Chapter 4: Theory of Explosives

Learning Objectives

• Describe the characteristics that make an explosive unique.
• Compare and contrast pure chemical (or molecular) explosives and chemical mixture explosives.
• Identify common types of explosives used in blasting.
• Describe detonation and deflagration.
• Identify the most common effects of rock blasting.
• Discuss the properties of commercial explosives.
• Explain the concepts of strength and energy as related to explosives.
• Describe velocity of detonation (VOD) and its principles.
• Discuss density and its importance to blasters.
• Describe the characteristics and behaviour of toxic gases in blasting operations.
• Discuss how temperature and water exposure can affect the performance of explosives.
• Explain the specific criteria for selecting an explosive.
• Explain conditions in which cartridge explosives should be used.
• Define dead pressing and conditions in which it is likely to occur.
• Define sympathetic propagation or detonation.
• Identify conditions in which misfires could occur.

Overview

Blasters must understand the basic theory of explosives and the tools of the blasting trade. This includes knowing the characteristics, effects, and properties of commercial explosives, as well as the general and specific criteria for selecting an explosive.

Characteristics

Consider the three characteristics that make an explosive unique:

• Method of initiation - What sets it off or initiates the reaction
• Composition - What it consists of
• Detonation or deflagration - What happens when it is initiated

Method of Initiation

An explosive is designed to be initiated by the shock effect of a detonator or another explosive. Many explosives are sensitive to heat, friction, and impact. For these reasons, they should be protected from effects that could cause premature or accidental detonation.

Composition

An explosive is a type of chemical that detonates when shock, heat, or impact is applied. Some explosives are pure chemicals that have a unique, consistent molecular structure. These chemical compounds typically include carbon (C), hydrogen (H), nitrogen (N), and oxygen (O) within each molecule.

Pure Chemical (Molecular) Explosives

Examples include:

• PETN - Used in detonating cord and detonator base charges
• TNT - Used with PETN to make cast boosters or on its own
• HMX - Used in shock tube
• Lead azide - Used in detonators

Chemical Mixture Explosives

Other explosives come in the form of chemical mixtures. Chemical mixtures are combinations of fuels and oxidizers. Examples of mixtures include ammonium nitrate and fuel oil (ANFO) and emulsions. The more thoroughly the fuel and the oxidizer are mixed together, the higher the energy output. Other ingredients can be added to further affect overall performance. For example, guar can be added for water resistance (such as in water-resistant ANFO or WR) or aluminum for increased energy.

Explosives manufacturers make chemical mixtures for reasons such as the following:

• To make an explosive from two non-explosive compounds
• To achieve proper oxygen balance (the ratio of oxygen to fuel in the explosive)
• To change chemical properties (such as toxicity)
• To change physical properties (such as density)
• To change explosive properties (such as velocity of detonation)
• To change explosive forms (shaped, plasticized, rubberized, etc.)

Mixtures give manufacturers the ability to make hundreds of types of explosives. The two types most common in drilling and blasting are ANFO and slurries (typically in the form of emulsions).

Detonation

Detonation is an explosive reaction that moves through an explosive at a velocity greater than the speed of sound.

Upon detonation, most commercial explosives are capable of producing gases with temperatures ranging from 1,600 to 3,800°C (3,000 to 7,000°F) and pressures ranging from 1,379,000 to 103,425,000 kPa (200,000 to 1,500,000 psi).

Method of initiation + Explosive = Detonation

Deflagration

Deflagration is an explosive reaction, such as a rapid combustion, that moves through an explosive at a velocity less than the speed of sound.

Effects

In most applications, detonation of commercial explosives produces a shock wave and a sudden release of heat and explosive gases.

Explosives used in rock blasting typically have four common effects:

1. Fragmentation of material - Breaking up the rock
2. Displacement of material - Moving the rock
3. Vibration of ground
4. Concussion (air blast)

The gas pressure released in an explosion is the main cause of fragmentation and displacement of rock, as well as vibration of ground. Gas pressure can also increase the concussion, depending on the site conditions.

The shock wave produced by an explosive is not a major cause of fragmentation, displacement, or vibration. However, the shock wave may have an effect on the concussion in rock blasting, depending on the site conditions.

Specialty Applications

Explosives are used in many industries for specialty applications. A few examples include:

• Plasticized or rubberized explosives are used to harden metals in the explosive forging of railway frogs. (A railway frog is the point where two rails cross as part of a switch.)
• Electrical line workers use explosive connectors to fuse high-voltage transmission lines together.
• A shaped charge (as in a perforator) is designed to cut or penetrate metal or rock.
• A special crimping technique uses the heat and pressure of an explosive to bond metal pipes together.
• Some explosives used in pyrotechnic work (special effects) create a flash effect.

Properties

Each commercial explosive has a unique combination of properties. To determine whether or not an explosive is suitable for a specific application, a blaster needs to understand its properties. Always refer to the manufacturer's technical data sheet for proportions and specifications.

