Lecture13A(BlastingforRockTunneling)

8/13/2019 Lecture13A(BlastingforRockTunneling)

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Blast Effects on Structures and Slopes


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Outline

Examples of Blasting for Slope and Tunnel Formation

General Principles of Blasting in Slope and Tunnel

Limitations of Current Vibration Limit Criterion

Other Parameters required for better understanding of

Blast Effects on Slope and Tunnels

Simplified Finite Element Model for studying Effects of

Blasting near Slopes

Future Monitoring Works required to improve the Currentunderstanding of Vibration Limits

Conclusions


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318 August 1995

Example 1: Controlled Blasting for Rock Slope Trimming


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Road Widening at Tai Lam Section of Tuen Mun Highway

Boulder Fall Fatality in 18 August 1995


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Extensive Rock Mapping after Accident


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Risk of Large

Wedge formed by

Sheeting Joints


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Scheme Design

before Tendering

For

Constructability

Purposes


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Direction of blasted rock

0

0

1

25

2

50

3

67

4

75

5

92

6

117

7

134

Need to create two Free

Faces to control

direction of blasting


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Upper shafts

Diaphragm Wall

Lower shafts

Drill and Blast

Excavated diameters range between 2.5 m to 50 m

(SCI) and at a maximum depth over 150 m

Below the toe of the diaphragm walls 1.5 m long

blasting holes were used.

As the ground conditions improved with the

depth of excavation the shot hole length was

increased to 2.4 m.

Two types of full face blasting patterns namely

wedge cut and parallel hole cut (with relief

holes) were used.

Example 2: Controlled Blasting for Shaft Excavation


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Plan View Looking Down

Sequence of Initiation

55

35

18

14

10

8

5

2

Wedge Cut to create Free Faces


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ICI manufactured explosives Powegel 2500UG (bulk emulsion)

Pexgel and Tovex are cartridge explosives in 25 mm and 32 mm diameter sizes.

Pexgel and Excel non-electric detonators, Nonel GT/T non-electric detonators.

The emulsion was pumped into the blast holes.

A primer charge with one cartridge and detonator was placed at the bottom of the hole before filling with emulsion.

The specific charge (powder factor) was approximately 3.6 kg/m3 with the bulk emulsion

The specific charge was approximately 2.7 kg/m3 with cartridge explosives.

For the SSDS shaft excavation, the following was used:


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Explosives: The purpose is to generate peak gas pressure and heat in crushing and

plastically deforming weak/soft rocks, without creating excessive throw, over break,

ground vibration and air vibration

Each blast hole should be primed, charged and stemmed so that explosion gases are confined

for a reasonable period of time.

Explosives: should have a low cost and a high energy yield per unit weight coupled

with:

Insensitivity to initiation by friction, mechanical impact and fire;Totally reliable sensitivity to initiation by the detonator or primer for which the

explosive has been designed;

Unlimited resistance to water and low temperatures;

Oxygen balance and hence minimal yield of poisonous explosion gases;

Excellent handling characteristics, and

Unlimited shelf life.


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Explosives: grouped into two principal categories:

High explosives

(detonator-sensitive)

1.Emulsion explosive

2.Watergel explosive

3.Dynamites (NG (nitroglycerine) sensitized)

4.Cast Pentolite-type Boosters

Blasting agents

(detonator-insensitive)

Detonating explosives are divided into Primary

and Secondary. Primary (used in the blasting

caps and cast primers (mercury fulminate,

PETN, Pentolite etc.) are used as initiators for

the Secondary. Secondary are those applied to

the breakage of rocks.

Normally can be initiated by No. 8 strength detonator

or by a strand of 10 g/m detonating cord

1.Emulsion explosive water-in-oil type (consists of micro

droplets of super-saturated oxidizer solution within an oil matrix.

