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TRAINING PROGRAMME ON
CONTROLLING ENVIRONMENTAL IMPACT OF
ROCK EXCAVATION BY BLASTING
Design of Blasting Pattern for Different Applications
By
R.R. Shirke
CENTRAL WATER AND POWER RESEARCH STATION,
KHADAKWASALA, PUNE-411 024
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PRESENTATION OVERVIEW
Blasting Patterns for Different Applications
Open Excavation
Secondary Blasting
Tunnel Excavation
Shaft Excavation
Perimeter Controlled Blasting
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CONTROLLED BLASTING
DESIRABLE EFFECTS UNDESIRABLE EFFECTS
Ground Vibration
Airblast
Flyrocks
Over-breakage
Breaking Rock into
Specific Shape and Size
Blasting
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FOR CIVIL ENGINEERING PROJECTS ROCK IS
EXCAVATED FOR VARIOUS APPLICATIONS :
DAM STRENGTHENING WORKS
DAM TOE POWER HOUSE
NUCLEAR PLANTS
TRENCHES & CANALS
QUARRYING FOR AGGREGATES
TUNNELS , SHAFTS
UNDER WATER BLASTING
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BLAST DESIGNING IS NOT PRECISE SCIENCE
WIDE VARIATION IN GEOLOGIC STRATA &
EXPLOSIVE MATERIALS
NO SIMPLE EQUATIONS TO DESIGN IDEAL
BLAST WITHOUT FIELD TESTING
BLAST DESIGNING IS MODIFIED & IMPROVED
AFTER TRIALS
BLAST DESIGN- TWO BASIC PRINCIPLES
1. EXPLOSIVE PERFORM BEST WHEN FREE FACE
PARALLEL TO EXPLOSIVE COLUMN
2. ADEQUATE SPACE MUST BE AVAILABLE FOR
BROKEN ROCK MASS TO MOVE & EXPAND
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TYPES OF EXPLOSIVES
Dry Blasting Agents
Slurry & Emulsion Explosives
Nitroglycerin based High Explosives
LOW EXPLOSIVES
HIGH EXPLOSIVES
Black Powder
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IMPORTANT EXPLOSIVE PROPERTIES
Density
Velocity of Detonation
Strength
Water Resistance Properties
Fume Characteristics
Detonation Pressure
Borehole Pressure
Sensitivity
Sensitiveness
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IN GENERAL EXPLOSIVES SHOULD HAVE
High
Strength
VOD
Detonation Pressure
Borehole Pressure
Sensitiveness
Low Sensitivity
Good
Water Resistance Property
Fume Characteristics
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ROCK PROPERTIES OF IMPORTANCE
Strength
Blastability: Resistance of rock to blasting
Wave velocity
Density
Characteristic impedance
Saturation of various layers
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Qualitative Strength of RockUCS
(MPa)
Very Strong 100
Strong 50100
Moderately Strong
12.5
50
Moderately Weak 512.5
Weak 1.255
Very Weak Rock and hard Soil .61.25
STRENGTH
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IGNEOUS ROCKSBASALT 2.75 - 3.20
GABRO 2.75 - 3.15
GRANITE 2.60 - 2.80
SEDIMENTARY ROCKSDOLOMITE 2.60 - 3.20
LIMESTONE 2.40 - 3.00
METAMORPHIC ROCKS
QUARTZITE 2.65 - 2.70
MARBLE 2.60 - 2.70SANDSTONE 1.53 - 2.95
DENSITY OF SOME COMMON ROCK
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CHARACTERISTIC IMPEDANCE
Product of compressional wave velocity and density
Useful parameter for analyzing the transfer of energy
from explosive to rock
Energy transfer is optimum when explosive impedance
matches with rock impedance.
2
2
)(
)(1
re
re
II
II
= Energy Transmission YieldIe = Explosive Impedance
Ir = Rock Impedance
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SATURATION OF VARIOUS LAYERS
Blast effects are more intensified in saturated rocks.
Pore water pressure reduces the compressive and tensile
strength of rock.
Propagation velocity is more in saturated rock media.
Higher level of ground vibration is observed in saturated
ground.
Presence of water around decoupled charge inside borehole
can increase the level of ground vibration.
