Page 415 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.15

If special explosives are purchased from a contractor or a quarry, it may be necessary to handle
the transaction through a dealer in explosives to comply with state laws.
Permissible dynamites are those approved by the U.S. Bureau of Mines for use in gassy and
dusty coal mines. Their most important feature is minimum flame in the explosion.
Black Powder. The explosives which explode by burning are called low explosives. Black powder
is the only commercially important member of this class, and is the oldest explosive known.
Black powder is ordinarily composed of sodium nitrate, sulfur, and charcoal, finely ground
and combined in grains of various sizes. The grains are then coated with graphite or other glazing
to make the powder free-running. A more expensive powder for special purposes uses potassium
nitrate instead of the sodium compound.
Fine-grain powders burn and explode faster than coarse-grain ones. Somewhat more powder can
be packed in a borehole by mixing two or more grain sizes.
Black powder can also be obtained in pellets, which are short cylinders of compressed powder
with a center hole. They are wrapped into 8-inch cartridges resembling dynamite in appearance.
They are more convenient to use in small boreholes than the loose powder, and are somewhat
safer to handle.
Black powder may be ignited or exploded by flame, heat, sparks, or concussion, and requires
more careful handling than most dynamites. A special hazard is that powder spilled on the ground
or on the magazine floor may ignite if stepped on or scuffed.
The blasting action will depend on the degree of confinement, the bulk, the grain size, and the
closeness of packing. Unconfined powder will flash-burn, without explosion; and poorly confined
powder will waste much of its energy along the path of least resistance.
Black powder produces considerable smoke and quite toxic fumes, the quantities of which vary
considerably in different blasting procedures.
Black powder can be used to advantage when large, firm pieces of rock are desired, or when
the material being blasted is soft and resilient enough to absorb the shattering blow of high
explosives.
It cannot be used underground where ventilation is poor, or where the air may contain inflam-
mable gas which may be ignited by the flame from the powder, or in wet holes. It has been
replaced by other explosives in most applications.
Dynamite. Dynamite is the best known and one of the most widely used commercial high explo-
sives. The name includes several different chemical groups, wrapped and marketed in about the
same manner.
The “straight” dynamites consist primarily of a mixture of nitroglycerin, sodium nitrate, and com-
bustible absorbents such as wood pulp, wrapped in strong paper to make a cylindrical cartridge.
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Although a wide variety of sizes are available, the most popular are 8 inches long and 1 ⁄ 8 or 1 ⁄ 4
inches in diameter.
The percentage of nitroglycerin by weight contained in the mixture is used to identify it, accord-
ing to strength. From 15 to 60 percent may be used.
Strength does not increase in proportion to the percentage of nitroglycerin because the other
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ingredients also contribute gas and heat. For example, a 60 percent dynamite is about 1 ⁄ 2 times as
strong as a 20 percent.
Higher percentages are faster and more sensitive. Speed is desirable in hard rock and where
the explosive is not confined, as in mud-capping boulders. Sensitivity is necessary when blasting
mud ditches by the propagation method.
Straight dynamites have fair water resistance. Their fumes are poor, however, and they are
never recommended for underground work.
Any type of dynamite of the general-purpose 40 percent strength will explode if subjected to
sharp concussion, such as explosion of a blasting cap; from impact of a rifle bullet; from exces-
sive heat, whether produced by fire, friction, or impact; and from sparks.
When dynamite is burned—usually to destroy surplus or deteriorated stock—it is spread in a
thin layer on straw or other combustible material, which is ignited. All personnel should keep a