Oxidizer within the micro droplets consists of mainly ammonium

nitrate (AN)

Excellent water resistance

Densities usually lie in the range of 1.1 to 1.2

g/cm3

2.Watergel explosive based on saturated aqueous solution of AN in which

fuels (aluminum powder), sensitizers are dispersed. Have been largely

replaced by emulsion explosives.

3.Dynamite explosive are NG-based explosive of high sensitivity

4.Cast Pentolite-type Boosters boosters to initiate ANFO-type, emulsion and

watergel blasting agents.

- make principally of cast pentolite (a mixture of TNT and PETN)

- completely waterproof

-ANFO (consists of AN (ammonium

nitrate) and diesel oil)-Slurries

-Emulsion

-Watergel

-Heavy ANFO

Blasting agents (detonator-insensitive) consists largely

of AN (ammonium nitrate is classified as an oxidizing

agent and not considered to be an explosive).


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ANFO (consists of AN (ammonium nitrate) and diesel oil) AN is a strong supporter for combustion, but it is not flammable itself

- cost is lower for ANFO and energy yield per ton is usually higher than other explosives- the micro voids within each porous prill of the diesel oil absorb and retain the optimum amount of oil

- usually used in dry blast hole situations

Heavy ANFO consists of a mixture of porous prilled AN, oil and an emulsion blasting agent.

- If the emulsion can fill up the micro void within the ANFO, then the density can reach 1.00 to 1.25 g/cm3

- should be used in hole diameters of about 100 mm

- generates about 20% to 50% more energy than an equal volume of ANFO

Emulsion-based Blasting Agents

- maintain the properties of the slurries but with improved strength and water resistance

- cartridged emulsion-based blasting agents

- bulk emulsion-based blasting agents

Watergel-based Blasting Agents

- cartridged watergel-based blasting agents require high-explosive boosters (e.g., PENTEX) to initiate them reliably

- pumped watergel -based blasting agents

Slurries are blasting agents based on saturated aqueous solution of AN, often with other

oxidizers such as sodium nitrate in which fuels, sensitizers, gellants are dispersed toavoid segregation of the solids

Blasting agents

1 When an explosive is detonated it is converted within a fewWh d d


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Mechanism of

Rock Breakage byExplosion

1. When an explosive is detonated, it is converted within a few

thousandths of a second into a high temperature gas, pressure

exceeds about 18000 atmospheres.

2. This energy is transmitted into the rock in the form of a

compressive strain wave at a velocity of 2000-6000 m/s.

3. The rock within 1 to 2 charge radii is crushed by compression.

4. As the compressive wave front expands, the strain quickly

decays and beyond this point (pulverized zone), the rock is

subject to intense radial compression where tangential strains

start to develop.

5. When these tangential strains exceed the tensile strength of the

rock, radial fractures began.

6. When the compressive wave reaches a free surface, it is

reflected as a tensile strain wave.

7. If the reflected tensile strain wave is sufficiently strong,

spalling occurs progressively from any effective free face

back toward the blast hole.

8. This causes unloading of the rock mass, producing an extension

of previously formed radial cracks.

9. Rock is much weaker in tension than in compression, thereforethe reflected strain wave is very effective in fracturing the rock.

10. The expanding gases will wedge open the strain-wave generated

cracks and expel the rock mass.

11. The fragmentation process will depend on:

1. Degree of confinement provided by the stemming(inadequate will cause energy lost).

2. Charges within the blast hole (inadequate will result in

poor transmission of strain wave).

3. Amount ofburden (excessive burden will choke the gas

expansion).

4. Sequence of blasting (ensure development of freesurfaces for the rock to move into).

Why do we need

to create a free

face before

massive

explosion?

I iti ti D i d S t


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Initiating Devices and Systems

Initiating devices and systems are designed to activate explosive charges:

1. From a safe distance

2. At a pre-determined time

3. In a pre-determined sequence and

4. With pre-determined time intervals between successive detonations

Initiating systems explosive and inert components which transmit signals to explosive charges by

non-electric or electric means.