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BLAST DESIGN PARAMETERS FOR OPEN EXCAVATION
HOLE DIAMETER
BURDEN
SPACING
HOLE DEPTH
SUB-DRILLING
CHARGE PER HOLE
STEMMING LENGTH
POWDER FACTOR
DELAY PERIOD
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LOADING OF HOLES
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BLASTING SITE
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BLASTHOLE DIAMETER (D)
Choice of blast hole diameter governed by:
Bench height
Structure of rock mass
Fragmentation requirement
Type of explosive material used
Size of area to be blasted
Rock mucking method adopted ( Mechanical or Manual)
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Blast hole Diameter Influence the Burden
At construction site commonly used blast hole diameters
varies between 32 and 125mm.
Small diameter holes produce better fragmentation.
Use of large diameter holes are cost effective as more
area covered in a single blast.
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BURDEN (B)
Too large burden : Explosive energy insufficient to move rock.
Excessive over-breakage
Inadequate fragmentation
Excessive ground vibration
Too small burden :Less energy available for fragmentation
Excessive airblast
Excessive flyrock
Burden is defined as the distance between a blasthole and the
nearest free surface at the instant of detonation.
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BURDEN Contd.
e - DENSITY OF EXPLOSIVE (gm/c.c.)
r - DENSITY OF ROCK (gm/c.c.)D - BLASTHOLE DIAMETER (mm)
DB
r
e
33.0
8.37
RELATIONSHIP BETWEEN BURDEN AND
BLASTHOLE DIAMETER
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APPROXIMATE B/D RATIOS FOR BENCH BLASTING
Ratio
ANFO (0.85 g/cc)
Light rock (2.2 gm/cc) 28
Average rock (2.7 gm/cc) 25
Dense rock (3.2 gm/cc) 23
Slurry, Dynamite (1.2 gm/cc)
Light rock (2.2 gm/cc) 33
Average rock (2.7 gm/cc) 30Dense rock (3.2 gm/cc) 27
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SPACING (S)
Spacing is defined as the distance between adjacent blastholes.
Spacing is a function of burden and has significant effect on
rock fragmentation
Close spacing :Crushing and cratering between holes
Boulders in burden
Toe problems
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SPACING Contd.
Wide spacing :
Inadequate fracturing between holes
Humps on the faces
Toe problems between holes
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COMMONLY USED SPACING VALUES
For Simultaneous Firing in a Row
BS 2
For Delays Between Holes in a Row
BtoBS 8.12.1
Large diameter blastholes require smaller spacing-to-burden
ratios (usually 1.2 to 1.5) than small diameter holes (usually
1.5 to 1.8).
S =1.5 B is a good approximation to start blasting work.
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DEPTH OF BLASTHOLE (H)
Blasthole depth less than 1.5 B should not be used
Shallow holes are often associated with excessive air blast
and dangerous flyrocks. Shallow hole also results
Coarse and uneven fragmentation.
Hole depth more than 4.0 B should not be used.
Deeper holes are associated with drilling error.
Hole depths around 3.0B give good blasting results withminimum back-break and toe problems.
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SUBDRILLING (J)
Subdrilling is defined as the distance drilled below the floor
level.
Required to insure that full face of the rock is removed.
Commonly used value of J is 0.1 to 0.3 times the burden.
Too much subdrilling causes excessive ground vibration.
Sub drilling should not be used where the damage to the floor
level is a concern.
Rock with well defined weakness plane may not require
subdrilling.
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STEMMING (T)
Stemming is defined as the distance between the top of the
explosive column and collar of the blasthole.
This zone of the blasthole is usually filled with inert material.
It gives confinement to explosive gases useful for rock breakage.
Best material is an angular coarse material of mixed size.
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STEMMING (T) contd..
Often drill cuttings are used. Best size is about 0.5 to 1 cm
and should not be larger than 0.1 D
Commonly used values between 0.7B and 1.0B
Too small stemming results in excessive airblast, flyrock and
may cause back break.
Too large stemming creates boulders in the upper part of the
bench.
