Rock Blasting Fundamentals

ROCK BLASTING
FUNDAMENTALS
QA/QC Department
Infrastructure Projects
Division
DESCON Engineering
Limited

CONTENTS
 Explosives and their types
 Initiation Systems and Delays
 Circuits, Power Sources and Cords
 Blast Hole Loading
 Blast Design
 Geology of Rock Mass
 Blasting Parameters
 Control Blasting

EXPLOSIVES
 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 more stable substances, mostly
gases, and a large amount of heat.
 The very hot gases produce extremely high pressures within the
borehole, and it is these pressures that cause the rock to be
fragmented.
 The principal reacting ingredients in an explosive are fuels and
oxidizers.

TYPES OF EXPLOSIVES
 Products used as the main borehole charge can be divided into
three categories:
 Nitroglycerin (or nitro starch) based high explosives,
 Dry Blasting Agents, and
 Slurries, which may also be referred to as Water Gels or Emulsions.
 If a product is sensitive to No. 8 blasting cap, it will be termed as
Explosive, while a product not sensitive to No. 8 blasting cap is
referred to as Blasting Agent.
 If an explosive reacts faster than speed of sound, it is a high
explosives while if the reaction is slower than speed of sound, it is
a low explosive.
 Blasting agent is any material or mixture consisting of a fuel and
an oxidizer.
 Blasting agent can be made cap-sensitive by pulverizing it to a fine

TYPES OF EXPLOSIVES
 Nitroglycerine based High Explosives
 Categorized on the basis of Nitroglycerine content.
 Most sensitive commercial products used today.
 More susceptible to accidental detonation.
 Dry Blasting Agents
 AN-FO (ammonium nitrate – fuel oil) is the most common product.
 Cheapest source of explosive energy available today.
 Slurries
 Mixture of nitrates such as ammonium and sodium nitrate, a fuel sensitizer
and water.
 Slurries are made water resistant through the use of gums, waxes and cross-
linking agents.
 May be classified as either explosives or blasting agents.

TYPES OF EXPLOSIVES
 Two Component Explosives
 Pulverized ammonium nitrate and nitromethane is the most common two
component mixture.
 Components are carried in separate containers to the job site and mixed
before use.
 Permissible Explosives
 Designed for underground coal mines where explosive gases and dust are an
hazard.
 Both nitroglycerine and slurry based permissibles are available.
 Primers and Boosters
 Primer is a unit of cap-sensitive explosives used to initiate other explosives
or blasting agents.
 Primer contains a detonator.
 Booster is used to intensify an explosive reaction.

EXPLOSIVES SELECTION CRITERIA
 The blaster should select a product that will give lowest cost per unit of
rock broken, while assuring that fragmentation and displacement of rock
are adequate for the job at hand.
 Following factors should be taken in to account while selecting the
explosives;
 Explosive Cost
 Charge Diameter
 Cost of Drilling
 Fragmentation Difficulties
 Water Conditions
 Adequacy of Ventilation
 Atmospheric Temperature
 Propagating Ground
 Storage Considerations
 Sensitivity Considerations

INITIATION SYSTEMS
 High Explosives are initiated by a detonator, which is a capsule
containing series of relatively sensitive explosives which can
be initiated by outside energy source.
 Blasting Agents are detonated by a primer (unit of cap-
sensitive explosives with a detonator).
 An initiation system consists of three basic parts;
 An initial energy source.
 An energy distribution network that conveys energy into the individual
blast holes.
 An in-the-hole component that uses energy from the distribution
network to initiate a cap-sensitive explosive.
 Commercial detonators vary in strength from No. 6 to No. 12.
Although No. 6 and No. 8 detonators are the most common.

DELAY SERIES
 In most types of blasting, time intervals are required between
the detonation of various blastholes or even between decks
within a blastholes.
 To accomplish this, a delay element containing a burning
powder is placed immediately before the priming charge in the
detonator.
 The delay time is determined by the length and burning rate of
delay powder column. Slow delay caps may be quite long
whereas lower period millisecond delays are shorter in length.
 Millisecond delays provide improved fragmentation, controlled
throw, reduced ground vibrations and air blast as compared
with simultaneous firing.
 Typical delay intervals for surface connectors are 5, 9, 17, 25,
35, 45 and 65 ms.