Non-electric initiating systems utilize chemical reactions, which can range from slow burning torapid violent detonation, to initiate explosive charges either directly or via non-electric detonators.

Electric initiating systems require a device which can generate or store electrical energy and

transmit to electric detonators by a circuit of insulated conductors.

Initiating Devices

Detonators contain relatively sensitive high explosives which are initiated by an energy signal from an external source.

Delay detonators incorporate a controlled time delay to sequence the detonations of charges.

Shock-tube Detonators are Non-electric Detonators assembled from:

1. A high strength, non-electric detonator which features a PETN base charge and pyrotechnic delay

elements inside an aluminum shell;

2. A length of shock tube (one end of the tube is crimped into the detonator shell and the other is closed off

by a waterproof seal) and

3. An inert plastic connector (for connecting the shock tube to a trunk line of a detonating cord).

The energy required to fire a Shock-tube detonator is transmitted by means of a stable shock wave which

travels through the tube at approximately 2000 m/s. It cannot be initiated by flame, friction or impact andeliminates the accidental initiation by stray electrical currents, static electricity and radio frequency.


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Electric Detonators An instantaneo s electric detonator consists of an al min m hich is closed at


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Electric Detonators An instantaneous electric detonator consists of an aluminum which is closed at

one end and contains a base charge of high-explosive PETN, a sensitive priming charge and an electric

fuse head. The fuse head consists of a fine metal bridge wire which is surrounded by a sensitive

flashing composition, and is soldered across the ends of two insulated lead wires. The lead wires pass

through a rubber plug which is securely crimped into the shell to provide a water-resistant seal.

When sufficient electrical energy is passed through the lead wires, the fine bridge wire becomes hot

enough to ignite the fuse head, which initiates the priming charge and the powerful PETN base charge.

Instantaneous electric detonators with a No. 8 or higher strength base charge are recommended as

starter detonators for initiating detonating cord trunk lines.

No. 8 strength detonators have explosive mixture equivalent to 2 g of 80% mercury fulminate and 20% potassium chlorate.

Electric Detonator Electric Detonator with Delay

Aluminum powder

Electronic Detonator


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Detonating Cords they are strong, flexible linear explosives which consists of a continuous core of high

explosive, covered by a seamless plastic jacket which may be over-wrapped with other textile yarns.

All cords suitable for use in construction blasting contain PETN, a high-explosive powder, and detonate at 6.0 to7.5 km/s. Detonating cords designed for specific tasks have PETN core charges ranging from 1.5 g/m to 85 g/m,

enclosed in appropriate outer cover.

Energy sources such as static electricity, stray electrical currents or radio frequency transmissions will not

initiate detonating cord.

Detonating cords can be reliably initiated by means of a No. 8 strength detonator which is firmly attached to a

dry section of cord at least 150 mm from the cut end.

Detonating Cord

Contains explosive charge of 10 g of PETN per meter run

Diameter nominally 4.65 mm


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Detonating Cord Trunk Line they are cords with core load of 3.6 to 10.0 g/m that will allow

transmission of signal from one point to another even though joins are made by simple knots.

Detonating Cord Down Line detonating cord can be reliably initiated by non-electric trunk linedelays.

Initiating Systems


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Initiating Systems

Initiating Systems for Bench-Type Blasts

1. MS (millisecond)-delay shock-tube detonators are used inside blast holes. Short delay

times are used between blast holes, to ensure that they interact effectively to producegood fragmentation.

2. A trunk line of 3.6 to 5.0 g/m detonating cord is used to initiate the in-hole shock-tube

detonators.

3. Two instantaneous No. 8 strength electric detonators are used to initiate the detonating

cord trunk line.

Initiating Systems for Tunnelling

1. LP (long-period)-delay shock-tube detonators are used inside blast holes. Relatively

long delay times are required between blast holes, to ensure that broken rock iseffectively ejected during the blast.

2. A trunk line of 3.6 to 5.0 g/m detonating cord is used to initiate the in-hole shock-tube

detonators.