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POWDER FACTOR
42
10854.7
HSB
LDPF
PF : Powder Factor, kg/m3
D : Hole diameter, mm
L : Length of explosive charge, m
: Density of explosive charge, gm/cc
B : Burden, m
S : Spacing, m
H : Bench height, m
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EXAMPLE ON BLAST DESIGNDATA:
Bench height : 8 m, Hole Dia. : 10 cm, e : 1.4 & r : 2.65
Compute Burden, Spacing, Sub-drilling, stemming length and
powder factor
mB 0.3056.365.2
4.11008.3733.0
mBS 5.40.35.15.1
mBJ 9.03.0
mT 1.20.37.0
Amount of rock fragmented/ hole =
BS H = 3.04.5 8.0 =108 m3
L = H+J-T = 8+0.9-2.1 = 6.8 m
Holes are filled with cartridge
explosives of 2.78 kg/cartridge,
length 40 cm
Total Charge/hole = 17 sticks= 47.26 kg
Powder Factor = Explosive Weight/ Excavated rock volume
= 47.26/ 108 = 0.44 kg/m3
DB
r
e
33.0
8.37
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BLASTING PATTERNS WITH DELAYS
Delay interval between the holes in a row should be 3 to 15 ms per
meter of burden depending on rock type.
Lower delays should be used for harder rocks and higher for softer
rocks.
A delay ratio of 9 ms/m of burden gives good results in many kinds
of rock.
Delay period between rows should be 2 to 3 times that of between
holes in a row to avoid flyrock.
To control airblast, the delay between holes in a row should be at
least 6 ms/ meter of spacing.
ELECTRICAL DETONATORS d
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ELECTRICAL DETONATORS contd..
Instantaneous
Detonators Delay Detonators
Long Delay Detonators
Short delay Detonators
Long Delay Detonators : 500 ms interval between each successive delay number
Short Delay Detonators:25ms to 100 ms
BLASTING PATTERNS WITH DELAYS
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BLASTING PATTERNS WITH DELAYS
Typical rectangular drilling pattern with delays
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BLASTING PATTERNS WITH DELAYS contd..
Same delay in a row
BLASTING PATTERNS WITH DELAYS td
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BLASTING PATTERNS WITH DELAYS contd..
Alternate delays in a row:
AS G A S A S
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BLASTING PATTERNS WITH DELAYS contd..
Progressive delays in a row:
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NON ELECTRICAL INITIAING DEVICES
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NON ELECTRICAL INITIAING DEVICES
Non electrical initiating systems used for blasting are
designed to over come various short comings of electrical
initiation systems.
Various form of non electrical initiating systems.
Non Electrical Detonator system (NONEL)
Detonating Cord and Delay connectors
NONEL DETONATOR SYSTEM
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NONEL DETONATOR SYSTEM
Total non-electric initiation
system
Has the advantages of
initiation by electric
detonators or detonating cords
but none of the disadvantages.
Include the NONEL detonator
connected NONEL tube along
with surface and down hole
delays and surface connectors.
NONEL DETONATOR SYSTEM Contd..
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Main component of this system is shock tube, a hollow tube made
with advanced material designed to withstand field conditions.
The tube consists of two layers, inner layer is made of special
material coated with a very fine layer of explosive.
Explosive used is in the range of 14 to 16 mg/m of the tube.
Outer layer is designed to withstand stress during field use.
On initiation by D-cord/ detonator, shock wave results from tube
coating of explosive layer propagate inside the tube.
One end of the shock tube is fitted with a detonator and the other
end fitted with a connector with a delay.
ADVANTAGES OF NONEL SYSTEM
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Very simple to handle and store
Immune to accidental initiation by hazardous stray current
Can be used in underwater also
Provides more possibilities regarding choice of delay interval
Provides shooting larger blast rounds, no hole limit, reduction in air
blast/ ground vibration
DISADVANTAGES OF NONEL SYSTEM
Expensive
Lack of circuit testing
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Nonel Delay Pattern
ELECTRONIC DETONATORS
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ELECTRONIC DETONATORS
Computer chip is used to control delay timing which uses
electrical energy stored in one or more capacitors to providepower for timing clock and initiation energy.
Delay is achieved electronically and not pyrotechnically.
With the electronic detonators, it is possible to provide timing
precision in the microsecond range and to get better blastingresult.
Electronic detonators with its precise delay times provide
new opportunities for more precise control of ground
vibrations.
