Chapter One - Explosives Products

Chemistry and Physics of Explosives

Explosive

An explosive is a chemical compound or mixture of compounds that undergoes a very rapid decomposition when initiated by energy in the form of heat, impact, friction, or shock. This decomposition produces mostly gases and a large amount of heat. The very hot gases produce extremely high pressure within the borehole, and these pressures cause the rock to be fragmented. If the speed of reaction of the explosive is faster than the speed of sound in the explosive (detonation), the product is called a high explosive. If the reaction of the explosive is slower than the speed of sound (deflagration), the product is called a low explosive.

Reactive Ingredients

The principal reacting ingredients in an explosive are fuels and oxidizers. Common fuels in commercial products include fuel oil, carbon, and aluminum. TNT, smokeless powder, monomethylamine nitrate, and monoethanolamine nitrate are also used as fuels although they contain their own oxygen. Fuels often perform a sensitizing function. Common explosive sensitizers are nitroglycerin, nitrostarch, aluminum, TNT, smokeless powder, monomethylamine nitrate, and monoethanolamine nitrate.

Microballoons and aerating agents are sometimes added to enhance sensitivity. The most common oxidizer is ammonium nitrate, although sodium nitrate and calcium nitrate may also be used. Other ingredients of explosives include water, gums, thickeners and cross-linking agents used in slurries, gelatinizers, densifiers, antacids, stabilizers, absorbents, and flame retardants. In molecular explosives such as nitroglycerin, Trinitrotoluene (TNT), and Pentaerythritoltetranitrate (PETN) the fuel and oxidizer are combined in the same compound.

Elements in Explosives

Most ingredients of explosives are composed of the elements: oxygen, nitrogen, hydrogen, and carbon. In addition, metallic elements such as aluminum are sometimes used. For explosive mixtures, energy release is optimized at zero oxygen balance. Zero oxygen balance is defined as the point at which a mixture has sufficient oxygen to completely oxidize all the fuels it contains but there is no excess oxygen to react with nitrogen oxides.

Oxygen Balance

Theoretically, at zero oxygen balance the gaseous products of detonation are water (H₂O), carbon dioxide (CO₂), and nitrogen (N₂), although in reality small amounts of nitrogen oxide (NO), carbon monoxide (CO), ammonia (NH₃), methane (CH₄), and other gases are generated. Partial oxidation of carbon to carbon monoxide, which results from an oxygen deficiency, releases less heat than complete oxidation to carbon dioxide. The oxides of nitrogen, which are produced when there is excess oxygen, are "heat robbers"; that is, they absorb heat when generated. Free nitrogen, being an element, neither absorbs nor releases heat upon liberation.

It should be noted that the gases resulting from improper oxygen balance are not only inefficient in terms of heat energy released, but are also poisonous. Although the oxidation of aluminum yields a solid, rather than a gaseous product, the large amount of heat released adds significantly to the explosive's energy. Magnesium is even better from the standpoint of heat release, but it is too sensitive to use in commercial explosives.

ANFO and Oxygen Balance

The principle of oxygen balance is best illustrated by the reaction of ammonium nitrate (NH₄NO₃) and fuel oil (CH₂) mixtures. Commonly called ANFO (where AN represents ammonium nitrate and FO represents fuel oil), these mixtures are the most widely used blasting agents. From the reaction equations for ANFO, one can readily see the relationship between oxygen balance, detonation products, and heat release. The equations assume an ideal detonation reaction, which in turn assumes thorough mixing of ingredients, proper particle sizing, adequate confinement, charge diameter and priming, and protection from water. Fuel oil is actually a variable mixture of hydrocarbons and is not precisely CH₂, but this identification simplifies the equations and is accurate enough for the purposes of this manual. In reviewing these equations, keep in mind that the amount of heat produced is a measure of energy released.

Although the simple ANFO mixture is optimum for highest energy release per unit cost of ingredients, products with higher energies and densities are often desired. The common high-energy production additives, which may be used in both dry blasting agents and slurries, fall into two basic categories: explosives such as TNT, and metals such as aluminum.

Both of these mixtures release more energy, based on weight, than ammonium nitrate-carbonaceous fuel mixtures and have the added benefit of higher densities. These advantages must be weighed against the higher cost of such high-energy additives. The energy of aluminized products continues to increase with larger percentages of metal, even though this "overfueling" with metals is not economical except for such specialty products as high-energy boosters.

Detonation Process

The chemical reaction of an explosive creates extremely high pressures. It is these pressures which cause rock to be broken and displaced. To illustrate the pressures created in the borehole, let's take a brief look at the detonation process as pictured by Dr. Richard Ash of the University of Missouri-Rolla.

Figure 1 (adapted from Ash's work) shows a column of explosive or blasting agent that has been initiated. Detonation has proceeded to the center of the column. The primary reaction occurs between a shock front at the leading edge and a rear boundary known as the Chapman-Jouquet (C-J) plane. Part of the reaction may occur behind the C-J plane, particularly if some of the explosive's ingredients are coarse. The length of the reaction zone, which depends on the explosive's ingredients, particle size, density, and confinement, determines the minimum diameter at which the explosive will function dependably (critical diameter). High explosives, which have short reaction zones, have smaller critical diameters than blasting agents.

