Rock blasting for mining

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ROCK BLASTING FOR MINING
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore
Mysore-6

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Objectives:
You are all aware that Mining is a major
economic sector of any country.
Mining is a geotechnical industry.
Geologist explore the economic mineral
deposits for mining.
A lot of methods are adopted to exploit the
valuable economic mineral resources through
open cast mining or underground mining
methods.

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In all these methods, there is a need to dislodge
the ore or mineral deposit from the main body
of the rocks.
Dislodging requires heavy energy application
and machineries.
To simplify the work, rock blasting is the
easiest way.

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In mining operations, blasting has become a
routine work. With proper understanding of
the rocks and blasting methods, the mining
work can be done effectively.

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The basic objectives of this report is to
highlight the principles and practices adopted
in rock blasting operations for mining
sectors.

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The aspects concentrated in this report are:
a) Purpose and scope of blasting
b) Steps involved in blasting
c) Blast design and patterns
d) Methods of blasting
e) Controlling factors of Blasting.

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1.0 Introduction:
Most of the economic mineral deposits occur in
association with massive hard rocks. These
rock masses should be fragmented to obtain the
valuables and separate the materials for further
processing. Breaking a rock mass involves
energy. This can be achieved by drilling and
blasting.

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Hence, drilling and blasting are considered to be
the first phase of the production cycle in most
of the mining operations.
Today, technology has shown much
advancement in several activities of mining.
Inspite of that, there is no alternative to
blasting or drilling.

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Quiet a lot of improvements have been shown
by experts in controlled blasting operations
which are more beneficial not only on
production point of view, but also on the safety
point of view of mine workers.
Let us see the details of blasting methods
adopted in mining operations.

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This image shows a large scale open cast mine.
Large scale mining in hard rock zones are
difficult without the application of drilling and
blasting operations.

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This is an areal view of a mine.

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This illustration shows the schematic view of a
quarry operation.

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This diagram shows the extension of an ore
body to be mined after removing the over-
burden/ waste lying both sides of it. The bench
slope is about 45 to 72 degrees. Systematic
blasting is necessary to mine this ore body.

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1.1 What is blasting?
Blasting is the process of breaking of bulk rock
masses into loose forms, using explosive
compounds.
Here, the primary role is played by the
explosives.
The explosives are the substances or devices
used in blasting.

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The explosives are used to produce a volume of
rapidly expanding gas that exerts sudden
pressure on its surroundings and break the mass
into pieces.
There are three common types of explosives
used for blasting as chemical, mechanical, and
nuclear explosives.
About 100 years ago, the Chinese invented
explosives.

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The first chemical explosive was gunpowder.
Germans manufactured gunpowder in the early
1300s.
A detonator is a device used to trigger this
explosive device. Detonators can be
chemically, mechanically, or electrically
initiated.
Different explosives require different amounts
of energy to detonate.

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Detonation is a necessity for the explosive to
get triggered for blasting.
1.2 Purpose and scope of blasting:
Blasting is employed for breaking the massive
rocks in quarries, open pits, surface and
underground mining works.

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Sometimes, blasting is also carried out for
removing the overburden from the surface of
the earth for construction purposes.
An optimum blast is associated with the most
efficient use of blasting energy in the rock-
breaking process.
The mine blasting expenditure should also be
very less.

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The process should not only reduce the
blasting cost but also reduce the consumption
of explosives, and less wastage of explosive
energy in blasting.
During any mining-related blasting, there
should be less throw of materials, and there
should not be much blast-related vibration.
All these should provide greater levels of safety
and stability to the nearby structures and also
for all the people working around the mines.

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1.3 Blasting- An essential part of the mining
cycle:

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This diagram illustrates the overall
organisational structure of a mine cycle. Once
the ore body is investigated through detailed
surveys, the next steps include drilling and
blasting, followed by loading, hauling, primary
crushing and other ore-dressing operations.
Blasting helps in not only in rock breaking, but
also in primary crushing.
So, Blasting is an essential aspect in the mining
cycle.

