This article is geared to providing basic
practical knowledge about commercial explosives, blast physics, rock properties,
construction blast design, controlled blasting techniques, ground vibration,
and air blast exposures.
Liberty Mutual Group
Understanding these fundamentals will assist construction managers, project
personnel, and company risk managers to better understand the process, define
the risks involved, and set parameters for an injury- and risk-free work environment.
An explosive is a compound or mixture of compounds which, when initiated
by heat, impact, friction, or shock, undergoes a rapid decomposition, releasing
tremendous amounts of energy in the form of heat and gas. This decomposition
is a self-propagating, exothermic reaction called an explosion.
Chemically, there are two fundamentally different types of explosive materials:
molecular and composite explosives. Molecular explosives are substances that
contain all that is needed for reaction within each well-defined molecule. Trinitrotoluene
(TNT) and nitroglycerin (NG) are examples of molecular explosives. Composite
explosives are mixtures that might contain fuels and oxidizers, and other self-explosive
ingredients. Most rock blasting explosives fall into this category, with ammonium
nitrate-fuel oil (ANFO) being the classic example. Some composite explosives
also contain ingredients such as water or ballast materials that do not add
energy to the reaction but modify the mixture's flow properties or consistency.
Without exception, composite explosives must contain some mixture of carbon,
oxygen, and nitrogen.
Upon detonation, maximum energy release occurs when the explosive mix is
formulated for oxygen balance. When explosives are oxygen-balanced, they generally
form water vapor (H2O), carbon dioxide, and nitrogen (N2).
Commercial explosives rarely detonate under ideal conditions, so in actual practice,
small amounts of toxic gases—such as oxides of nitrogen (NO and NO2),
carbon monoxide (CO)—are produced.
In commercial blasting, explosives and blasting agents are characterized
by various properties that define how they will perform under field conditions.
These properties include fume class, density, water resistance, temperature
effects, detonation velocity, detonation pressure, borehole pressure, sensitivity,
Fumes are toxic and noxious gases that are produced by the detonation of
explosives. Limiting and controlling these gases is most important in underground
and confined workings. Factors such as explosive age, water penetration, and
chemical reaction with the host rock will affect the amount of fumes produced
by an explosive detonation or conflagration.
The density of a material is defined as its weight per unit volume for blasting.
Explosive densities are expressed in grams per cubic centimeter (g/cc). Blasters
can relate to these metric units because water density is 1 g/cc. Therefore,
explosives with densities higher than 1 g/cc will sink in clear water. This
is important when blasting in wet areas, where the blaster needs to have the
blasting material sink to the bottom of the bore hole.
There are two distinct different reasons for measuring and knowing the relative
sensitivity of various explosive compositions. From a safety perspective, it
is important to know how sensitive an explosive is to impact, friction, and
heat. From a performance standpoint, measures such as gap sensitivity, critical
diameter, and minimum primer sensitivity define the "functional sensitivity"
When compared with newer water-based explosives, nitroglycerin (NG) explosives
are much more sensitive to detonation by impact or friction. Of the four classes
of explosives, NG has the highest sensitivity, followed by high explosive water-gels,
ANFO, and then emulsion explosives. NG can be set off by shock, whereas ANFO
emulsion blends cannot.
The measure of an explosive's sensitivity determines the minimum size of
the primer or detonator that is needed to reliably detonate it under normal
conditions of use. The ignition sensitivity varies widely for various explosive
types. A blast can be initiated with high-sensitivity explosives by using a
detonator. Low-sensitivity explosives, such as ANFO emulsion blends, require
a booster (usually a very small amount of high explosives) that is initiated
by a detonator to cause an explosion. Obviously, low-sensitivity explosives
are safer to use because they cannot be set off accidentally.
When explosives detonate, they produce shock and heave energy. The shock
energy is produced in the form of stress waves, driven by the explosive's detonation
front. The heave energy is produced by the rapidly expanding gases that follow
detonation. Actual explosive yields can be measured by a variety of field tests.
The total theoretical energy of explosives can be expressed in calories per
unit weight or calories per unit volume.
There are other considerations that are important to blasting. One of these
is water resistance of the blasting agent, since in many applications explosives
are loaded into damp or wet holes. Some blasting agents, such as ANFO, have
poor water resistance and will not fire when wet. Temperature levels and fluctuations
can affect the stability as well as the performance of explosives.
Whenever possible, contractors typically use ammonium nitrate-fuel oil (ANFO)
mix blasting agents for construction blasting. When used properly, ANFO can
produce good blasting results with relatively low cost. Number 2 diesel oil
is commonly used as the fuel. ANFO can be purchased and handled in either packaged
or bulk form. In large scale projects, storing the bulk ingredients requires
little special consideration, and loading efficiency is improved by using mechanized
systems to handle and load bulk ANFO.
Initiating explosives are designed to safely activate larger explosive charges
at a controlled time and in a predetermined sequence. Initiating systems are
generally classified as electric or non-electric, depending on their signal
transmission method. Electric systems use wire to transmit an electric current
from a power source to detonators. Non-electric systems use plastic shock tube
technology, detonating cords with narrow trains of high explosives, or slow
burning pyrotechnic compounds to transmit initiation signals. Blast initiation
sequences can be controlled by using timing systems, but delay timing is usually
produced by means of pyrotechnic delay elements inside detonators. Delay detonators
are available with millisecond or long delays, with approximately ½-second timing
Long-period (½-second) delays are primarily used in underground blasting
applications. For surface blasting, detonators with relatively short-period
timing intervals from 25-50 milliseconds (short period) produce the best blasting
results. These types of blasts generally produce better rock fragmentation.