Pressure Profiles

The pressure profiles in Figure 1 show the explosive forces applied to the rock being blasted. A general comparison is given between an explosive and a blasting agent, although it should be understood that each explosive or blasting agent has its own particular pressure profile depending on its ingredients, particle size, density, and confinement.

The initial pressure, called the detonation pressure (Pd), is created by the supersonic shock front moving out from the detonation zone. The detonation pressure gives the explosive its shattering action in the vicinity of the borehole. If the explosive reacts slower than the speed of sound, which is normally the case with black powder, there is no detonation pressure.

The detonation pressure is followed by a sustained pressure called explosive pressure (Pe), or borehole pressure. Borehole pressure is created by the rapid expansion of the hot gases within the borehole. The detonation pressure of high explosives is often several times that of blasting agents, but the borehole pressures of the two types of products are of the same general magnitude. The relative importance of detonation pressure and borehole pressure in breaking rock will be discussed in the "Properties of Explosives" section of this chapter.

Types of Explosives and Blasting Agents

This section will cover all explosive products that are used for industrial rock blasting, with the exception of initiators. Products used as the main borehole charge can be divided into three categories:

1. Nitroglycerin (or nitrostarch) based high explosives
2. Dry blasting agents
3. Slurries (also referred to as water gels or emulsions)

These products can also be broadly categorized as explosives and blasting agents. For ease of expression, the term "explosives" will often be used in this manual to collectively cover both explosives and blasting agents.

High Explosives

A high explosive is any product used in blasting that is sensitive to a No. 8 cap and that reacts faster than the speed of sound in the explosive medium. A low explosive is a product in which the reaction is slower than the speed of sound. Low explosives are seldom used in blasting today.

Blasting Agents

A blasting agent is any material or mixture consisting of a fuel and an oxidizer intended for blasting and not otherwise classified as an explosive, provided the finished product, as mixed and packaged for shipment, cannot be detonated by a No. 8 blasting cap in a specific test prescribed by Energy Mines and Resources Canada. Slurries containing TNT, smokeless powder, or other explosive ingredients are classified as blasting agents if they are insensitive to a No. 8 blasting cap.

ANFO, which in normal form is a blasting agent, can be made cap sensitive by pulverizing it to a fine particle size, and a slurry can be made cap sensitive by including a sufficient amount of finely flaked paint-grade aluminum. Although neither of these products contains an explosive ingredient, because of their cap sensitivity they are explosives.

Nitroglycerin-Based Explosives

Nitroglycerin-based explosives can be categorized by their nitroglycerin content. As a group, nitroglycerin-based explosives are the most sensitive commercial products used today (excluding detonators). Because of this sensitivity they offer an extra margin of dependability in the blasthole but are somewhat more susceptible to accidental detonation. This is a tradeoff that many operators using small-diameter boreholes must make. Nitroglycerin dynamites account for less than 5 percent, by weight, of the explosives market, and almost all of that is in small-diameter work. Dynamite is available in cartridges of various sizes and shapes.

Properties of Nitroglycerin-Based Explosives

Nitroglycerin

Nitroglycerin (NG), the first high explosive, is the sensitizer in dynamites and is seldom used alone. It has a specific gravity of 1.6 and a detonation velocity slightly over 25,000 feet per second. Its extreme sensitivity to shock, friction, and heat make it hazardous to use.

Straight Nitroglycerin Dynamite

Straight dynamite consists of nitroglycerin, sodium nitrate, an antacid, a carbonaceous fuel, and sometimes sulphur. The term "straight" means that a dynamite contains no ammonium nitrate. The weight strength, usually 50 percent, indicates the approximate percentage of nitroglycerin or other explosive oil. The use of straight dynamite is limited because of its high cost and sensitivity to shock and friction. Fifty percent straight dynamite is referred to as ditching dynamite and is used in propagation blasting.

High-Density Ammonia Dynamite

High-density ammonia dynamite, also called extra dynamite, is the most widely used dynamite. It is like straight dynamite, except that ammonium nitrate replaces part of the nitroglycerin and sodium nitrate. Ammonia dynamite is manufactured in grades of 20 to 60 percent weight strength, although these grades are not truly equivalent to straight dynamites of the same weight strength. Ammonia dynamite is less sensitive to shock and friction than straight dynamite.

Low-Density Ammonia Dynamite

Low-density ammonia dynamite is manufactured in a weight strength of about 65 percent. The cartridge (bulk) strength ranges from 20 to 50 percent, depending on the bulk density of the ingredients. A high-velocity series and a low-velocity series are manufactured. Low-density ammonia dynamite is useful in very soft or prefractured rock or where coarse rock such as riprap is required.

Blasting Gelatin

Blasting gelatin is a tough rubber-textured explosive made by adding nitrocellulose, also called guncotton, to nitroglycerin. An antacid is added to provide storage stability and wood meal is added to improve sensitivity. Blasting gelatin is the most powerful nitroglycerin-based explosive.

Straight Gelatin

Straight gelatin is basically a blasting gelatin with sodium nitrate, carbonaceous fuel, and sometimes sulphur added. It is manufactured in grades ranging from 20 to 90 percent weight strength and is the gelatinous equivalent of straight dynamite. Straight gelatin has been used mainly in specialty areas such as seismic or deep well work, where a lack of confinement or a high head of water may affect its velocity. To overcome these conditions a high-velocity gelatin is available which is like straight gelatin except that it detonates near its rated velocity despite high heads of water.