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Now, blasting has become a technology by
itself.
Many Mega engineering projects need some
kind of blasting of basement rocks.
The blasting technology is the process of
fracturing the material by the use of a calculated
amount of explosive so that a predetermined
volume of material is broken down into pieces.

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Everything can be visualized by proper
calculations and execution, with reference to
time and space.
2.0 Steps involved in blasting:
In surface mining operations, most of the rocks
require blasting prior to their excavation.

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The process of blasting includes the following
stages:
1. Creation of blast holes and cleaning by using
suitable tools.
2. The charge of explosives and place them at
the breaking ends.
3. Filling of the remaining portion with clay
and tamping.
4. Insertion of Fuse and keep it projecting 15-
20 cm above the rock surface.

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5. Firing from the free end of fuse by using a
suitable detonator.
(after Joseph M. Pugliese, 1972)

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This diagram shows the drill hole charged with
explosives at a depth.
The burden face is shown as B.
The left-hand side is the bench over which the
fragmented mass will fall.
H is the depth of the borehole.

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This diagram shows the schematic arrangement
of blast holes.

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This photograph shows the drilling works being
done on a bench.
You can see the location of boreholes in the
form of a design.

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Selection of explosives, drilling pattern,
blasting pattern and delay patterns are the major
aspects in mining.

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2.1 Drilling and Blasting:
There are two basic operations that are
necessary to achieve success in mining. One
operation is drilling and the other one is
blasting. Drilling is mainly done for three
reasons. The first reason is to investigate the
depth-wise distribution of minerals during
prospecting. The second reason is to create the
blast-holes for rock blasting.

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The third reason is to drill for dislodging the
soft and friable minerals like magnesite with
shallow depth drills.

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For surface explorations, in opencast mining
operations, drillings rigs may be used.

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For subsurface explorations, rock boring
machines are used.

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A shallow-hole driller may also be used for
preparing blast holes in underground mines.
If we do not do these operations, the
expenditure of mining would be more and the
time taken for recovering the economic deposit
will also be more.

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In order to cut down the cost of the production
and speed up the processes, optimal
fragmentation of the ore body is necessary.
Fragmentation is a necessity for mining and ore
dressing also. Fragmentation helps rapid
excavation of resources. Through a properly
designed blasting pattern and safe execution, in
a mine, it is possible to achieve the targeted
productions. Rock blasting is an interesting
aspect of study.

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2.2 Concept of Blast:
Mine Blast looks like a very simple but
effective, thought provoking process. The
physical characteristics of the rocks are more
important than the characteristics of the
explosives used in blasting.

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The success lies in proper design of the blasting
process. The following are the important points
to be kept in mind:
•When an explosive charge is detonated,
chemical reaction is activated.
•This chemical reaction will very rapidly
change the explosive mass into a hot gas
which generate shockwaves. These are
compressive waves, which also expand on all
sides.

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•They act on the borehole wall and propagate
through the explosive column.
•If a series of holes are detonated
simultaneously, the entire line of masses will
be fragmented.
•Due to this effect, blasting is always carried
out in an array of interlinked blastholes.
•Waves generate cracks and make the entire
process very effective.

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3.0 Good blast design and execution:
A good blast design and proper execution of it ,
are very much essential for a successful
mining operation.
Improper design or a poor design planned in
blasting, can have a severely negative impact
on the economics of a mine.
Sometimes, it may complicate the situation and
may need to excess work and expenditure.

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Unexpected hazards may also crop out in the
environment.
The use of excessive explosives at a mine site
will result in full damage of the rock structures
and cause unwanted caving and large increases
in support costs.

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3.1 Factors of Blast design:
The procedures developed for a blast are aimed
at the desired fragmentation and area of
coverage.
Any kind of specific thermodynamic break
should take into account of the following
aspects:
1. Explosive and energy factors
2. Type and Quantity of explosives used

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3. Diameter of blast hole
4. Orientation of the ore body
5. Dynamic rock/ore properties.
There is a need to have effective free face for
blasting.