ELECTRIC INITIATION
 An electric blasting cap consists of two insulated leg wires
that pass through a waterproof seal and into a metal
capsule containing a series of explosive powder.
 Inside the capsule, two leg wires are connected by a fine
filament bridge wire embedded in a highly heat-sensitive
explosive.
 Upon application of electric current, the bridge wire heats
sufficiently to initiate the ignition mixture, which in turn
initiates a series of less sensitive, more powerful
explosives.
 Detonators are available in strengths ranging from about
No. 6 to No. 12, with No. 6 and No.8 being most common.

TYPES OF CIRCUITS
 There are three types of electric blasting circuits. In order of
preference, they are;
 Series
 Parallel Series
 Parallel
 Series Circuit
 All caps are connected consecutively and current from power source has only
one path to follow.
 The series circuit is recommended because of its simplicity.
 All caps receive same amount of current.
 Electric resistance of series of caps is equal to the sum of resistances of
individual caps.
 For most blasting machines, number of caps in a single series is limited to 40
to 50 depending upon length of the leg wire.

CIRCUITS RESISTANCES
SERIES
PARALLEL-SERIES
PARALLEL

TYPES OF CIRCUITS
 Parallel-Series Circuit
 When the number of caps in a round exceeds 40 to 50, parallel-series circuit
is recommended.
 The caps are divided into a number of individual series.
 Each series shall contain equal number of caps to assure even current
distribution.
 Two bus wires are placed in such a position that each end of each series can
be connected.
 To assure equal current distribution to each series, one bus wire should be
reversed.
 Parallel Circuit
 Parallel circuit is not recommended because it cannot be checked. Broken leg
wires or faulty connections cannot be detected once the circuit is hooked up.
 Also, because the available current is divided by the number of caps in the
circuit, powerline firing must often be used to provide adequate current for

POWER SOURCES
 Electric blasting circuits can be energized by generator-type
blasting machines, condenser-discharge blasting machines, and
powerlines.
 Storage and dry cell batteries are definitely not recommended for
blasting because they cannot be depended on for a consistent
output.

DETONATING CORD
 Detonating cord consists of a core of high explosives, usually PETN
(pentaerythritol tetranitrate), contained in a water proof plastic sheath
enclosed in a reinforced covering of various combinations of textile,
plastic, and waterproofing.
 Detonating cord is available with PETN core loadings ranging from 1 to
400 gr/ft.
 All cords can be detonated with a blasting cap. Millisecond delay surface
connectors are used for delaying detonating cord blasts. A time of 1 ms is
required for 21 ft of cord to detonate.
 The most common strengths of detonating cord are from 25 to 60 gr/ft.
 These strengths are used for trunklines, which connect the individual
blastholes into pattern, and for downlines which transmit the energy from
trunkline to the primer cartridge.

BLASTHOLE LOADING
 Blasthole loading involves placing all of the necessary ingredients into the
blasthole, including the explosive charges, initiation systems, primers and
stemming.
 It is generally recommended that charge completely fills the borehole
diameter.
 Blasthole shall be stemmed to improve the efficiency of the explosive and
to reduce airblast and flyrock. As a rule of thumb, the length of stemming
should be from 14 to 28 times the borehole diameter.
 When small diameter blastholes are loaded, the primer cartridge is
normally loaded at the bottom of the hole for maximum confinement and
prohibiting undetonated cartridge issues if the hole gets chocked.
 Cartridge dynamites and slurries are commonly used in small diameter
blastholes. One or two cartridges should be loaded after the primer,
before temping.
 AN-FO may be loaded into small diameter blastholes by pouring from a
bag or through pneumatic loading.

BLAST DESIGN
 Throughout the blast design, two principles shall be kept in mind;
 Explosives function best when there is a free face approximately parallel to
the explosive column at the time of detonation.
 There must be adequate space into which the broken rock can move and
expand.
 Decisions on explosive selection, blast design, and delay pattern
must take first hand knowledge of the rock mass into account.
 The direction, severity and spacing between the joint sets, strike
and dip of bedding planes, major zones of weaknesses such as
faults, open beds, solution cavities, or zones of incompetent rocks
shall be considered.
 A detailed drill log indicating the depth at which these various
conditions exist can be very helpful to the person designing the
blast.