Ammonia Gelatin

Ammonia gelatin, also called special gelatin or extra gelatin, is a straight gelatin in which ammonium nitrate has replaced part of the nitroglycerin and sodium nitrate. Manufactured in weight strengths ranging from 40 to 80 percent, it is the gelatinous equivalent of ammonia dynamite. Ammonia gelatin is suitable for underground work, in wet conditions, and as a toe load, primarily in small-diameter boreholes. The higher grades (70 percent or higher) are useful as primers for blasting agents.

Semigelatin

Semigelatin has a weight strength near 65 percent. The cartridge (bulk) strength ranges from 30 to 60 percent with variations in the bulk density of the ingredients. Semigelatin is versatile and is used in small-diameter work where some water resistance is required. It is useful underground, where its soft, plastic consistency makes it ideal for loading into holes drilled upward.

Nitrostarch Explosives

Nitrostarch explosives are sensitized with nitrostarch, a solid molecular explosive, rather than an explosive oil. They are manufactured in various grades, strengths, densities, and degrees of water resistance to compete with most grades of nitroglycerin-based dynamites. They are similar to dynamites in many ways, with their most significant differences being somewhat higher impact resistance and their "headache free" nature.

Ammonium Nitrate - Fuel Oil (ANFO)

Dry Blasting Agent

Early dry blasting agents employed solid carbon fuels combined with ammonium nitrate in various forms. Through experimentation it was found that diesel fuel oil mixed with porous ammonium nitrate prills gave the best blasting results. Hence, the term ANFO (ammonium nitrate-fuel oil) has been synonymous with dry blasting agent. An oxygen-balanced ANFO mixture is the cheapest source of explosive energy available today. Adding finely divided or flaked aluminum to dry blasting agents increases the energy output but at an increase in cost.

Aluminized dry mixes are sometimes used in combination with primers as primers for ANFO. Aluminized mixes may also be used as a high-energy toe load and as the main column charge where blasting is difficult.

Properties of Dry Blasting Agents

It is difficult to give precise numerical values for the properties of dry blasting agents because the properties vary with ingredient particle size, density, confinement, charge diameter, water conditions, and coupling ratio.

Coupling Ratio

Coupling ratio is the percentage of the borehole diameter filled with explosive. Poured bulk products are completely coupled, which increases their efficiency. Cartridge products are partially decoupled, and lose some efficiency.

ANFO Mixes

ANFO's theoretical energy is optimized at oxygen balance (approximately 94.5% AN and 5.5% FO), where the detonation velocity approaches 5000 meters per second in large charge diameters.

• Excess fuel oil (8% or more) can seriously reduce sensitivity to initiation
• Inadequate fuel oil causes an excess of harmful nitrogen oxide fumes in the detonation gases
• Specific gravities of ANFO range from 0.5 to 1.15; 0.80 to 0.85 is the most common range

The lighter products are useful in easily fragmented rock or in eliminating the need for alternate decks of explosive and stemming where a low powder factor is desirable. The densified dry mixes are packaged in waterproof containers for use in wet blastholes.

Specific Gravity and ANFO

Densification is necessary to enable the cartridges to sink in water. To obtain a higher specific gravity, part of the prills are pulverized and then the mixture of whole and pulverized prills is vibrated or compressed into rigid cartridges or polyburlap bags. The sensitivity of ANFO decreases with increased density. The "dead press" limit, about which detonation is undependable, is about 1.25 grams per cubic centimeter.

Detonation Velocity of ANFO

The detonation velocity of ANFO is strongly affected by charge diameter. The critical diameter is near 25 mm with a normal prill and oil mixture. Although ANFO has been detonated at 25 mm, it is not recommended for blastholes diameter below 25 mm unless pneumatically loaded. The velocity increases with diameter and levels off near a 375 mm diameter, at a velocity of nearly 5000 meters per second.

The minimum primer required for ANFO increases as charge diameter increases. There is a tendency to underprime in large-diameter boreholes. A good rule of thumb is: when in doubt, overprime. Many operators claim improved results when they use primers that fill, or nearly fill, the blasthole diameter.

Wet Holes

The undesirable effects of water on dry blasting agents has often been seen in poor blasts where ANFO was used in wet boreholes with insufficient external protection. Excess water adversely affects the velocity, sensitivity, fume class, and energy output of a dry blasting agent. The extreme result is a misfire. It is essential when using ANFO in wet conditions that positive protection in the form of a waterproof package or a borehole liner be used.

Purchased Forms of Dry Blasting Agents

Dry blasting agents can be purchased in three forms. In increasing costs they are as follows:

1. As separate ingredients in bulk form for onsite mixing
2. Premixed in bulk form for onsite delivery (a premixed product may cost about the same as separate ingredients)
3. In polyethylene packages for pouring into the boreholes

Slurries, Water Gels, Emulsions

Composition of Slurries

A slurry is a mixture of nitrates such as ammonium nitrate and sodium nitrate, a fuel sensitizer either explosive or nonexplosive, and varying amounts of water. A water gel is essentially the same as a slurry and the two terms are frequently used interchangeably. An emulsion is somewhat different than a water gel or slurry in physical character but similar in many functional respects. The principal differences are an emulsion's generally higher detonation velocity and a tendency to wet or adhere to the blasthole. In some cases this may affect its bulk loading characteristics. In this section slurries, water gels, and emulsions will be treated as a family of products.

