Performance Characteristics of Explosives
In the explosive selection process, the environmental conditions at the site can eliminate certain types of explosives from consideration. After the environmental conditions have been considered, one must consider the performance characteristics of explosives. Characteristics of main concern are: (1) sensitivity, (2) velocity, (3) density, (4) strength, and (5) cohesiveness.
a. Sensitivity.
The sensitivity of an explosive product is defined by the amount of input energy necessary to cause the product to detonate reliably. This is sometimes called the minimum booster rating or minimum priming requirements. Some explosives require little energy to detonate reliably. The standard No. 8 blasting cap will detonate in dynamite and some of the cap sensitive slurry explosives. On the other hand, a blasting cap alone will not initiate bulk loaded ANFO and slurry that has not been altered by water. Cycled ANFO can be more sensitive than unaltered ANFO. For reliable detonation, one would have to use a booster or primer in conjunction with the blasting cap. Hazard sensitivity defines an explosive’s response to the accidental addition of energy, such as bullet impact. Table 2-7 lists the sensitivity of commonly used explosives.
b. Velocity.
(1) The detonation velocity is the speed at which the reaction moves through the column of explosive. It ranges from 5,000 ft/s to 25,000 ft/s for commercially used products. Detonation velocity is an important consideration for applications outside a borehole, such as plaster shooting, mud capping or shearing structural members. Detonation velocity has significantly less importance if the explosives are used in the borehole. Table 8 lists the detonation velocities of commonly used explosives.
c. Density.
(1) The density of an explosive is important because explosives are purchased, stored, and used on a weight basis. Density is normally expressed in terms of specific gravity, which is the ratio of explosive density to water density. The density of an explosive determines the weight of explosive that can be loaded into a specific borehole diameter. On a weight basis, there is not a great deal of difference in energy between various explosives. The difference in energy on a unit weight basis is nowhere near as great as the difference in energy on a volume basis. When hard rock is encountered and drilling is expensive, a denser product of higher cost is often justified. Table 10 lists the density of commonly used explosives.
(3) An easy method to calculate loading density is:
d. Strength.
(1) Strength refers to the energy content of an explosive, which in turn is the measure of the force it can develop and its ability to do work. Strength has been rated by various manufacturers, both on an equal weight and an equal volume basis, and is commonly called weight strength and cartridge or bulk strength. There is no standard method to measure strength universally used by the explosives manufacturers. Instead many different strength measurement methods exist such as the ballistic mortar test, seismic execution values, strain pulse measurement, cratering, calculation of detonation pressures, calculation of borehole pressures, and determination of heat release. However, none of these methods can be used satisfactorily for blast design purposes. Strength ratings are misleading and do not accurately compare rock fragmentation effectiveness with explosive type. In general, one can say that strength ratings are only a tool used to identify the end results and associate them with a specific product.
(2) One type of strength rating, the underwater shock and bubble energy test used to determine the shock energy and the expanding gas energy, is used by some for design purposes. The bubble energy tests produce reliable results that can be used for approximating blast design dimensions.
(3) In the United States, explosives are commonly rated by methods called relative weight strength and relative bulk strength. Relative weight strength refers to an arbitrary index that compares the strength of equal weights of the explosive being rated and the standard explosive, which is ANFO. Relative bulk strengths compare to relative strengths of equal volumes of explosives. An arbitrary scale is used to compare the weight of a fixed volume of the explosive being rated to a fixed volume of ANFO. Normally, these rating numbers are given as either decimal fractions, or by arbitrarily setting the weight of ANFO as 100 and comparing other explosives against ANFO. Therefore, their values would be either somewhat greater or less than 100.
e. Cohesiveness.
Cohesiveness is defined as the ability of the explosive to maintain its original shape. There are times when explosive must maintain its original shape and others when it should flow freely. For example, when blasting in cracked or broken ground, one definitely wants to use an explosive that will not flow into the cracked area causing holes to be overloaded. Conversely, in other applications such as in bulk loading, explosives should flow freely and should not bridge the borehole nor form gaps in the explosive column.