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This diagram shows the effective free face.
The location of first blast hole, second blast
hole and the third one also seen here.

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3.2 Gun Powder and Dynamite:
In the earliest days, blasting was carried out
using black powder which was also called as
gunpowder. This compound is a mixture of
saltpeter (potassium nitrate), sulfur, and
charcoal (carbon).
There have been steady developments in
explosives, detonating and delaying techniques
and in the understanding of the mechanics of
rock breakage by explosives.

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The subsequent development was the invention
of dynamites made by the Swedish chemist and
engineer Alfred Nobel in 1867.
Dynamite is an explosive made
of nitroglycerin, sorbents (such as powdered
shells or clay) and stabilizers.
Dynamites gained popularity due to its wide-
scale use as a safer alternative to gun
powder and nitroglycerin.

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3.3 Selection of explosives:
Usually four types of explosives are used in
surface mining operations.
They are
lurries,
dry mixes,
emulsions and the hybrid heavy
ANFO(ammonium nitrate/fuel oil).

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Selection of explosives depends on many
factors. The primarily factors include the
critical diameter, hydrostatic pressure,
temperature, minimum primer weight, density
weight strength, bulk strength, gap sensitivity,
water resistance, loading procedures, coupling
or decoupled properties, shelf life, reliability for
bulk operations and overall drilling and blasting
economics.

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3.4 Drilling Pattern:
Drilling is performed in order to blast the
overburden, ore deposit, coal seams etc., so that
the power requirement for excavators to extract
the materials becomes less.
This also reduces the wear and tear of the
excavators, increases their life, reduces clearing
time of materials, and decreases operation cost.

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Drilling holes are usually made in a zig-zag
pattern. They are done in the form of rows and
columns on the surface like grids. But inside
their disposition may be dipping differently.
The spacing between the rows and column
should be of equal length. This is called as
drilling pattern. Selection of the drilling pattern
varies with the type and size of the drill’s used,
depth of holes, kinds of rocks handled, quantity,
rapidity of the explosive & amount of steaming.

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An array is the line of drill holes made for
blasting works.
4.0 Blasting Pattern:
The basic blast hole arrays may be single-rows,
square-grid, or rectangular-grid and staggered
arrays.

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A typical surface blast design will have a set of
blast holes charged with explosives.
In this illustration, we can see the layout. The
rows are designated with sequential numbers.

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The symbol B denotes the burden thickness
and the symbol S denotes the spacing between
blast holes. If this sequence is executed, the
array of line 1 will get triggered first, followed
by the other arrays, one after the other. The
mass gets fragmented and will get fragmented
towards the direction of arrow mark shown in
the diagram.

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4.1 Types of blast patterns:
Regular symmetrical or Irregular arrays are
used for blasting.
To break any irregular areas at the edge of a
regular array, the blast holes may be distributed
irregularly.
The standard blast patterns are of three major
types. They are:

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• Square Grid pattern
• Rectangular grid pattern
• Staggered pattern.
This illustration shows the square grid pattern:

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This is a rectangular grid network:

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This is the staggered pattern used for blasting:
In staggered pattern the alignments are
diagonal, as you can see in this diagram.

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First row blast holes and third row blast holes
are in alignments.
The drill holes may be vertical, inclined or
horizontal. In opencast mines, both vertical
and inclined holes parallel to the bench face is
practiced.

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This illustration shows the vertical blast hole
designed to remove the excessive burden on the
left.
This is an inclined blast hole. But has an
excessive inclination. Not a good method.

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You can see that it burst at the bottom. the rest
of the burden may come as a big block.

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Here is another example. The location and
design may produce much air blast. There may
be a failure in blast operations, which may incur
loss of explosives.

