Page 413 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.13

FIGURE 9.11 Charge placement for cushion blasting.

have a much greater volume. Heat is generated by the change, and serves to expand the gases
greatly.
Explosion by rapid burning is called deflagration, and by almost instantaneous decomposition
is called detonation. High explosives detonate.
Handling explosives requires experience. A widely used reference is Blaster’s Handbook,
published by International Society of Explosives Engineers.
Properties to be considered in selecting an explosive include sensitivity, density, strength, velocity,
water resistance, fumes, price, and availability.
Sensitivity is a measure of the ease with which a substance can be caused to explode and its
capacity to maintain an explosion through the length of a borehole. It is also a measure of safety—
the higher the sensitivity, the greater the risks of handling.
Nitroglycerin is so sensitive that it must be mixed with other substances before it can be used in
commercial explosives. Compounds such as fulminate of mercury and lead azide that are used in det-
onators are so sensitive that they will explode at a light hammer blow or when exposed to moderate
heat. At the other extreme, ammonium nitrate is so insensitive that few precautions and no permits
are required for shipping and storing it.
Density is the volume of an explosive in proportion to its weight. It is measured in pounds per
1
cubic foot, or in the number of 1 ⁄ 4 8 inch cartridges in a 50-pound case for wrapped dynamites.
Such a cartridge contains about 9.78 cubic inches. A count of 100 sticks to the case is roughly
equivalent to a density of 77 pounds per cubic foot. A cubic foot of water weighs 62.4 pounds.