Water Resistance of Slurry Type Products

Although they contain large amounts of ammonium nitrate, slurries are made water resistant through the use of gums, waxes, and cross-linking agents. The variety of possible slurry formulations is almost infinite. Frequently a slurry is specially formulated for a specific job. The list of possible fuel sensitizers is especially long, although carbonaceous fuel, aluminum, and amine nitrates are the most common.

Classification of Slurry Type Products

Slurries may be classified as either explosives or blasting agents. Those that are sensitive to an E.B. detonator are classified as explosives, even though they are less sensitive than dynamites. It is important that slurries be stored in magazines appropriate to their classification.

Efficiency

Except for their excellent water resistance and higher density and bulk strength, slurries are similar in many ways to dry blasting agents. Good oxygen balance, decreased particle size and increased density, increased charge diameter, good confinement and coupling, and adequate priming all increase their efficiency. Although slurry blasting agents tend to lose sensitivity as their density increases, some explosive-based slurries function well at densities up to 1.6. The effect of charge diameter on the detonation velocity of slurries is not as pronounced as it is on ANFO.

Sensitivity

Most non cap-sensitive slurries depend on entrapped air for their sensitivity and most cap-sensitive varieties are also dependent, to a lesser degree, on this entrapped air. If this air is removed from a slurry through pressure from an adjacent blast, prolonged periods of time in the borehole, or prolonged storage, the slurry may become desensitized.

Slurries can be delivered as separate ingredients for onsite mixing, premixed for bulk loading, in polyethylene bags for bulk loading or loading in the bag, or they may be cartridged. Their consistency may be anywhere from a liquid to a cohesive gel.

Cartridged Products

Cartridged slurries for use in small-diameter blastholes (25 mm diameter or less) are normally made cap-sensitive so they can be substituted for dynamite. However, their lower sensitivity as compared with dynamite should be kept in mind.

Cost

In wet conditions where dewatering is not practical, and the rock is not extremely difficult to fragment, a low-cost slurry can be competitive with ANFO. Aluminized slurries or those containing significant amounts of other high-energy sensitizers develop sufficient energy for blasting in hard, dense rock. However, the economics of using total column charges of highly aluminized slurry are doubtful because of the significantly higher cost of these products. High energy slurries have improved blasting efficiency when used in combination with the primer at the toe or in another zone of difficult breakage.

Detonating Cord Downlines and Blasting Agent Slurries

Detonating cord downlines can have a harmful effect on the efficiency of blasting agent slurries, depending on the size of the blasthole and the strength of the cord. When using detonating cord downline, the slurry manufacturer should be consulted concerning the effect of the cord on the slurry. The technology of slurries is very dynamic. New products are continually being developed. Blasters should check the technical literature to be aware of developments that affect their blasting program.

Heavy ANFO

A new area of technology referred to as "Heavy ANFO" is emerging in the blasting industry. Heavy ANFO is a mixture of normal ANFO with one or more common blasting materials. Generally, these materials are emulsions, water gels, or slurries. The percentage of the individual components of the mixture can vary greatly, but generally in increments of no less than 5 percent. This flexibility permits the blaster to vary the explosive energy and water resistance not only for an individual borehole in the pattern, but also along the entire length of the borehole.

Heavy ANFO allows the blaster to increase energy per foot of borehole because the more dense products fill the void between adjacent ammonium nitrate prills. The emulsion, water gel, or slurry adds to the water resistance of the mix by encapsulating the prills.

To take advantage of this new technology, a modified loading vehicle is required. The vehicle needs compartments to store each component separately. Metering and mixing devices are also needed to blend the desired mixture. It is expected that there will be many changes in this emerging technology, not only in formulation, but also in application, particularly in underground operations.

Primers and Boosters

Primer

The terms "primer" and "booster" are often confused. According to MSHA, a primer, sometimes called a capped primer, is a unit of cap-sensitive explosive used to initiate other explosives or blasting agents. A primer contains a detonator. A booster is often, but not always, cap sensitive, but does not contain a detonator. A booster is used to perpetuate or intensify an explosive reaction.

Cast Primers

Although various products have been used as primers and boosters, an explosive with a high detonation pressure, such as a high-strength ammonia gelatin or a cast military explosive (composition B or pentolite), is recommended. Cast primers have a sensitive inner core that will accept detonation from a detonator or detonating cord. Cast primers with built-in millisecond delay units are available. These primers, when strung on a single detonating cord downline, enable the blaster to place as many delayed decks in the blasthole as the blast design requires.

Primers and Borehole Size

Although small 4.54 gram cast primers are popular, even in large boreholes, a primer functions best when its diameter is near the size of the borehole. A two-stage primer, with a charge of high energy dry blasting agent or slurry poured around a cast primer or ammonia gelatin, is frequently used in large-diameter blastholes. In small-diameter work, excellent results have been reported with a high-strength blasting cap used to initiate ANFO, thus eliminating the need for a primer.