Depending on the application, a blaster should consider the following properties when selecting an explosive:

• Strength (energy)
• Velocity of detonation (VOD)
• Density
• Toxic gases
• Sensitivity
• Temperature
• Water resistance

Strength (Energy)

Strength is the amount of energy produced by a unit weight or volume of an explosive (typically, 1 gram or 1 cubic centimetre). It expresses the capacity of an explosive to perform work.

Strength may be a convenient yardstick for comparing various explosive products, but there is no recognized standard for measuring strength. The classic measurements based on nitroglycerine (NG) explosives (e.g., dynamites) do not accurately reflect the relative energy output of non-NG explosives. For this reason, strength alone may not be a reliable basis for comparing products.

Many blasters compare explosives by the amount of energy in unit weight or volume. Strength is defined as the thermomechanical heat of an explosive's reaction. Strength is measured in calories per gram (cal/g).

The energy produced by an explosive is ultimately determined under actual blasting conditions. Several techniques can be used to obtain a relative comparison and estimate of blast performance. The four main indicators of strength are:

Absolute Weight Strength (AWS)

Measures the heat of reaction (explosion) available in each gram of explosive (i.e., cal/g).

Absolute Bulk Strength (ABS)

Measures the heat of reaction available in each cubic centimetre of explosive (i.e., cal/cc).

Relative Weight Strength (RWS)

Measures the heat of reaction per unit weight of an explosive compared to the energy of an equal weight of standard ammonium nitrate and fuel oil (ANFO). The RWS is the ratio of the AWS of an explosive to the AWS of ANFO. The RWS shows how much energy per gram the explosive has relative to ANFO.

Relative Bulk Strength (RBS)

Measures the heat of reaction per unit volume of an explosive compared to the energy of an equal volume of standard ANFO at a given density. The RBS is the ratio of the ABS of an explosive to the ABS of ANFO. The RBS shows how much energy per cubic centimetre an explosive has relative to ANFO.

Note: An explosive's energy rating is normally published by the manufacturer. Each manufacturer may use a different formula or code to rate its products. Be cautious with ratings of different products, since they are only meaningful if they come from the same manufacturer.

Velocity of Detonation (VOD)

Velocity of detonation is the speed at which the shock wave (also known as a detonation wave) travels through a column of explosives. Each type of explosive has a maximum or ideal VOD that depends on the explosive's composition and density. During detonation, higher-VOD explosives, such as detonating cord, tend to create a greater shock wave. Lower-VOD explosives, such as ANFO, tend to create more gas pressure.

The VOD of most commercial explosives ranges from 1,500 to 7,500 m (5,000 to 25,000 ft.) per second. The VOD is an important tool to ensure that the explosive works properly. For example, if an explosive is rated to fire at 2,000 m/s and instead fires at 1,500 m/s, the explosive is likely not detonating properly.

A lower-than-expected VOD suggests that problems have occurred with the explosive either in manufacturing or in use. These problems often result in the formation of toxic gases upon detonation. These gases apply less pressure on the blast-hole walls, resulting in poor blast performance. And a low VOD won't pressurize the hole instantaneously, which leads to breakage (i.e., shattering of rock) before all the explosive detonates. This results in reduced blast-hole pressure and blast underperformance.

Blasting with black powder (or other deflagrating explosives) is most likely to result in this breakage-before-pressurization problem. The risk can be reduced by shortening the length of explosive charges. This allows the entire charge to burn before breakage occurs.

In most applications, a VOD below 2,000 m (6,560 ft.) per second may not produce the desired results. In specialty applications, such as secondary blasting (mud capping) and perforating (shaped charges), a higher VOD is necessary to produce the desired effect.

Velocity of Detonation by Type of Explosive

Density

Density (or specific gravity) is a measurement of the weight/volume ratio of an explosive to an equal volume of water. Water has a density of 1 gram per cubic centimetre. Explosives with a specific gravity less than 1 are lighter than water. Those with a specific gravity greater than 1 are heavier than water. Most explosives have densities between 0.6 and 1.7.

Blasters should consider density when determining the most appropriate blast-hole charge. Since higher-density products have more explosive in a given volume, they have a greater potential for breakage (i.e., for shattering rock). This is because an increase in density leads to an increase in the volume of gas produced following a blast.

Density or Specific Gravity by Type of Explosive

Toxic Gases

An explosive detonation can produce toxic gases, including carbon monoxide and oxides of nitrogen. Carbon monoxide is colourless and odourless. Oxides of nitrogen have an orange-brown colour. Both types of gases tend to hover in the atmosphere. Blasters must not enter or allow others to enter the worksite until these gases have disappeared.

In surface blasting operations, gases and fumes (very fine particulates) quickly disperse to the atmosphere. In confined areas, such as underground workings, exposure to these contaminants can be minimized with low-fume explosives and effective ventilation systems.

ANFO and emulsions are designed to produce much lower amounts of toxic gases than many other types of explosives. However, even ANFO and emulsions can generate toxic gases in some cases. Examples of such situations include errors in manufacturing, loss of confinement, or, for ANFO, water in the blast hole.