Blast Geometry – A General Overview

GEOLOGY OF ROCK MASS
 Density of a rock is major factor in determining how much
explosive is needed to displace a given volume of rock (powder
factor).
 The average burden-to-charge diameter ratio of 25 to 30 is for
average density rocks such as limestone, schist or porphyry.
Denser rocks such as basalt and magnetite require smaller ratios.
Lighter material such as some sandstones or bituminous coal can
be blasted with higher ratios (low powder factor).
 When a blasthole intersects a void zone, explosive energy will seek
the path of least resistance. If not catered for properly, additional
charge will be filled resulting in excessive fly rock.
 The depth of voids and incompetent zones encountered by the drill
should be documented and filled with inert stemming material

 Alternate zones of competent and incompetent rock usually result
in unacceptable blocky fragmentation. Smaller blastholes with
smaller blast patterns shall be used in such situations.
 Jointing can have a pronounced effect on both fragmentation and
stability of the perimeter of excavation. Close jointing results in
good fragmentation, however widely spaced joints will produce
blocky muck pile.
 Perimeter holes of a blast shall be aligned with the principal joint
sets. Open corners shall be preferred as compared to tight corners
in order to prevent violence.
 Open bedding planes or beds of weak material should be treated
as zones of weakness. Stemming should be loaded at these
locations instead of explosives.

 In a bed of hard material, it is often beneficial to load an explosive of
higher density than is used in the remainder of the blasthole.
 To break an isolated bed of hard material near the collar of the blasthole,
a deck charge is recommended. Occasionally, satellite holes are used to
help break a hard zone in the upper part of the burden.
 When the beds dip into the excavation wall, the stability of the slope is
enhanced. However, when beds dip outward from the wall, they form slip
planes that increase the likelihood of slope deterioration.
 Although beds dipping into the face improve slope stability, they do
create toe problems.
 In some cases, advancing the opening perpendicular to the dipping beds
may be a good compromise.

BLAST HOLE DIAMETER
 The blasthole diameter, along with the type of explosive being
used and the type of rock being blasted, will determine the
burden. All other blast dimensions are a function of the burden.
 Practical blasthole diameters for surface mining range from 2 in to
17 in.
 As a general rule, large blasthole diameters yield low drilling and
blasting cost because large holes are cheaper to drill per unit
volume, also, cheap blasting agents can be used.
 However, large diameter holes also results in large burden and
spacing and collar distances and hence, they tend to give coarser
fragmentation.
 Geologic structure is a major factor in determining blasthole

BLAST HOLE DIAMETER
 Planes of weakness such as joints and beds, or zones of soft,
incompetent rock tend to isolate large blocks of rock in the
burden. The larger the blast pattern, the more likely these blocks
are to be thrown unbroken into the muckpile.
 As the blasthole diameter increases, the collar distance required to
prevent violence increases. The ratio of collar distance to blasthole
diameter required to prevent violence varies from 14:1 to 28:1,
depending on the relative densities and velocities of the explosive
and rock, the physical condition of the rock, the type of stemming
used, and the point of initiation.
 As the collar distance increases, the powder distribution becomes
poorer resulting in poorer fragmentation of the rock in the upper
part of the bench.

TYPES OF BLAST PATTERNS
 There are three commonly used drill patterns; square, rectangular,
and staggered.
 The square drill pattern has equal burdens and spacings, while the
rectangular pattern has a larger spacing than burden.
 In both the square and rectangular patterns, the holes of each row
are lined up directly behind the holes in the preceding row.
 In the staggered pattern, the holes in each row are positioned in
the middle of the spacings of the holes in the preceding row. In the
staggered pattern, the spacing should be larger than the burden.
 The staggered drilling pattern is used for row-on-row firing.
 The square or rectangular drilling patterns are used for firing V-
Cut or Echelon rounds.

BURDEN
 The burden is defined as the distance from a blasthole to the
nearest free face at the instant of detonation.
 It is very important that the proper burden be calculated, taking
into account the blasthole diameter, the relative density of the rock
and the explosive, and to some degree, the length of the blasthole.
 An insufficient burden will cause excessive air blast and fly rock.
Too large a burden will give inadequate fragmentation, toe
problems, and excessive ground vibrations.
 The burden dimension is a function of the charge diameter.
 When blasting with AN-FO or other low density blasting agents
with densities near 0.85 g/cu cm, in typical rock with a density
near 2. 7 g/cu cm, the normal burden is approximately 25 times
the charge diameter.