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The row of the holes may be in single or
multiple. Based on this, blasting pattern is also
classified into
a) Single Row blasting pattern and
b) Multi-row blasting pattern.

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4.2 Single& Multi- row firing pattern:
In single row blasting, the fragmentation will
be low and the explosive consumption may be
more than multi-row blasting.
The multi-row blasting pattern is better and
mostly preferred.

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Plan A Shows the multi-row blasting design.
The mass will move towards the direction of the
arrow, that is vertically upwards.
(after Joseph M. Pugliese, 1972)

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Plan B shows another pattern.
It is called as box-cut design.
In this, the area to be fragmented is not a
square.
Valley-like excavation is aimed at, in this
design.
Fragmented mass will fall back within the
trapezium.

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Plan C shows a corner-cut design. It is also
called as echelon design. The fragmented mass
will fall back on the cut bench itself.

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This is one more corner-cut design- Plan D. The
blast sequence is different from the previous
plan-C.

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Plan-E is another corner cut blast design.
The cut bench will expand leftwards.

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For any shallow-depth excavations, single-row
design is preferred as shown in this plan F.

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These blast designs are preferred for the
following kinds of rock materials:
a) The alternate delay pattern (used for softer
rocks),
b) Consecutive shot delay pattern (used for
rocks with medium hardness),
c) Short delay firing with a cut (used for hard
rocks).

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4.3 Bench blasting :
Bench blasting is a common blast technique
most often used for open pit mines.
By definition, bench blasting is blasting in a
vertical or sub-vertical hole or a row of holes
towards a free vertical surface.
More than one row of holes can be blasted in
the same round.

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A time delay in the detonation between the rows
creates new free surfaces for each row.
There are three methods involved in this
process as short-hole blasting, long-hole bench
blasting and ring drilling and blasting methods..
The short-hole blasting is usually limited to
drilling rounds of 1.2 m to 5.0m length and hole
diameters of up to 43 mm.

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Cut and fill and room and pillar underground
mining methods commonly employ short-hole
blasting.
The Longhole bench blasting is similar to
bench blasting in open pits, using long holes
drilled downward either parallel to each other or
in slight rings to cover the stope area.

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Initiation of the blast is with a booster down the
hole.
Ring drilling and blasting is done from a
series of sub level drill drifts developed in the
ore body. The drill pattern is designed to cover
off the extent of the ore in the stope.
This type of blasting will cause the ore to swell
by 30%, and this must be allowed for when
blasting otherwise the blast may freeze.

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4.4 Delay Patterns:
In addition to these aspects, there is yet another
factor which plays a dominant role.
That is the blast timing and triggering
sequences.
It is called as delay pattern.
It is also called as firing sequences.

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The delay patterns, and varying the hole array
to fit natural excavation requirement, allow for
an efficient use of the explosive energy in the
blast. This diagram shows the delay in
triggering with reference to arrays

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5.0 Methods of Blasting:
The major method adopted in mining is called
as controlled blasting.
Controlled blasting is a technique of blasting for
the purpose to reduce the amount of over break
and to control the ground vibrations.

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5.1 Controlled Blasting
Following are the different types of controlled
blasting techniques:
Pre-Splitting - this is an old but highly
recognized technique with the purpose to form a
fracture plane beyond which the radial cracks
from blasting cannot travel.
Other methods include

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Trim (Cushion) Blasting,
Smooth blasting (contour or perimeter
blasting) for underground mines and muffle
blasting as a solution to prevent fly-rock from
damaging human habitants and structures.
Presplitting
Presplitting is a technique that involves loading
a single row of holes that have been drilled
along a desired highwall crest or excavation
line.

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Here in this diagram, you can see the role
presplitting.

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When the final limit of excavation is known,
there is a need to restrict the blasting zone.
Presplitting helps to control the zone.
Presplitting is to create buffer holes and presplit
holes in a closed network.
When blasting is done, since the zone is already
weakened with series of holes, the blast has no
role to play after this line.

