lecture04 -rrs

7/28/2019 Lecture04 -RRS

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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



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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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Alternate delays in a row:

AS G A S A S


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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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