BURDEN
 When using denser products such as slurries or dynamites, with
densities near 1.2 g/cu cm, the normal burden is approximately 30
times the charge diameter.
 The burden-to-charge-diameter ratio is seldom less than 20 or
seldom more than 40, even in extreme cases.
 For instance, when blasting with a low density blasting agent, such
as AN-FO, in a dense formation such as iron ore, the desired
burden may be about 20 times the charge diameter. When blasting
with denser slurries or dynamites in low density formations such
as some sandstones or marbles, the burden may approach 40
times the charge diameter.
 High-speed photographs of blasts have shown that flexing of the
burden plays an important role in rock fragmentation. A relatively
long, slender burden flexes, and thus breaks more easily than a
short, stiffer burden.

SUB-DRILLING
 Sub-drilling is the distance drilled below the floor level to assure
that the full face of rock Is removed.
 Where there is a pronounced parting at floor level, to which the
explosive charge can conveniently break, sub-drilling may not be
required.
 A good first approximation for sub-drilling under average
conditions is 30% of the burden.
 Where the toe breaks very easily, the sub-drill can sometimes be
reduced to 10 to 20% of the burden. Even under the most difficult
conditions, the sub-drill should not exceed 50% of the burden.
 If the toe cannot be pulled with a sub-drill-to-burden ratio of 0.5,
the fault probably lies in too large a burden.

COLLAR DISTANCE (STEMMING)
 Collar distance is the distance from the top of the explosive charge to the
collar of the blasthole.
 This zone is usually filled with an inert material called stemming to give
some confinement to the explosive gases and to reduce air-blast.
 Too small a collar distance results in excessive violence in the form of
air-blast and fly-rock and may cause back-break. Too large a collar
distance creates boulders in the upper part of the bench. The selection of
a collar distance is often a tradeoff between fragmentation and the
amount of air-blast and fly-rock that can be tolerated.
 This is especially true where the upper part of the bench contains rock
that is difficult to break. Collar priming of blastholes normally causes
more violence than center or toe priming, and requires the use of a longer
collar distance.
 Field experience has shown that a collar distance equal to 70% of the
burden is a good first approximation except where collar priming is used.

COLLAR DISTANCE (STEMMING)
 A deck charge is an explosive charge near the top of the blasthole,
separated from the main charge by inert stemming.
 If boulders are being created in the collar zone but the operator
fears that less stemming would cause violence, the main charge
should be reduced slightly and a deck charge added.
 The deck charge is usually shot on the same delay as the main
charge or one delay later.
 In a delicate situation, it may be best to start with a collar distance
equal to the burden and gradually reduce this if conditions permit.
 Collar distances greater than the burden are seldom necessary.

SPACING
 Spacing is defined as the distance between adjacent blastholes,
measured perpendicular to the burden.
 Spacing is calculated as a function of the burden and also depends
on the timing between holes.
 Too close a spacing causes crushing and cratering between holes,
boulders in the burden, and toe problems. Too wide a spacing
causes inadequate fracturing between holes, accompanied by
humps on the face and toe problems between holes.
 When the holes in a row are initiated on the same delay period, a
spacing equal to twice the burden will usually pull the round
satisfactorily.

SPACING
 When millisecond delays are used between holes in a row, the
spacing-to-burden ratio must be reduced to somewhere between
1.2 and 1.8, with 1.5 being a good first approximation.
 Generally, large-diameter blastholes require lower spacing-to-
burden ratios (usually 1.2 to 1.5 with millisecond delays) than
small-diameter blastholes (usually 1.5 to 1.8).
 Because of the complexities of geology, the interaction of delays,
differences in explosive and rock strengths, and other variables,
the proper spacing-to-burden ratio must be determined through
onsite experimentation.
 Except when using controlled blasting techniques, spacing should
never be less than the burden.

HOLE DEPTH
 In any blast design it is important that the burden and the
blasthole depth be reasonably compatible.
 As a rule of thumb for bench blasting, the hole depth-to-burden
ratio should be between 1.5 and 4.0.
 Hole depths less than 1.5 times the burden cause excessive air
blast and fly rock and, because of the short, thick shape of the
burden, give coarse, uneven fragmentation.
 Where operational conditions require a ratio of less than 1.5, the
primer should be placed at the toe of the bench to assure
maximum confinement.
 Where it is necessary to use blast designs with large hole depth-
to-burden ratios, multiple priming should be used as insurance
against cutoffs.