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Such methods reduce the crushing effect
around the borehole and are shot before the
main production shot.

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The idea is to minimize or eliminate overbreak
from the primary blast and to produce a smooth
rock wall. Presplitting will add a large drilling
cost to an operation.
Borehole Diameters
Normally, the diameter of a borehole is limited
by the capabilities of the drill used to create it.
As a rule, open-pit and coal strip mines using
large drills will drill presplit holes that range
from 9 to 12-¼ inches in diameter.

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Quarry and construction presplits are much
smaller, generally ranging from 2 to 4 inches in
diameter.
Presplit holes may be drilled on an angle if the
geology and drill allow for it.
Spacing
Presplit spacing will vary, depending upon rock
characteristics, size of the operation, and bench
height. Trials should be conducted to determine
the optimal borehole spacing.

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Explosive Charge
Depending upon the rock characteristics and
spacing used for the presplit, charge loads will
vary.
Smooth Blasting
The term “smooth blasting” refers to lightly
loaded holes that have been drilled along
excavation limits and are shot after the main
excavation is removed.

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Typically, such holes are shot instantaneously
or with little delay, leaving a smooth wall with
minimum overbreak.
Smooth blasting is the most widely accepted
method for controlling overbreak in
underground headings and stopes.

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Line drilling
“Line drilling” provides a plane of weakness to
which a primary blast may break.
It may also protect a highwall by reflecting
some of the shock wave created by a blast.
The distance from the back row to the line drill
is normally 50 to 75 percent of the production
burden.

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Line drilling is normally limited to construction
projects, dimension stone quarries, and rock
sculpting activities.
Cushion blasting
“Cushion blasting,” or trim blasting, is similar
to smooth blasting. In this method, the holes are
shot after the main production shot.
Cushion blasting involves backfilling the entire
borehole with crushed stone to cushion the
shock from the finished wall.

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5.2 Major factors influencing blast
efficiency:
• Burden
• Spacing
• Bench height
• Powder column
• Blast hole diameter and depth
• Powder factor
• Stemming.

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5.3 Secondary Blasting:
Irrespective of the method of primary blasting
employed, it may be necessary to reblast a
proportion of the rock on the quarry floor so as
to reduce it to a size suitable for handling by the
excavators and crushers.

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5.4 Explosives based on chemical nature:
The Explosive used here, is a reactive substance
that contains a great amount of potential energy
that can produce an explosion if released
suddenly.
Chemical explosives are materials which under
ignition start rapid chemical reactions to
release gaseous products and energy.

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These gases under high pressure exert forces
against borehole walls which causes the rocks
get fractured.
Non-Explosive Rock Breaking is also a
possible method:
Non-explosives are used in areas very closed to
sensitive structures, like buildings.
These are mostly used in construction industry
for breaking oversize rocks, concrete etc.

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5.5 Calculating Powder Factors
The Powder Factor is a relationship between
how much rock is broken and how much
explosive is used to break it. It can serve a
variety of purposes, such as an indicator of how
hard the rock is, or the cost of the explosives
needed, or even as a guide to planning a shot.
Powder factor can be expressed as a quantity of
rock broken by a unit weight of explosives.

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Or, alternatively, it can be the amount of
explosives required to break a unit measure of
rock.
6.0 Tools for blasting:
There are lots of tools used in blasting.
The major tools used for blasting in mines are
as follows:
a) Dipper is the machine used to drill the hole
to the required depth.

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b) Jumper is the machine used to make small
blast holes & it is more effective in boring a
nearly vertical hole.
c) Priming needle is used to maintain the hole
while tamping is done & is in the form of a
thin copper rod with a loop at one end.
d) Scraping spoon is used to remove dust of
crushed stone from blast hole.
e) Tamping bar is used to tamp the material
while refilling a blast hole.

