Page 427 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.27

Spaced charges are more likely to require additional priming than solid ones. Deck charges
need a primer in each level.
Correct placing of primers, and even correct direction of the cap in the primer, is of importance
under some circumstances and makes little difference in others. Because of the speed and destruc-
tiveness of blasts, exact analysis of the mechanism and results is difficult.

Speed of Explosion. There are four classes of speed concerned in the firing of explosives. There is
the slow burning of a fuse, explosive burning of confined black powder, extremely rapid but some-
what variable detonation of high explosives, and almost instantaneous passage of electricity.
Black powder is little used, but still serves to illustrate the effect of point of ignition on explosive
performance in a blast hole. Its slow-explosive burning speed may be 1,000 to 3,000 feet per second,
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so that it would take ⁄ 5 to ⁄ 15 second for a 200-foot borehole to fire. If ignited at the top, the upper
rock might be moved a considerable distance before the bottom fired. In one way, this would act
to lighten the bottom burden and help it to pull, but it might also serve to “uncork” the borehole
and allow the bottom of the charge to blow upward rather than horizontally. On the other hand,
the force of the upper part of the explosion, reacting against a heavy burden, might press down on
the unexploded charge with great force and seal it.
If the column were set off from the bottom by electricity, the toe would be well kicked out
before the explosion reached the top. If several caps were placed at intervals in the column and
fired together, the whole burden would be moved out at approximately the same time.
The same action is found in high explosives, although the rapid detonation makes it less effective.
(See Fig. 9.21.) A 200-foot column of a 40 percent dynamite with a velocity of 10,000 feet per
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second, fired from the top, would explode at the bottom ⁄ 50 second afterward. If Primacord were
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used to the full depth, the detonation would take only ⁄ 100 second. If electric caps were used at top
and bottom, the time at the bottom would be about ⁄5,000,000 second after the top, but the lag to the
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center would be ⁄ 100 second.
At first glance, these small fractions of a second might not seem significant. In many cases
they are not, but sometimes they do have an important effect on both the performance and the con-
cussion of a blast.
If long-borehole blasts do not give the desired effect in fragmentation, throw, or any other way,
it may be advisable to change the location, number, or type of primers to try for better results.
Cameras can be obtained that will take pictures of the various stages of the explosion for study.
These may be high-speed streak cameras, which can take 550,000 pictures per second, or instan-
taneous pictures, as seen in Fig. 9.22.
Precautions. Precautions to be observed in regard to a primer in placing and leaving it in the
hole include: placing it in such a manner that it is not subject to shock or jar, and is not penetrated
by rock splinters or other sharp objects; that it is not to be wet for a longer time than the powder,
the cap, the fuse, or the wiring can stand; that the fuse and wires lead to the top without kinks, and
are held so as not to be damaged by placing and tamping of additional charges and stemming.
Water Resistance. This is the resistance of an explosive to penetration by moisture, and/or its
ability to explode when wet or damp. It is important when holes are wet, or when there is a long
time between loading and firing.
Gelatin dynamite and slurry are good to excellent. Other dynamites are good with intact wrap-
pings, fair to poor if cartridges are torn or punctured.
Ammonium nitrate has very poor resistance, but can be kept dry in plastic bags.

FIGURE 9.21 Loading with instantaneous cap primer.