3. Two instantaneous No. 8 strength electric detonators are used to initiate the detonating

cord trunk line. Two detonators are used to ensure reliable initiation.

LP Detonators long period delay intervals for tunneling

MS Detonators millisecond delay intervals for benching type operations

T & D Detonators - millisecond delay intervals with an additional connector to enable them to be used as Trunk Line

Delays between blast holes.


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Production Bench Blasting The production of a well-fragmented and loose

muck pile that can be handled easily requires the

understanding of the following parameters:

1. Type of explosive

2. Properties of the rock being blasted

3. Blast hole diameter

4. Blast hole inclination

5. Effective burden

6. Effective sub-drilling

7. Effective spacing

8. Stemming

9. Initiation sequence for detonation of explosive

charges

10. Delays between successive blast holes and/or

rows of blast hole

J i t b ddi l f lt ft


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Production Bench Blasting TerminologyJoints, bedding planes, faults, or soft seams may

allow the explosives energy to be wastefully

dissipated rather than performing the work.

Pre-existing fractures may tend to dominate the

nature of the fracture pattern produced by a

detonating blast hole.

In order to maintain a constant burden with

depth, all blast holes should be inclined.

Sub-drilling is necessary in order to break therock at bench level. Breakage of the rock usually

projects from the explosive charge in the form of

an inverted cone with sides inclined at 100 and

200 to the horizontal. Experience has shown that

effective sub-drilling equal to 8 times the blast

hole diameter is usually adequate to ensure

effective digging to grade.


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25Blasting to a a) free face, b) free end and c) buffered end

1 Free Face

2 Free Faces

Free Face away

from previously

blasted rock

Too small a burden allows the radial cracks to extend to the free face giving rise to rapid venting of explosion


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Too small a burden allows the radial cracks to extend to the free face, giving rise to rapid venting of explosion

gases with a consequent loss of efficiency and the generation of fly rock and air blast problems.

Too large a burden chokes the blast and gives rise to very poor fragmentation and a general loss of efficiency.

Experience has shown that the explosive charge is most efficient when theburden is equal to 25 to 35 times the

blast hole diameter.

Effective burden and spacing depends onblast hole pattern and sequence of firing

A square pattern which is fired row by row

gives an effective burden equal to the

spacing between successive rows

Identical pattern fired at differentsequence, resulting in different burden and

spacing


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When the free face is uneven, the use of

easer blast hole, to reduce the burden, is

recommended.


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Blast is broken down into a number of

successive detonations by means of delays

Most simplest but not the most satisfactory

For use in strong rock where near vertical joints

strike across the bench at an angle to the face.

For use in condition where the strike of two

joint sets intersect.

When free face is not available.

Golder Associates (1992)


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When the front row is detonated and moves away from the rock mass to create a new free face, time should be

allowed for this new face to be established before the next row is detonated.

Typically, minimum delay intervals of 5 to 8 ms per meter of burden are used. A typical blast with a burden of

2 m would have a delay of at least 15 ms between rows.


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Experience has shown that:

a staggered pattern with an effective spacing equal to 2.0 times the effective burden gives

good results.

The use of stemming is to direct explosives effort into the rock mass.

The optimum stemming length depends on the properties of the rock and can vary

between 0.7 to 1.0 times the burden.

The blasting sequence should always be such that the blast holes detonate in rows starting

at the free face.

When the front row is detonated and moves away from the rock mass to create a new free

face, time should be allowed for this new face to be established before the next row is

detonated.

Typically, minimum delay intervals of 5 to 8 ms per meter of burden are used. A typical

blast with a burden of 2 m would have a delay of at least 15 ms between rows.

Side-Hill Blasting


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Side Hill Blasting

Most side-hill cuts require that the

muck pile be retained within the

boundaries of the cut. By designing

the delay pattern with the V offset

from the centerline, the principal

direction of rock movement is largely

up the slope.

Design is based upon the use of

millisecond delay detonators located

well down within the blast holes.