DELAYS
 Millisecond delays are used between charges in a
blast round for three reasons:
 To assure that a proper free face is developed to
enable the explosive charge to efficiently fragment
and displace its burden.
 To enhance fragmentation between adjacent holes.
 To reduce the ground vibrations created by the blast.

POWDER FACTOR
 Powder factor is a necessary calculation for cost accounting
purposes.
 Powder factors for surface blasting can vary from 0.25 to 2.5 Ib/cu
yd, with 0.5 to 1.0 lb/cu yd being most typical.
 Powder factor is a function of type of explosive, rock density, and
geology. Typical powder factor for surface blasting
Degree of difficulty in rock
breakage
Powder Factor (lb/cu yd)
Low 0.25 – 0.40
Medium 0.40 – 0.75
High 0.75 – 1.25
Very High 1.25 – 2.50

SECONDARY BLASTING
 Some primary blasts, no matter how well designed, will leave
boulders that are too large to be handled efficiently by the loading
equipment or large enough to cause plug ups in crushers or
preparation plants.
 Secondary fragmentation techniques must be used to break these
boulders which can be accomplished in four ways:
 A heavy ball suspended from a crane may be dropped repeatedly on the
boulder until the boulder breaks.
 A hole may be drilled into the boulder and a wedging device inserted to split
the boulder.
 Loose explosive may be packed into a crack or depression in the boulder,
covered with damp earthen material, and fired.
 The most efficient method of secondary fragmentation is through the use of
small (1 to 3 in) boreholes loaded with explosives.

CONTROLLED BLASTING
 The term controlled blasting is used to describe several techniques for
improving the competence of the rock at the perimeter of an excavation.
 The best value for a given blasting job is a function of the geology,
specifically the number and severity of planes of weakness in the rock,
and the quality of rock surface that is required.
 Normal blasting activities propagate cracks into the excavation walls.
These cracks reduce the stability of the opening. The purpose of
controlled blasting is to reduce this perimeter cracking and thus increase
the stability of the opening.
 Four types of control blasting techniques include the following, with the
first two being more important in surface blasting;
 Line Drilling
 Pre-Splitting
 Smooth Blasting
 Cushion Blasting

LINE DRILLING
 Line drilling involves the drilling of a row of closely spaced holes
along the final excavation line. No charge is inserted in these
holes.
 The line-drilled holes provide a plane of weakness to which the
final row of blastholes can break and also reflect a portion of the
blast's stress wave.
 Line drilling is used mostly in small blasting jobs and involves
small holes in the range of 2 to 3 in diameter. Line drilling holes
are spaced (center to center) two to four diameters apart.
 The use of line drilling is limited to jobs where even a light load of
explosives in the perimeter holes would cause unacceptable
damage.
 The results of line drilling are unpredictable, the cost of drilling is
high, and the results are heavily dependent on the accuracy of

PRE-SPLITTING
 Presplitting, sometimes called pre-shearing, is similar to line
drilling except that the holes are drilled farther apart and a very
light load of explosive is used in the holes.
 Presplit holes are fired before any of the main blastholes adjacent
to the presplit are fired.
 In badly fractured rock, unloaded guide holes may be drilled
between the loaded holes.
 The light powder load may be obtained by using specially designed
slender cartridges, partial or whole cartridges taped to a
detonating cord down line, explosive cut from a continuous reel, or
even heavy grain detonating cord. A heavier charge of tamped
cartridges is used in the bottom few feet of hole.

PRE-SPLITTING
 It is desirable to completely stem around and between the
cartridges in the borehole.
 The maximum depth for a single presplit is limited by the accuracy
of the drill holes, and is usually about 50 ft.
 It is preferable to presplit a shorter section and dig that section out
so that the quality of the presplit can be checked. If the presplit is
unsatisfactory, adjustments can be made in subsequent blasts.
Average Specifications for Pre-Splitting
Hole Diameter (in) Spacing (ft) Explosive Charge (lb/ft)
1.50 – 1.75 1.00 – 1.50 0.08 – 0.25
2.00 – 2.50 1.50 – 2.00 0.08 – 0.25
3.00 – 3.50 1.50 – 3.00 0.13 – 0.50
4.00 2.00 – 4.00 0.25 – 0.75

Rock Blasting Fundamentals

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