SUMMARY
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Design initial blasting pattern using optimum
values of various blast design parameters.
Burden ( B ) = 30 X Hole Diameter ( D )
Spacing ( S ) = 2 X Burden ( B )
- Simultaneous firing between holes in a row
Spacing ( S ) = 1.5 X Burden ( B )
- Delays between holes in a row
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Sub-drilling ( J ) = 0.3 X Burden ( B )
Stemming ( T ) = 0.7 X Burden ( B )
Hole Depth ( H ) = 3 X Burden ( B )
As the excavation progresses review the initial blasting
pattern to make suitable changes
SUMMARY ---contd.:
SECONDARY BLASTING
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SECONDARY BLASTING
It is used for breaking big size boulders produced from
main blasts.
SECONDARY BLASTING
Plaster Shooting
Pop Shooting
PLASTER SHOOTING
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Plaster Shooting
Is also known as mud
capping.
An explosive charge is placed
above the boulder with very
close contact with the rock.
Explosive is covered with clay
and blasted.
It produces high degree
airblast and flyrock.
Should be used at far off
distances from built up areas.
POP SHOOTING
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Pop Shooting
Commonly jack hammer holes of depth
around 0.25 to 0.5 m are used.
Small charge is placed inside theborehole.
Like conventional blasting, holes are
stemmed .
Initiation of holes are done by
instantaneous electrical detonators.
It produces high degree airblast and
flyrock.
Should be used at far off distances from
built up areas.
TUNNEL BLASTING
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Tunnels could be blasted as a full face method or by heading
and benching method.For heading, the only free face is the surface from which holes
are drilled.
For benching there may be one or more additional free face.
Benching can be compared with open excavation.
Heading round differs from benching with that the initial cut
in heading is made at the most confined condition.
Essential function of the cut is to provide additional free faceto which rock will break.
TUNNEL BLASTING
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Opening Cuts
Angled Cuts Parallel Hole Cuts
Fan Cut V-Cut
TYPICAL FAN-CUT BLASTING PATTERN
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Horizontal Pattern Vertical Pattern
In the fan cut holes are located in the
form of a fan.
The fan can be horizontal or vertical.
V-CUT PATTERN FOR TUNNEL EXCAVATION
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Cut holes are drilled at an angle
to create a V-shaped opening.
Angle of the subsequent holes
are reduced.
Perimeter holes are slightly
looking forward.
PARALLEL-CUT PATTERN FOR HORIZONTAL DRIFT
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Parallel cut is also known as burn
cut.
A series of closely spaced parallel
holes are drilled.
Some of these holes are loaded
and some are unloaded.
These holes eject a cylinder of
rock to create an opening.
The burden of the remaining holes
can be broken to this opening.
SHAFT EXCAVATION
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Full Bottom Method
Benching Method
Shafts are used at construction site for excavating
tunnels at large depths.
It is of rectangular, elliptical or circular shape. However,
circular shafts are used more frequently.
Shaft excavation can be broadly be divided into three
groups;viz.,
FULL BOTTOM METHOD
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Circular Shaft
Several techniques are used for
placement of holes in which the
face is opened with angle or
parallel cut.
Normally the cut is drilled at the
center of the shaft.
For better results,the center of
the cut may be shifted to suit
the strata.
FULL BOTTOM METHOD
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Rectangular Shaft
In rectangular shafts holesare distributed similar to
horizontal drifts.
Most used pattern for
rectangular shaft excavation is
wedge or pyramid type.
BENCHING METHOD
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Two halves of the shaft bottom
are blasted alternatively.
It is suitable for rectangular
shafts.
One half of the shaft cross
section is drilled leaving the
other half as a free cavity.
Blasting pattern is similar to
bench blasting.
PERIMETER CONTROLLED BLASTING TECHNIQUES
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Pre-splitting
Like ground vibration, airblast and flyrock, over-breakage and
damage to adjacent rock mass in the form of fractured roof andwall are also undesirable effects.
Excessive over-breakage leads to:
Safety problems due to rock falls
Escalation in cost due to extra mucking and extra concrete to
back fill
Smooth Blasting Cushion Blasting Line Drilling
THEORETICAL BACKGROUND
D i ti l t PPV
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Damage is proportional to PPV
Particle velocity beyond which damage is initiated is called as
Critical Particle Velocity.
