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90
HARBOUR ENGINEERING.
The cutting edges, or “ bits,” of percussive drills, the widths of which
range downwards from 4, or even 5 inches in mechanical drills and from
Fig. 78. —Drill Steels.
2 inches in manual drills, are slightly wider than the
shanks of the bars on which they are worked, in
order to ensure the necessary clearance in driving.
For the same reason, the diameter of the drill is
diminished as the depth attained is increased, at the
rate of about T\ inch every 18 inches drilled by hand,
and about | inch every 2 feet drilled by machine.
Thus a 20-foot hole commencing with a diameter of
3| inches at the top would become reduced to 2 inches
by the time the bottom was reached.
Charging*.—After driving has been completed to
the required extent, the hole is cleared of débris and
moisture prior to the insertion of the charge. The
amount of the charge is calculated on the same
principle as that which underlies the preparation of
the borehole, viz., that the explosive acts in the
direction of the line of least resistance, or along the
shortest route from the charge to the nearest open
face; the hole should be considerably longer than
this distance, at least twice as long. On the basis
stated, we have the following.formula :—
Charge in Ibs. = (line of least resistance in
feet) 8 x coefficient,
in which the coefficient depends upon the nature of
the rock and of the charge, being only definitely
determinable by actual experience. For ordinary
biasting powder in granite it is approximately '04,
and in softer stone ’03. For higher explosives it will
be less, in proportion to their specific power. Thus,
for dynamite the coefficient becomes '005 or ’004.
Gunpowder exerts, according to its précise composi-
tion, an explosive force of from 18 to 40 tons per
square inch. For blasting purposes only the lower
power is used, and a cubic yard of quarry rock
ordinarily requires a charge of from | Ib. to 2 Ibs.
according to its nature and position; in tunnels and
shafts as much as 6 Ibs. per cubic yard has been used.
Blasting powder may be deposited in the borehole in bulk, but high
explosives are usually made up in the form of cartridges, and the diameter of
these is arranged so as to fit the hole exactly. The charge will consist of as
many cartridges as may be considered necessary, carefully pressed into contact
with one another by means of a blunt-ended wooden tamping-rod. The top-