]

FIG.3:-DRAG CUT