Page 419 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.19

Aquanal, Power Primer, Pentolite, gelatin dynamite, and perhaps TNT or water gel, but not regu-
lar dynamite.
To summarize: The efficient initiation of AN-FO requires a primer that has sufficient diameter
to nearly fill the borehole and has a minimum length of at least one borehole diameter; the deto-
nation pressure of the primer should be at least 80 kilobars.

Specifics. The du Pont company originally developed a family of high-explosive primers known
as Detaprime, which stress safety, high performance, economy, and ease of handling. These
primers have a velocity of 24,000 feet per second and a density of 1.50. They are insensitive to
mechanical impact, have no metal-to-metal contact between the primer and cap, and do not deto-
nate when exposed to open flame. There is no sensitivity to temperatures from 40 to 225°F.
Their water resistance is excellent, including resistance to moisture throughout shelf life. And
storage life is 3 to 5 years at ambient temperature. The Detaprime primers are about ⁄2 inch in
1
diameter and 2 to 3 inches long, with a hollow core for detonating cord, fuse, or cap insertion.
Poor confinement may also cause the detonation to slow or stop. A soft rock or mud seam between
layers of hard rock may not confine the AN-FO sufficiently for it to carry the explosion across.
In general there should be a primer at the bottom and at the top of the hole, and one at least every
20 feet. However, 50-grain Primacord and a single primer at the bottom may prove adequate.

Loading. Dry holes are loaded by placing the primers with the detonating cord or wire, then
pouring the mixture or the two ingredients separately until the proper amount has been placed.
Stemming is then added, as described later under LOADING.
There is a tendency to load holes higher with AN-FO than with dynamite because of its lower
cost, and to use less stemming. Overdoing this is likely to be a waste of the explosive, and to add
to the hazard of high flying rocks.
Unprotected AN-FO cannot be expected to perform dependably in wet holes. It can be protected
by putting it in plastic bags. It may be bought this way, or the bags made up from sheet plastic
with the help of a sealing machine.
Bagged AN-FO tends to float on any water in the hole, and must be forced down by the weight
of explosive above it. Even with firm tamping the bags will reduce the amount that can be placed
because they will not conform perfectly to the walls. Bags may tear and allow water to ruin their
contents, perhaps cutting off part of the blast.
Density. The density of AN ranges from 47 pounds per cubic foot for the prilled variety to 64
pounds for fine-grained types. This compares with dynamite densities of 37 to 90 pounds.
In loading calculations the low density of prilled AN-FO compared to heavier dynamites is off-
set to a variable degree by complete filling of all hole space by the free-running material. That is,
if a dynamite with a 60-pound density filled only 80 percent of the bore space in spite of slitting
and tamping the cartridges, then its effective density in the hole would be .8 60 or 48 pounds per
cubic foot, about the same as the prills.
The finer and denser types of ammonium nitrate are not well suited for use as blasting agents
with present techniques. They are more sensitive and less powerful than the prills, and are more
difficult to mix with fuel oil. Some increase in density of prills may be obtained by mixing two or
more sizes, but the heavier charge may still be offset by slower detonation.

SLURRY (WATER GEL)

Composition. Slurries, also known as water gels or dense blasting agents (DBAs), are usually mix-
tures of a “sensitizer,” an oxidizer, water, and thickener.
The sensitizer may be any of a number of reducing (oxygen-hungry) chemicals. This is usually
the explosive TNT (trinitrotoluene), but may be (or include) finely divided aluminum and/or other
substances that may or may not be explosives themselves. They may be in sand-size granules,
very fine powder, or other forms.
The oxidizer is an oxygen-rich chemical such as ammonium nitrate and/or sodium nitrate.