Black Powder

Black powder, a mixture of potassium or sodium nitrate, charcoal, and sulphur, dates back to ancient times. Once the principal commercial explosive, black powder is extremely prone to accidental initiation by flame or spark. When initiated, it burns at a very rapid rate. This rapid burning, called deflagration, is much slower than typical detonation velocities. Black powder has a specific gravity of 1.6 or less (depending on granulation), has poor water resistance, and emits large volumes of noxious gases upon deflagration. Black powder finds limited use in blasting dimension stone where a minimum of shattering effect is desired. It is not an efficient explosive for fragmenting rock.

Properties of Explosives

Explosives and blasting agents are characterized by various properties that determine how they will function under field conditions. Properties of explosives which are particularly important to the blaster include:

• Strength
• Detonation velocity
• Density
• Water resistance
• Fume class
• Detonation pressure
• Borehole pressure
• Sensitivity and sensitiveness

Numerous other properties can be specified for explosives but have not been included because they are not important to the field blaster.

Strength

The strength of explosives has been expressed in various terms since the invention of dynamite. The terms "weight strength" and "cartridge strength," which originally indicated the percentage of nitroglycerin in an explosive, were useful when nitroglycerin was the principal energy-producing ingredient. However, with the development of products with decreasing proportions of nitroglycerin, these strength ratings have become misleading and inaccurate and do not realistically compare the effectiveness of various explosives.

More recently, calculated energy values have been used to compare the strengths of explosives with ANFO being used as a base of 1.0. Although this system has not been universally adopted, it is an improvement over weight strength and cartridge strength in estimating the work an explosive will do. Other strength rating systems such as seismic execution value, strain pulse measurement, cratering, and the ballistic mortar have been used, but do not give a satisfactory prediction of the field performance of an explosive.

Underwater tests have been used to determine the shock energy and expanding gas energy of an explosive. These two energy values have been used quite successfully by explosive manufacturers in predicting the capability of an explosive to break rock.

Detonation Velocity

Detonation velocity is the speed at which the detonation front moves through a column of explosives. It ranges from 1,675 to 7,600 meters per second for products used commercially today. A high-detonation velocity gives the shattering action that many experts feel is necessary for difficult blasting conditions, whereas low-velocity products are normally adequate for the less demanding requirements typical of most blasting jobs. Detonation velocity, particularly in modern dry blasting agents and slurries, may vary considerably depending on field conditions.

Detonation velocity can often be increased by:

1. Using a larger charge diameter
2. Increasing density (although excessively high densities in blasting agents may seriously reduce sensitivity)
3. Decreasing particle size (pneumatic injection of ANFO in small diameter boreholes accomplishes this)
4. Providing good confinement in the borehole
5. Providing a high coupling ratio (coupling ratio is the percentage of the borehole diameter filled with explosive)
6. Using a large initiator or primer (this will increase the velocity near the primer but will not alter the steady state velocity)

Density

Density is normally expressed in terms of specific gravity, which is the ratio of the density of the explosive to that of water. A useful expression of density is loading density, which is the weight of explosive per unit length of charge at a specified diameter, commonly expressed in pounds per foot. Cartridge count (number of 1-1/4 by 8-inch cartridges per 50-pound box) is useful when dealing with cartridged high explosives and is approximately equal to 141 divided by the specific gravity. The specific gravity of commercial products ranges from 0.5 to 1.7.

The density of an explosive determines the weight that can be loaded into a given length of borehole. Where drilling is expensive, a higher cost, dense product is frequently justified. The energy per unit volume of explosive is actually a more important consideration, although it is not a commonly reported explosive property.

Water Resistance

Water resistance is the ability of an explosive product to withstand exposure to water without losing sensitivity or efficiency. Gelled products such as gelatin dynamites and water gels have good water resistance. Ammonium nitrate prills have no water resistance and should not be used in the water-filled portions of a borehole. The emission of brown nitrogen oxide fumes from a blast often indicates inefficient detonation frequently caused by water deterioration, and signifies the need for a more water-resistant explosive or external protection from water in the form of a plastic sleeve or a waterproof cartridge.

Fume Class

Fume class is a measure of the amount of toxic gases, primarily carbon monoxide and oxides of nitrogen, produced by the detonation of an explosive. Most commercial blasting products are oxygen balanced both to minimize fumes and to optimize energy release per unit cost of ingredients. Fumes are an important consideration in tunnels, shafts, and other confined spaces.

Certain blasting conditions may produce toxic fumes even with oxygen-balanced explosives:

• Insufficient charge diameter
• Inadequate priming or initiation
• Water deterioration
• Removal of wrappers
• Use of plastic borehole liners

MSHA standards limit the volume of poisonous gases produced by a permissible explosive to 2.5 cubic feet per pound of explosive.

Detonation Pressure

The detonation pressure of an explosive is primarily a function of the detonation velocity squared times the density. Detonation pressure is the head-on pressure of the detonation wave propagating through the explosive column, measured at the Chapman-Jouquet (C-J) plane.

