Page 414 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.14 THE WORK

FIGURE 9.12 Angle drilling.

Strength is the energy content of an explosive in relation to its weight. In general, maximum
explosive power can be obtained from a given borehole by using a high-density, high-strength
explosive in it.
Velocity is a measure, expressed in feet per second, of the speed at which the burning or the
detonation wave travels through an explosive. It varies from 1,000 to 3,000 feet per second for
black powders to 23,000 feet per second for blasting gelatin.
Low-velocity explosion has a heaving and separating effect, while high velocity crushes and
shatters.
Water resistance is an important factor in wet rock, and varies with not only the character of
the explosive but the manner in which it is packed and wrapped. Manufacturers are able to put
water resistance in the explosive rather than in the wrapper.
The gases resulting from explosions vary in toxic and irritating qualities. This is very important
in underground work, particularly if ventilation is poor. Explosives are rated by the manufacturers
according to fumes as excellent, good, fair, and poor.
Explosives vary widely in the length of time they can be kept under various conditions before
deterioration makes them dangerous or useless. Dynamite was formerly damaged by freezing, but
this difficulty has been entirely overcome. Spoiling may be a serious factor if use is subject to
delay, particularly in hot, wet weather.
Different explosives vary widely in price. The most economical one for a certain use may be
one with a high cost. It is important to select an explosive on the basis of all the related factors,
rather than purchase cost alone.
In many areas, very few types of explosive may be available, and because of the complications
of shipping, delivery of special orders may be delayed weeks or months. Under such conditions,
use of the standard dynamite may be advisable even if it is not exactly suited to the job.