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6.1 Time design / delay design :
Time design refers to the selection of time
between holes in a row based on one third to
one half the time between rows.
Delay intervals between holes in a row less than
3 milli seconds per meter of spacing are not
recommended due to air blast and fragmentation
considerations.

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Delay intervals between rows less than 6 ms per
of burden can cause stemming ejection, fly
rock, and excessive back break.
Multiple row blast (> 4 rows) use longer
intervals in back rows.
Bottom delay has generally the shortest delay
and delay between decks in the same hole
should range 10 to 50 milli seconds.

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6.2 Selection of explosives:
Five characteristics are considered in the
selection of explosives. They are:
concern environmental factors,
1. sensitiveness,
2. water resistance,
3. fumes,
4. flammability and
5. temperature resistance.

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Sensitiveness is the characteristic of an
explosive which defines its ability to evaporate
through the entire length of the column charge
and controls the minimum diameter for
practical use.
Water Resistance:
Water resistance is the ability of an explosive to
withstand exposure to water without it suffering
detrimental effects in its performance.

100
Explosive products have two types of water
resistance, internal and external water
resistence.
Internal water resistance is defined as water
resistance provided by the explosive
composition itself.
As an example. some emulsions and water gels
can be pumped directly into boreholes filled
with water.

101
These explosives displace the water upward, but
are not penetrated by the water and show no
detrimental effects if fired within a reasonable
period of time.
External water resistance is provided not by the
explosive materials itself, but by the packaging
or cartridging into which the material is placed.

102
The fume class :
The fume class of an explosive is the measure
of the amount of toxic gases produced in the
detonation process.
Carbon monoxide and oxides of nitrogen are the
primary gases that are considered in the fume
class ratings.

103
Although most commercial blasting agents are
near oxygen-balanced to minimize fumes and
optimize energy release, fumes will occur and
the blaster should be aware of their production.
In underground mining or construction
applications, the problems which can result
from producing fumes with inadequate
ventilation is obvious.

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Flammability:
The flammability of an explosive is defined as
the characteristic which deals with the ease of
initiation from spark. fire or flame.
Some explosive compounds will explode from
just a spark while others can be burned and will
not detonate fast.

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Temperature Resistance:
Explosive compounds can suffer in
performance if stored under extremely hot or
cold condition .
Cold Resistance:
Extreme cold conditions can also effect the
performance of products.

106
Most dynamites and blasting agents will not
freeze under ordinary exposure under the lowest
temperature encountered in the country.
This is because the manufacturers have added
ingredients to these products which allow them
to perform properly, in spite of the cold
weather.

107
6.3 Performance of Explosives:
In the explosive selection process, the
environmental conditions can eliminate certain
types of explosives from consideration on a
particular project. After the environmental
conditions have been considered, one must
consider the performance characteristics of
explosives. Characteristics of main concern are
sensitivity, velocity, density, strength and
cohesiveness.

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6.4 Integration of all factors:
Blast engineering is an integrated approach. It
considers the internal environment, external
environment, drill pattern and loading and
haulage of the fragmented ore materials.

109
A good fragmentation needs this integrated
approach. Planning is a must to achieve success
in this process.
The good design will produce good results or
release of broken masses.
A bad/ poor method gives a poor output as
shown here:

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6.5 Cleaning operations in blast holes:
The blast holes are to be cleaned before loading
the explosives. It is also a time consuming
process. Now-a-days, automatic drilling and
cleansing systems are available.

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This diagram shows the schematic of such an
operation available within the drilling
equipment.

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7.0 Conclusion:
Fragmentation control through effective blast
design and its effect on productivity are the
major challenging tasks in mining.
The easiest way to mine the valuable ore bodies
is through the use of explosives and well-
planned blasting layouts which can produce
good fragmentation profiles.

113
The design of blasting patterns is specific to not
only the explosives that are being used, but also
to the rock or ore type that is being fragmented.
In this lesson we have seen most of the
important aspects related to rock blasting for
mining.

Rock blasting for mining

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