General Formula to Determine Detonation Pressure:

P = 4.18 × 10⁻⁷ × D × C² / (1 + 0.8D)

Where:

• P = detonation pressure in kilobars (1 kb = 14,504 psi)
• D = specific gravity
• C = detonation velocity in feet per second

Some authorities feel that a high detonation pressure resulting in a strong shock wave is of major importance in breaking very dense rock. Others, including Swedish experts, feel that it is of little or no importance. Detonation pressures for commercial products range from about 5 to over 150 kilobars.

Borehole Pressure

Borehole pressure, sometimes called explosion pressure, is the pressure exerted on the borehole walls by the expanding gases of detonation after the chemical reaction has been completed. Borehole pressure is a function of confinement and the quantity and temperature of the gases of detonation.

Borehole pressure is generally considered to play the dominant role in breaking most rocks and in displacing all types of rocks encountered in blasting. This accounts for the success of ANFO and aluminized products which yield low detonation pressures but relatively high borehole pressures. The 100 percent coupling obtained with these products also contributes to their success.

Borehole pressures for commercial products range from less than 10 to 60 kilobars or more. Borehole pressures are calculated from hydrodynamic computer codes or approximated from underwater test results, since borehole pressure cannot be measured directly. Many ANFO mixtures have borehole pressures larger than their detonation pressures. In most high explosives the detonation pressure is the greater.

Swedish Formula for Relative Rock-Breaking Capability:

S = 1/6 (Vx / Vo) + 5/6 (Qx / Qo)

Where:

• S = strength of the explosive
• V = reaction product gas volume
• Q = heat energy
• Subscript x = explosive being rated
• Subscript o = standard explosive

Although the complexity of the fragmentation process precludes the use of a single property for rating explosives, more and more explosives engineers are relying on borehole pressure as the single most important description in evaluating an explosive's rock-breaking capability.

Sensitivity

Sensitivity and sensitiveness are two closely related properties that have become increasingly important with the advent of dry blasting agents and slurries, which are less sensitive than dynamites.

Sensitivity is defined as an explosive's susceptibility to initiation. Sensitivity to a No. 8 testing blasting cap, under certain test conditions, means that a product is classified as an explosive. Lack of cap-sensitivity results in a classification as a blasting agent. Sensitivity among different types of blasting agents varies considerably and is dependent upon:

• Ingredients
• Particle size
• Density
• Charge diameter
• Confinement
• Presence of water
• Temperature (particularly with slurries)

Manufacturers often specify a minimum recommended primer for their products, based on field data. In general, products that require larger primers are less susceptible to accidental initiation and are safer to handle.

Sensitiveness

Sensitiveness is the capability of an explosive to propagate a detonation once it has been initiated. Extremely sensitive explosives, under some conditions, may propagate from hole to hole. An insensitive explosive may fail to propagate throughout its charge length if its diameter is too small. Sensitiveness is closely related to critical diameter, which is the smallest diameter at which an explosive will propagate a stable detonation. Manufacturer's technical data sheets give recommended minimum diameters for individual explosives.

Explosive Selection Criteria

Proper selection of the explosive is an important part of blast design needed to ensure a successful blasting program. Explosive selection is dictated by economic considerations and field conditions. The blaster should select a product that will give the lowest cost per unit of rock broken, while ensuring that fragmentation and displacement of the rock are adequate for the job at hand.

Factors to Consider in Explosive Selection

• Explosive cost
• Charge diameter
• Cost of drilling
• Fragmentation difficulties
• Water conditions
• Adequacy of ventilation
• Atmospheric temperature
• Propagating ground
• Storage considerations
• Sensitivity considerations
• Explosive atmospheres

Economics of ANFO

No other explosive product can compete with ANFO on the basis of cost per unit of energy. Both of the ingredients, ammonium nitrate and fuel oil, are relatively inexpensive, both participate fully in the detonation reaction, and the manufacturing process consists of simply mixing a solid and a liquid ingredient. The safety and ease of storage, handling, and bulk loading add to the attractive economics of ANFO. It is because of these economics that ANFO now accounts for approximately 80 percent by weight of all the explosives used in the United States.

By the pound, slurry costs range from slightly more than to about four times the cost of ANFO. The cheaper slurries are designed for use in large-diameter blastholes and contain no high-cost, high-energy ingredients. They are relatively low in energy per pound. The more expensive slurries are:

1. Those designed to be used in small diameters
2. High-energy products containing large amounts of aluminum or other high-energy ingredients

Dynamite costs range from four to six times that of ANFO, depending largely on the proportion of nitroglycerin or other explosive oil.

Despite its excellent economics, ANFO is not always the best product for the job because it has several shortcomings: ANFO has no water resistance, it has a low specific gravity, and under adverse field conditions it tends to detonate inefficiently.

Charge Diameter under 50 mm

The dependability and efficiency of ANFO are sometimes reduced at smaller charge diameters, especially in damp conditions or with inadequate confinement. In diameters under 50 mm, ANFO functions best when pneumatically loaded into a dry blasthole. When using charge diameters smaller than 50 mm, many blasters prefer the greater dependability of a cartridge slurry or dynamite despite the higher cost. The cost savings that ANFO offers can be lost through one bad blast.

