Charging blast holes

1. Presented by:
Dr Clement Kweku Arthur

2. Learning Objective
 Having worked through this chapter, the student will
be able to:
 Discuss the process of charging a blasthole;
 Discuss the different ways of loading a charge into a
blasthole; and
 Discuss blasting theory.

3. Background
 Charging blastholes involve:
 The loading of the explosives into the blast hole,
 Using an appropriate initiating system and
 Thereafter firing the assemblage.

4. Charging Blastholes Procedures
 Check and ensure that there is no blockage and water
in the hole, using a linen tape with a weight attached
to the end.
 Prepare the primer.
 Introduce primer into the bottom of hole, and add
booster charge, if required.
 Add main or column charge onto the primer/bottom
charge.
 Then add the stemming (or tamping) onto the column
charge to complete the loading operation.

5. Charging Blastholes Procedures
 Connect up the firing system and check the circuit.
Correct all faults thus detected.
 After this, ensure all personnel are withdrawn to safety,
and post guards at all the entry points to the blast area.
 Give the appropriate prearranged signal to indicate
firing is about to take place.
 The check again to ensure that there is no one in the
vicinity of the blast.
 Give the signal again and receive a reply from the
guards, indicating all is clear. Then fire.

6. Charging Blastholes Procedures
 After firing, allow adequate time for the blast fumes to
clear (about 5 minutes for surface blasts, but much
longer for underground-30 minutes or more).
 Then inspect the blast for its quality (presence of
boulders, etc.), and any misfires.
 Note that the first person to enter any workings after a
blast is the blastman or the certified officer in charge
of the blasting crew.
 Normal work may then be resumed.
 Any boulders encountered may then be put aside,
during loading/mucking operations, for subsequent
secondary reduction or blasting.

7. Presence of Water and Hole Blockage
 In all situations the depth or length of holes drilled
should be known.
 Charge stick/rod or linen tape with a weight attached
to the end can be used to detect any blockage or
presence of water by dropping this assemblage into the
hole.
 In underground blasting, where shallow holes are
usually drilled, the holes are usually cleared/cleaned
by blowing out, using water under adequate pressure,
and barring that, water and compressed air.

8. Presence of Water and Hole Blockage
 In surface operations, deeper holes usually drilled are
more difficult to blow or cleaned out.
 Where there is a blockage that cannot be dislodged
that hole should be ignored and either a new hole is
drilled close to the existing one or the adjacent holes
are loaded with slightly larger charges to effect the
fragmentation.
 Should water be present, either use more water-
resistant explosives to above the water level before
continuing with the less water-resistant explosives or
use water-resistant explosives throughout.

9. Placing the Primer into the Blasthole
 The primer cartridge should be introduced into the
hole-bottom with the base of the detonator facing the
column of explosive.
 It should be noted that bottom priming gives the best
confinement at the initiating point and makes sure
that no explosive is left undetonated at the bottom of
the hole should it be blocked during charging.
 The primer cartridge should never be dropped into
the hole but should be lowered gently so as not to
dislodge the detonator or the detonating-cord from
the cartridge.

10. Priming The Charge
 Priming is of significant importance to blasting.
 A primer is generally an explosive that accepts
initiation from a detonator or detonating cord, with
the resulting detonation being transmitted to an equal
or less sensitive explosive.
 Its primary function therefore is to initiate a full
explosive reaction in the main explosive charge in the
blasthole by providing the initial detonating shock
wave.
 The primer should therefore have a VOD higher than
or equal to that of the main or column charge being
initiated.

11. Primer Location
 In most cases the primer will be located at the bottom
of the blasthole because it is at this region that the
rock must not only be shattered but must also be
displaced so as to produce the desired breakage.
 Priming may also be located at the collar region of the
blasthole.
 In either case there are disadvantages.
 With bottom priming it has the disadvantage of
scattering or spreading the broken rock over the floor
of the bench or working place, while with collar
priming it has the disadvantage of promoting high
muck-piles directly against the vertical.

12. Primer Location
 Generally, multiple priming is not encouraged
however, under certain circumstances multiple
priming may be undertaken, eg.,
 In deck loading of charges;
 In very high benches in surface operations;
 As a safety measure to assure total column
detonation.

13. Loading the Charge into the Blasthole
 This may be done in one of two ways:
 Column Loading, and
 Deck Loading

14. Column Loading
 This is the most commonly used method of loading
explosives into blastholes in mining operations.
 In the method the explosive column is continuous
from the bottom of the hole to the stemming.
 The explosive column will be made up as follows:
 Bottom charge, and
 Column charge.

15. Column Loading
 When loading, the entire explosive column may
comprise the same type of explosive such as:
 All emulsion {with the primer being made of a
higher density/strength than the main column}, or
 It may be of different types of explosives, with the
bottom charge being of higher strength (such as
80% gelatine) than the column charge of, say,
ANFO.
 In any case, it is necessary to have a higher energy
explosive at the bottom of the hole than the main
column to assure good breakage at the toe, since that is
the region where most work is done in the breaking
process.

16. Deck Loading
 This involves alternating explosives and stemming, air
cushion or plug, spreading over the total explosive
column in the blasthole.
 In this case, instead of having a continuous column of
explosive, some of the explosive is replaced with either
the stemming material, air cushion or plug.
 In deck loading the main explosive quantity is divided
into smaller units and when these are placed into the
blasthole they are interspersed with stemming
material or air cushion, resulting in an overall smaller
quantity of explosive charge being used in the hole.

17. Deck Loading
 The length of stemming between the charges should
be 10-20 times the hole diameter, i.e., TL = (10 - 20) D,
where TL = Stemming length, and D = Hole diameter.
 For example: If the total quantity of explosive to be
used in the main column of the hole is 50 kg, and 5
decks are required, then the charge/deck = 10 kg;
 so that the total charge that be used in the deck
column becomes only 3 x 10 = 30 kg., with 2 lengths of
stemming in the column.

18. Reasons for Deck Loading
 Decking may be adopted for the following reasons:
 To spread a fixed quantity of charge uniformly along
the length of the blasthole, and thereby reducing
the amount, especially as the hole length/depth
increases.
 To locate a satellite charge in a hard band of rock
along the hole or to break up a boulder¬ prone cap-
rock in the stemming area of the blast.
 In order to enable the use of a smaller weight per
delay so as to reduce ground vibration and other
adverse environmental effects.

19. Reasons for Deck Loading
 Generally, deck loading may be preferred as the hole
depth increases, from the point of view of controlling
cut-offs and misfires, air blast, ground vibration, over
break, fragmentation sizing and excessive fly rock.
 It must be noted however, that any savings made in
explosives consumption by the decking method may
be offset by the extra cost of priming, initiation and
costs of labour incurred in putting the additional
stemming into the blast holes.

20. Deck Loading

21. QUESTION TIME
???