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32Principal direction of rock movement changes

as the angle within the V (angle ) increases.

Square Pattern

Staggered Pattern

Rectangular Pattern


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Principal direction of rock movement is down

the hillside.

Purposely dispose the rock down the hill for

bench blasting


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Smooth Wall Blasting using Pre-splitting (Pre-shearing) Technique

Requires greater amount of

drilling, higher drilling accuracy

Involves drilling a row of closely

spaced 38-89 mm diameter blast

holes along the design excavation

limit.

Blast holes are lightly charged

and detonated simultaneously.

Pre-split blast holes can be fired

either as a separate shot or with

the production blast (pre-split

should detonate about 50 ms

before the earliest-firing

production blast holes).


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Spacing between pre-split

blast holes normally

increases with the blast

hole diameter.

Heavily charged detonating

cord can also be used as the

entire pre-splitting chargerather than attaching half

cartridges or whole

cartridges of explosives to

a detonating cord.

Creation of Free Face for Blasting in Tunnels


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Nomenclature used in Tunnelling

Hole Type used in a Round


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Blasting in Tunnels and Caverns

General rules for design of burn-cut:

Maximize the number and diameter of

(uncharged relief holes A and B)

Locate blast holes such that each isshielded by a relief hole. Shielding

refers to the practice of drilling relief

holes directly in line between blast

holes that may have an adverse effectto one another

Initiate charges with long period so that

any compression of a later-firing

charge is given the opportunity to relax.

Blast hole 1, 2 and 3 are shielded bythe relief holes

Each charge has two or three relief

holes to which it can shoot

Any ground movement or

deformation and gas venting tends to

be into the relief holes rather than into

a blast hole on a later delay


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38Wedge Cut


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Width of blast round 4.6 m

Height of blast round 7.6 m

Blast hole diameter 43 mm

Blast hole length 1.5 m

Number of blast holes 55

Diameter of relief holes 102 mm

Length of relief holes 1.7 m

Number of relief holes 3

Types of explosives: 32x300 mm cartridge of

Powergel

Exel (non-electric) detonators

MS delay numbers 4 to 30

Exel LPD number 10 to 15

TLD (trunk line delay) 0 ms, 17 ms and 34 ms

Powder factor: 1.51 kg/m3

Maximum charge per delay 1.138 kg

Estimated PPV at sensitive location

(43 m away) 11 mm/s


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Exel LPD number 10 to 15

MS delay numbers 4 to 30

TLD (trunk line delay) 0 ms,

17 ms and 34 ms


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Tunnels with small cross sectional area generally

require higher energy factor/powder factor than those

with large cross sectional area because of the high

concentrations of explosives in the cut.

Powder factor is affected by:

Blast hole diameter

Type and geometry of cut

Mean advance per round

Blasting Shafts


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Blasting Shafts

Blasting pattern for small round(this case 1.2 m x 1.8 m)

Short advance of 0.9 m to 1.0 m


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Blasting pattern for largerrectangular and square shafts

A V cut is used


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Blasting pattern for larger

circular shafts (4 m or greaterdiameter)

A pyramid cut is used

Influence of Rock Mass Properties on Blasting


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Influence of Rock Mass Properties on Blasting

Brady and Brown (1985)

Empirical correlation between explosive

type and rock mass properties. Bradyand Brown (1985) suggested that uniaxial

compressive strength may be used to

represent the ease of generating new

fractures in the medium.


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Rzhevsky and Novik (1971)

Generalized classification adopted in Soviet

Union, based on unit consumption of

Ammonite No. 9 (AN based blasting agent).


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Kutuzov (1979)

Generalized correlation between compressive strength and

powder factor in bench blastings


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49Ashby (1977)

Empirical correlation

between fracturefrequency, shear

strength and powder

factor of the explosive


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Blastability Index (Lilly, 1986)

correlates to geomechanical

parameters

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Lecture13A(BlastingforRockTunneling)