Charge Diameter from 50 mm to 200 mm

At intermediate charge diameters, between 50 mm and 200 mm, the use of dynamite is seldom justified because ANFO and slurries function quite well at these diameters. Slurries designed for use in intermediate charge diameters are somewhat cheaper than small-diameter slurries and are more economical than dynamite. The performance of ANFO in a 200 mm diameter blasthole is substantially better than at 50 mm. Where practical, bulk loading in intermediate charge diameters is economical.

Charge Diameter of 200 mm and Larger

In blasthole diameters larger than 200 mm, a bulk-loaded ANFO or slurry should be used unless there is some compelling reason to use a cartridged product. ANFO's efficiency and dependability increase as the charge diameter increases. Where the use of a slurry is indicated, low-cost varieties function well in large charge diameters.

Cost of Drilling

Under normal drilling conditions, the blaster should select the lowest cost explosive that will give adequate dependable fragmentation. However, when drilling costs increase (typically in hard, dense rock), the cost of explosive and cost of drilling should be optimized through controlled, in-the-mine experimentation with careful cost analysis.

Where drilling is expensive, the blaster will want to increase the energy density of the explosive, even though explosives with high-energy densities tend to be more expensive. Where dynamites are used, gelatin dynamites will give higher energy densities than granular dynamites. The energy density of a slurry depends on its density and the proportion of high-energy ingredients such as aluminum used in its formulation. Because of the diverse varieties of slurry on the market, the individual manufacturer should be consulted for a recommendation on a high-energy slurry.

In small diameter blastholes, the density of ANFO may be increased by up to 20 percent by high-velocity pneumatic loading. The loading density (weight per meter of borehole) of densified ANFO cartridges is about the same as that of bulk ANFO because of the void between the cartridge and the borehole wall. The energy density of ANFO can be increased by the addition of finely divided aluminum. The economics of aluminized ANFO improve where the rock is more difficult to drill and blast.

Drilling and Fragmentation Difficulties

Expensive drilling and fragmentation difficulties frequently go hand in hand because hard, dense rock may cause both. Despite the controversy as to the importance of detonation velocity in rock fragmentation, there is evidence that high velocity does help in fragmenting hard, massive rock. With cartridged dynamites, the detonation velocity increases as the nitroglycerin content increases, with gelatin dynamites having higher velocities than their granular counterparts. Several varieties of slurries, and particularly emulsions, have high velocities. The individual manufacturer should be consulted for a recommendation on a high-velocity product. In general, emulsions exhibit higher velocities than water gels.

Detonation Velocity of ANFO

The detonation velocity of ANFO is highly dependent on its charge diameter and particle size. In diameters of 225 mm or greater, ANFO's detonation velocity will normally exceed 4000 meters per second, peaking near 4600 meters per second in a 375 mm diameter hole. These velocities compare favorably with velocities of most other explosive products. In smaller diameters the detonation velocity decreases, until at diameters below 50 mm the velocity is less than half the 4600 meters per second maximum.

In these small diameters, the velocity may be increased to nearly 3050 meters per second by high velocity pneumatic loading, which pulverizes the ANFO and gives it a higher loading density. As a cautionary note, pressures higher than 14.6 kg per square cm should never be used with a pressure vessel pneumatic loader. Full line pressures of 41 to 50 kg per square cm are satisfactory for ejectors.

In many operations with expensive drilling and difficult fragmentation, it may be advantageous for the blaster to compromise and use a dense, high-velocity explosive in the lower position of the borehole and ANFO as the top load.

ANFO Water Resistance

ANFO has no water resistance. It may, however, be used in blastholes containing water if one of two techniques are followed:

1. Waterproof Packaging: The ANFO may be packaged in a water-resistant, polyburlap container. To enable the ANFO cartridge to sink in water, part of the prills are pulverized and the mixture is vibrated to a density of about 1.1 gram per cubic centimeter. Of course, if a cartridge is ruptured during the loading process, the ANFO will quickly become desensitized.

2. Borehole Dewatering: The blasthole is dewatered by using a down-the-hole submersible pump. A waterproof liner is then placed into the blasthole and ANFO is loaded inside the liner before the water returns to the hole. Again, the ANFO will quickly become desensitized if the borehole liner is ruptured.

The appearance of orange-brown nitrogen oxide fumes upon detonation is a sign of water deterioration, and an indication that a more water-resistant product or better external protection should be used.

Slurries

Slurries are gelled and crosslinked to provide a barrier against water intrusion, and as a result, exhibit excellent water resistance. The manufacturer will usually specify the degree of water resistance of a specific product. When dynamites are used in wet holes, gelatinous varieties are preferred. Although some granular dynamites have fair water resistance, the slightly higher cost of gelatins is more than justified by their increased reliability in wet blastholes.

Factors Influencing the Amount of Toxic Fumes

Although most explosives are oxygen balanced to maximize energy and minimize toxic detonation gases, some are inherently "dirty" from the standpoint of fumes. Even with oxygen balanced products, unfavorable field conditions may increase the generation of toxic fumes, particularly when explosives without water resistance get wet. The use of plastic borehole liners, inadequate charge diameters, removal of a cartridged explosive from its wrapper, inadequate priming, or an improper explosive ingredient mix may cause excessive fumes.

ANFO Underground

In areas where efficient evacuation of detonation gases cannot be ensured (normally underground), ANFO should be used only in absolutely dry conditions. Most small-diameter slurries have very good fume qualities. Large-diameter slurries have variable fume qualities. The manufacturer should be consulted for a recommendation where fume control is important.