Critical Particle Velocity is a function of compressional wave
velocity, modulus of elasticity and tensile strength and could be
estimated from the following relation.
E
VTV
CSC
P
For VC=5000 m/s, Ts=15 Mpa and E=75GPa, the critical particle velocity isfound to be 1000 mm/s
PPV for formation of new cracks is in the range of 600 to 1000 mm/s
VPC is the critical particle velocity,TS is the
tangential modulus of elasticity and E is the
modulus of elasticity.
THEORETICAL BACKGROUND contd..
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20.2
P
Q
R3700V
For Q, 0.125 kg, VP would be 600 mm/s at 0.81 m and 1000 mm/s at 0.64 m.
Thus, in conventional blasting rock will suffer excessive damage to considerable
distances from the blastholes at the perimeter of excavation.
In addition to charge weight, the damage zone around a blasthole would also
depend on charge concentration and decoupling of charge inside the borehole.
Based on past data collected from different project sites at very short distances
(
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The damage induced in the rock mass is mainly due to
the pressure applied to the borehole wall.
Any method which help in reducing this pressure will also
be useful in arresting damage to surrounding rock mass.
Decoupled charge: charges are not in contact with
borehole wall.
Coupling Ratio (CR) = Charge Radius/ Blasthole Radius.
Use of CR less than 1, helps in controlling the
overbreakage
DAMAGE TO ROCK MASS BEYOND EXCAVATION LINE
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EFFECTS OF USING PERIMETER CONTROLLED
BLASTING
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LINE DRILLING
A f l l d h l
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A row of closely spaced holes are
drilled along the final excavation
line.
The spacing between the holes is 2 -
4 times the dia. of holes and are not
loaded with explosive.
The row of unloaded holes provides
a plane of weakness to which main
blast can breakBlast holes close to the line drilling
holes are also loaded with smaller
charge than other holes.
Also the burden and spacing of the
row of holes adjacent to line drilling
holes are different from other
holes.
PRE-SPLITTING
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Pre-splitting holesThe technique uses a row of
closely spaced holes.
Spacing is 8-12 times thediameter of blast hole and
burden is infinite.
These holes are loaded with small
quantity of explosives.
Holes are fired before the mainblast or with earliest delay of the
main blast.
The light decoupled charges used
produce a single continuous
narrow crack along the pre-
splitting line.
SMOOTH BLASTING
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Widely used for underground
applications
It is similar to pre-splitting in
respect of drilling and loading
of holes.
Unlike pre-splitting, it is fired
with the last delay of the main
blast.
Burden for smooth blasting holes
is lower than that for other holes.
Main advantages of smooth
blasting are reduced over
breakage than conventionalmethod and requirements of less
back support.
CUSHION BLASTING
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Commonly used with large diameter of holes for open
excavation.
Holes are loaded with light, well distributed charge,
completely stemmed and fired after the main blast.
Stemming is placed in the void space around the charges.
Stemming cushions the shock from the finished wall as the
burden is blasted, thus minimizing fracturing and
shearing of finished wall.
Larger the diameter of hole, the more cushioning would be
realised.
COMPARISON OF PERIMETER CONTROLLED
BLASTING TECHNIQUES
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Q
Perimeter controlled blasting techniques are commonly used to minimize
over-breakage and damage to adjacent rock mass
Pre-splitting method differs from other methods in that the holes are
fired in most confined conditions before the main blast is fired
Smooth blasting is commonly applicable in underground application and
helps in minimizing the over-breakage
Cushion blasting is similar to smooth blasting and is applicable normally
during surface excavation. Large diameter of holes are used.
Line drilling involves more closely spaced holes than other methods and
explosives are not used in these holes. However, because of close spacing
this technique is associated with higher drilling cost.
CONCLUSIONS
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Secondary blasting used at construction sites for breaking large
size boulders are associated with high risk of fly rock.
Blasting for tunnel and shaft excavation are significantly
different from those used for open excavation.
Application of perimeter controlled blasting techniques help in
minimising the over-breakage and damage produced to
adjacent rock mass.
For shaft excavation commonly full bottom method or benchingmethod is used.
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