Detonators are compact devices designed to safely and efficiently initiate
and control the performance of larger explosive charges. They contain relatively
sensitive high explosives, which are initiated by a signal or energy from an
external source. Delay detonators incorporate components which introduce a controlled
time delay to sequence blastholes for optimal results.
The most common initiation system is the shock tube (non-electric) detonator.
These convey the signal by a shock front generated by the reaction of powdered
aluminum and other chemicals that coat the inside surface of extruded plastic
tubes. Field hook-up of modern shock tube units is very easy, and various delay
combinations allow for an infinite variety of blasting sequencing. These detonators
provide a high level of safety against accidental initiation by static electricity,
stray electric currents, and radio frequency energy. They also cannot be initiated
by flame, friction, or impact normally encountered in construction blasting
operations. The shock tube has been accidentally initiated by stretching the
tube until it breaks. If the tube is cut or slit, then a misfire will result,
so the tubes must be handled with care.
Commercial explosives release tremendous amounts of energy when detonated.
The blast design challenge is to effectively use this energy in the most efficient
way to fragment rock in a controlled way. The great energy release of explosive
can also cause less desirable effects, such as excessive ground vibration and
concussion, overbreaking of the rock, and possible pre-compression failure of
the adjacent explosive loads. Blast results may be difficult to predict since
they are greatly influenced by the in situ rock conditions. All rocks contain
fissures—like bedding planes, partings, and joints—that are opened up by explosion
gases and rock movement.
The orientation of structures with respect to blastholes and open faces greatly
influence the fracturing process and the potential for overbreak. The heave
energy from explosive gases cleaves open the natural joints and bedding planes.
When blast direction is parallel to major open joints, potential heave energy
is lost when gases prematurely vent to the face through open joints. Jointed
or otherwise weakened rock masses can also cause misfires when loaded holes
are cut off by premature ground movement and gas penetration. On large projects,
it is advisable to perform some small scale test blasts to provide the most
complete information about existing rock types, physical properties, structure,
and blasting characteristics.
Many interacting factors are involved in the general blast planning process.
Blast fragmentation size will influence equipment selection, and visa versa,
if equipment already exists. Excavation schedules and drill bench dimensions
will influence blast hole size, explosive selection, and labor requirements.
The proximity of blasting to structures can profoundly affect blast planning.
Blasting near structures or urban areas adds a special set of additional
concerns. The need for pre-blast structure surveys, vibration and air-blast
monitoring, stringent blast effect control measures, and effective blast area
security and warning methods must be evaluated for close-in blasting. Geological
conditions will also influence blast planning. Physical rock properties and
structural conditions will present special blast control challenges.
Over the years, bench blast design rules of thumb have been established to
help blast designers prepare initial blast designs—or estimators to make preliminary
cost estimates—if actual data is not available. These rules of thumb are not
intended to predict optimum blast performance. Blast managers should continue
to evaluate blast performance to continually improve their design and cost performance.
Bench blast design includes the following:
Blast designers have a virtually unlimited variety of drill patterns and
hole orientation that they can use to design construction blasts. The drill
pattern burdens and spacing are bound by practical limits based on hole size.
Large blast holes may be as large as 9 inches or 225 mm. in diameter, while
small holes may be as small as 1.5 inches or 38 mm. in diameter. Hole orientation
is influenced by project topography, excavation boundary geometry, bench height,
bench access, and many other factors. Equipment limitations will also influence
Another factor in blast design is the delayed (millisecond) timing of the
initiation of the explosives in relation to one another. Millisecond timing
patterns almost always alter the rock movement geometry, and hence, the timing
creates "effective" burden and spacing values that are often quite different
from the burden and spacing that define the drill pattern. Timing will affect
fragmentation, rock throw, and overbreak. Proper millisecond timing will enhance
fragmentation and improve excavation productivity.
Delay blasting techniques can also be used to manage adverse geologic conditions
and control of vibration and air blast effects. Major advantages of millisecond
(MS) blasting are:
Many easy-to-use non-electric systems that allow blasters almost infinite
timing flexibility are offered by various manufacturers. A combination of surface
and in-hole delays are widely used in construction blasting operations.
Another factor in blasting is rock-swell. This can vary from 20-40 percent
of the bench volume. This can impact the blast performance if there is limited
room. It can also affect the transportation requirements, which can impact both
time and cost.
When explosive charges detonate in rock, most of the energy is used in breaking
and displacing the rock mass. However, some of the energy is released in the
form of ground vibration and air overpressure or "airblast." Airblast pulses
are usually in frequencies below the threshold of human hearing, but this energy
can be felt. Human response to vibration and airblast is very subjective, and
people can often feel very low levels of vibration and blast noise.
When explosives detonate in rock, energy is generated in the form of various
surface and body vibration waves. Vibration wave energy decays with distance
as it spreads through a rock mass or travels along the ground surface. Some
waves pass through the "body" of the rock mass. These body waves are called
primary (p) and shear (s) waves. Other waves—like Rayleigh and Love waves—travel
along the ground surface. In an ideal isotropic and homogenous rock mass, wave
energy would travel evenly in all directions. However, most rock masses are
far from ideal, so wave energy is reflected, refracted, and differentially attenuated
by various geological and topographical conditions. The elastic property of
rock greatly influence vibration magnitude and attenuation rate.
The U.S. Bureau of Mines recommends the following:
Excessive airblast is controlled by ensuring that all charges are properly
confined. Excessive airblast is generated by the same poor confinement conditions
that cause flyrock. Conditions that cause high over-pressure levels:
Air blast from detonating chord trunklines can be significantly reduced if
it is covered with at least 20 cm. (8 in.) of dirt or sand.
*This article is based on a presentation
made to Liberty Mutual consultants in 1995.
Note: Safe blasting procedures and risk
management are discussed in "Construction
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