Of the cartridged dynamites:

• Ammonia gelatins and semigelatins have the best fume qualities
• High-density ammonia dynamites are rated good
• Low density ammonia dynamites are fair
• Straight dynamites are poor

In permissible blasting where fumes are a concern, care should be exercised in selecting the explosives because many permissibles have poor fume ratings. Permissibles with good fume ratings are available.

Atmospheric Temperature

Until the development of slurries, atmospheric temperatures were not an important factor in selecting an explosive. For many years, dynamites employed low-freezing explosive oils which permit their use in the lowest temperatures encountered in the United States. ANFO and slurries are not seriously affected by low temperatures if priming is adequate. A potential problem exists with slurries that are designed to be cap-sensitive. At low temperature, many of these products may lose their cap-sensitivity although still function well if adequately primed. If a slurry is to be used in cold weather, the manufacturer should be asked about the temperature limitation on the product.

The effect of temperature is alleviated if explosives are stored in a heated magazine or if they are in the borehole long enough to achieve the ambient borehole temperature. Except in permafrost or in extremely cold weather, borehole temperatures are seldom low enough to render slurries insensitive.

Propagating Ground

Propagation is the transfer or movement of a detonation from one point to another. Although propagation normally occurs within an explosive column, it may occur between adjacent blastholes through the ground. In ditch blasting, a very sensitive straight nitroglycerin dynamite is sometimes used to purposely accomplish propagation through the ground. This saves the cost of putting a detonator into each blasthole. Propagation ditch blasting works best in soft, water-saturated ground.

Effects of Propagation Between Holes

In all other types of blasting, propagation between holes is undesirable because it negates the effect of delays. Propagation between holes will result in:

• Poor fragmentation
• Failure of a round to pull properly
• Excessive ground vibrations
• Airblast
• Flyrock

The problem is most serious when using small blastholes loaded with dynamite. Small blastholes require small burdens and spacings, increasing the chance of hole-to-hole propagation, particularly when sensitive explosives are used. Water saturated material and blasthole deviation compound the problem. When propagation is suspected, owing to poor fragmentation, violent shots, or high levels of ground vibrations, the use of a less sensitive product usually solves the problem.

Explosives in decreasing order of sensitivity:

1. Straight nitroglycerin dynamite (most sensitive)
2. Other granular dynamites
3. Gelatin dynamites
4. Cap-sensitive slurries
5. Blasting agents (least sensitive)

Dead Pressed ANFO or Slurry

A different problem can occur when ANFO or slurry blasting agents are used at close spacings in soft ground. The shock from an adjacent charge may dead press a blasting agent column and cause it to misfire.

Storage Considerations

Federal requirements for magazine construction are less stringent for blasting agents than high explosives. Magazines for the storage of high explosives must be well ventilated and must be resistant to bullets, fire, weather, and theft; whereas a blasting agent magazine need only be theft and weather resistant. Although this is not an overriding reason for selecting a blasting agent rather than an explosive, it is an additional point in favor of blasting agents.

Two-Compound Explosives

Federal regulations do not require ingredients of two-component explosives to be stored in magazines nor is there a minimum distance requirement for separation of the ingredients from each other or from explosive products. Even though there is no Federal regulation requiring magazine storage, two-component explosives should be protected from theft. The use of two-component explosives eliminates the need for frequent trips to a magazine. However, when large amounts of explosives are used, the higher cost and the time consuming process of explosive mixing begin to outweigh the savings in travel time.

Sensitivity Considerations

Sensitivity affects both the safety and the dependability of an explosive. More sensitive explosives, such as dynamites, are somewhat more vulnerable to accidental initiation by impact or spark than blasting agents. Slurries and nitrostarch-based explosives are generally less sensitive to impact than nitroglycerin-based dynamites. However, more sensitive explosives, all conditions being equal, are less likely to misfire in the blasthole.

For instance, upon accidental impact from a drill bit, a blasting agent is less likely to detonate than a dynamite. This does not mean that the blasting agent will not detonate when accidentally impacted. Conversely, under adverse situations such as charge separation in the blastholes, very small charge diameters, or low temperatures, dynamites are less likely to misfire than blasting agents. This tradeoff must be considered primarily when selecting an explosive for small-diameter work.

Other selection criteria usually dictate the use of blasting agents when the blasthole diameter is large.

It can be concluded from explosive consumption and field observations that most of the dynamite still used in this country is used in construction, small quarries, and underground mines, where many operations consider a more sensitive explosive beneficial in their small-diameter blasting.

When safely handled and properly loaded, dynamites, dry blasting agents, and slurries all have a place in small-diameter blasting.

Explosive Atmospheres

Blasting in a gassy atmosphere can be catastrophic if the atmosphere is ignited by the flame from the explosive. All underground coal mines are classified as gassy; some metal-nonmetal mines may contain methane or other explosive gases; and many construction projects encounter methane. Where gassy conditions are suspected, MSHA or OSHA should be consulted for guidance.

Permissible explosives offer protection against gas explosions. Most permissible explosives are relatively weak explosives, and will not do an adequate job in most rock, although some relatively powerful permissible gelatins, emulsions, and slurries are available.