Sensitivity

Sensitivity is a measurement of how easily an explosive can be initiated by an external force such as a detonator, a primer, or a projectile impact. An explosive with high sensitivity may detonate when the shock wave from a nearby hole reaches it.

In some cases, blasters may also need to consider the explosive's packaging and stability (i.e., during handling or long-term storage).

Temperature

Temperature can affect how explosives respond and perform:

• Dynamites can become quite hard or frozen under very cold conditions. However, they can still be reliably used and fired successfully.
• Packaged emulsions can also freeze at cold temperatures, but they may fail to detonate, even after being allowed to warm up.
• ANFO does not suffer performance issues as long as it remains dry. But it can break down if it is often exposed to wide temperature changes during storage.

Water Resistance

Water resistance is the ability of an explosive to withstand exposure to water without losing sensitivity or efficiency. The level of resistance depends on the following:

• The composition of the explosive product
• The product's packaging
• The environmental conditions to which the product is subjected

Some manufacturers describe water resistance in general terms. Others specify how long a product may be exposed to water and still detonate. Such descriptions are guidelines only. Water resistance can be significantly affected by the following:

• Water depth and movement
• Damaged wrappings
• Exposure to cold temperatures

General Criteria for Selecting an Explosive

Under normal use, explosives should do the following:

• Remain intact during the period of storage
• Not freeze or break down chemically (dissociate) under normal temperatures
• Be suitably packaged for the intended use
• Be safe to handle, transport, and store
• Remain sensitive
• Detonate properly on initiation

Explosives should function at their ideal velocity of detonation. They should also have the following other properties:

• Adequate strength for the intended use
• Sufficient release of high temperature and gases
• Suitable density for the particular application
• Adequate water resistance
• Minimal toxic gases, particularly in confined areas

Specific Criteria for Selecting an Explosive

Explosive selection is a critical part of every blast. In construction blasting applications, choosing the right explosive product should be based on the risk level and the site conditions, not the cost.

The following recommendations for explosive selection are based on site conditions. The explosive products covered include packaged and bulk explosives. Packaged explosives include packaged blasting agents (such as ANFO/WR) and cartridge explosives (such as emulsions and dynamites). Bulk explosives include pumped blasting agents or emulsions.

Explosives for Surface Blasting

Before the start of a blasting project, only limited geological information is typically available. Groundwater conditions and fractures in the rock are largely unknown until drilling begins. The blaster should communicate with the driller and select the correct explosives and loading methods for each and every hole.

When to Use Cartridge Explosives

Fractured rock or voids: When the rock is fractured or contains significant voids and soft seams, cartridge explosives should be used. Cartridge explosives are better controlled (i.e., contained by their packaging), which helps avoid overloading. When pouring blasting agents (such as ANFO or WR), it's easy for the prills to find their way into fractured rock or soft seams and voids. This means it's easy to load far too much blasting agent, which causes avoidable and unnecessary risks. For this reason, using ANFO in such situations should never be considered.

Wet holes: When blast holes contain water, cartridge explosives should be used. In some cases, such as in solid rock, blasters can use cartridges to displace low levels of water in a blast hole. Once the water reaches its highest level and the remainder of the hole is dry, the blaster can determine whether to top up the hole with ANFO. However, wet holes (i.e., holes that are full or mostly full of water) must be dewatered and remain dry to use ANFO/WR because water renders these products useless.

Blast control requirements: In projects that require maintaining good control over blast effects (such as backbreak and ground vibration), use a packaged product. If such control is not required, then cartridge explosives, bulk explosives, and ANFO are all practical choices.

What Can Go Wrong

Dead Pressing

Dead pressing is the failure or underperformance of an explosive when its density increases past a point at which it can reliably detonate.

Dead pressing can occur with less-sensitive explosives such as ANFO and emulsions. But it does not commonly occur with more-sensitive explosives such as dynamites.

Explosives such as ANFO and emulsions are sensitized (i.e., made to initiate more easily) by a reduction in their density. Manufacturers typically achieve this reduction by inserting air into the explosives. This allows the explosives to sustain a detonation.

Dead pressing often occurs when charges are spaced close to each other, especially if the blast holes are wet. The detonation of the nearby explosives may increase the density by expelling the air. This can lead to dead pressing.

Sympathetic Propagation

Some types of explosives are so shock sensitive that the explosive in one charge will initiate as a result of the detonation of a charge in a nearby hole. The term sympathetic propagation or sympathetic detonation is sometimes used to describe such initiation.

Sympathetic propagation may occur with some explosives over considerable distances, depending on the following:

• The type of material being blasted
• The type of explosive
• The size of the charge
• The distance between charges
• Other factors, such as the presence of water

Under most conditions, individual charges should independently detonate at set delay intervals, not sympathetically propagate.

ANFO, emulsions, and water gels are at very low risk of sympathetic propagation. Typically, even two emulsion cartridges separated by several inches in a blast hole will not set each other off. For this reason, if a blaster does not properly load emulsions, and gaps are left between cartridges, misfires will occur. Sensitive explosives, such as dynamites, are at much higher risk of sympathetic propagation.