Blast Design

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Bia.st Design

Calvin J. Konya, Ph.D . Precision Blasting Services Montville, Ohio

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Intercontinental Development, Montville, Ohio 44064 ,~

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1995 by Intercontinental Development Corporation Montville, Ohio 44064, U.S.A.

All rights reserved. No part of this book may be reproduced, in any form or by any means, without permission in writing from the publisher.

Printed in the United States of America 10987654321

ISBN 0-9649560-0-4

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CONTENTS Preface .................................................................................. vii 1 Explosives Engineering ...................................................... 1 Introduction .................................................................. 1 Sources of Explosive's Energy ......................... 2 Shock Energy .................................................. 4 Gas Energy ...................................................... 5 Chemical Explosives ....................................... 6 ......................... 11 Identification of Problem Mixtures

2 Mechanics of Rock Breakage ........................................... 13 Shock Energy in Rock Breakage ................................ 13 Confined Charges in Boreholes .................................. 14 Bench Stiffness .......................................................... 16 Breakage Process ...................................................... 18

3 Explosive Products ........................................................... 19

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Environmental Characteristics of Explosives .............. 19 Sensitiveness ................................................ 19 Water Resistance .......................................... 20 Fumes .............................................·.............. 22 Flammability .................................................. 23 Temperature Resistance ................................ 24 The Cycling of Ammonium Nitrate ........... 24 Cold Resistance ...................................... 25 Performance Characteristics of Explosives ................ 27 Sensitivity ...................................................... 27 Velocity .......................................................... 28 Detonation Pressure ...................................... 29 Density .......................................................... 30 Strength ......................................................... 31 Cohesiveness ................................................ 32 Commercial Explosives .............................................. 32

Dynamite ....................................................... 33 Granular Dynamite ........................................ 34 Straight Dynamite .................................... 34 High Density Extra Dynamite ................... 35 Low Density Extra Dynamite .................... 35 Gelatin Dynamite ........................................... 35 Straight Gelatin Dynamite ........................ 35 Ammonia Gelatin Dynamite ..................... 36 Semigelatin Dynamite ............................. 36 Slurry Explosives ........................................... 36 Cart ridged Slurries ................................... 38 Bulk Slurries ............................................ 38 Dry Blasting Agents .................................................... 39 Cartridged Blasting Agents ............................ 40 Bulk ANFO .................................................... 41 Water Resistance of Ammonium Nitrate ........ 41 Energy Output of ANFO ................................. 42 Properties of Blasting Prills ............................ 43 Heavy ANFO ................................................. 45 Two Component Explosives ....................................... 46 4 INITIATORS & BLASTHOLE DELAY DEVICES ................ 47

Introduction ................................................................ 47 Electric Blasting Caps ................................................ 47 Instantaneous EB Caps .................................. 49 Long Period Delay Electric Caps .................... 49 Millisecond Delay Electric Blasting Caps ........ 49 Electronic Delay Blasting Caps ................................... 49 Magnadet ................................................................... 50 Magnadet Electric Detonator & Magna Primer Working Principle .................................... 50 Initiation Source ............................................. 50 Detonator Description .................................... 50 Magnadet Sliding Primers .............................. 51 Safety Features Claimed ............................... 53 Operational Advantages Claimed .................. 53 Sequential Blasting Machine ...................................... 53 Non-Electric Initiation Systems ................................... 54 Detaline Initiation System .............................. 55 Detaline Cord ................................................. 55

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Detaline MS Surface Delays .......................... 55 Detaline MS In-Hole Delays ........................... 56 Detonating Cord & Compatible Delay Systems ........... 56 Delayed Primers ......................................................... 57 Shock Tube Initiation Systems ................................... 58 LP Series Shock Tube Initiators ..................... 58 S.L. Series Primadets .................................... 58 L.L.H.D. Series Shock Tube Initiators ............ 59 Shock Tube Trunkline Delays ........................ 59 EZ Det (Ensign Bickford) ............................... 60 5 PRIMER AND BOOSTER SELECTION.............................. 61

Primer Types .............................................................. 61 Determination of Numbers Needed ................ 63 Selection Criteria for Primer. .......................... 63 Primer Selection Guidelines ........................... 65 Booster ...................................................................... 65 Effects of Detonating Cord on Energy Release ........... 66

6 BLAST DESIGN ................................................................. 68 Burden ...................................................................... 68 Adjustments for Rock & Explosive Type ........ 70 Corrections for Numbers of Rows .................. 72 Geologic Correction Factors .......................... 73 Stemming Distance .................................................... 75 Subdrilling .................................................................. 77 Selection of Blasthole Size ......................................... 80 Blasting Considerations ................................. 80 Initiation Timing and Cap Scatter ................... 82 Timing Effects on Fragmentation ............................... 84 Hole-To-Hole Delays ...................................... 85 Row-To-Row Delays ...................................... 85 Borehole Timing Effects ............................................. 87 Fragmentation Size ....................................... 87 Piling or Casting Material... ............................ 87 Air Blast and Flyrock ...................................... 87 Maximum Vibration ........................................ 88 Firing Time Overlap ....................................... 88

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Effects of Time and Distance ......................... 90 Cap Scatter ................................................... 92 Overbreak, Backbreak and Endbreak ............ 94 7 PA ITERN DESIGN ...........................•...•.•••••....•..••............. 95 Principle of Production Blasting Patterns .................... 95 Instantaneous Initiation Low Benches ............ 97 Instantaneous Initiation High Benches ........... 98 Delayed Initiation Low Benches ..................... 98 Delayed Initiation High Benches .................... 99 Maximum Fragmentation ......................................... 100 Rock Fragmentation ................................................. 102 Fragmentation ............................................. 102 Kuznetsov Equation ..................................... 103 Size Distribution ........................................... 104 Field Results ................................................ 105 Limitations of the Kuz-Ram Model ............... 106 Effects of Blasting Parameters on "n" .... 107 The Effects of Stronger Explosives ........ 107 Fragmentation Effects on Wall Control ........ 107 Rip-Rap Production .................................................. 126 Rock Piling Considerations ....................................... 127 Sinking Cuts ............................................................. 129 Hillside or Sliver Cuts ............................................... 132 Utility Trench Design ................................................ 133 Secondary Blasting .................................................. 135 Mud Capping (Boulder Busting) ................... 135 Blockhoting (Boulder Busting) ...................... 135 Air Cushion Blasting .................................... 136 8 OVERBREAK CONTROL ..•..............•......••...•.................. 137 Controlled Blasting ................................................... Principles of Operation ................................ Effects of Local Geologic Conditions ........... Presplitting ................................................... Trim (Cushion) Blasting ............................... Trim Blasting With Detonation Cord ............. Line Drilling .................................................

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Assessment of Results ................................. 150 Causes of Overbreak ............................. 152 Backbreak ............................................. 153 Endbreak ............................................... 155 Flyrock Control ...................................... 155

9 UNDERGROUND BLAST DESIGN .................................. 157 Introduction .............................................................. 157 Shafts ...................................................................... 157 Ring Drilled Vertical Hole Design ................. 158 Burden Determination ............................ 159 Number of Rings ................................... 159 Burden Actual. ....................................... 159 Spacing of Holes in Ring (Estimate) ...... 160 Number of Holes/Ring ........................... 160 Spacing Actual /Ring ............................. 160 Depth of Advance .................................. 160 Subdrill .................................................. 161 Stemming .............................................. 161 Look Out ............................................... 161 Timing ................................................... 161 Tunneling ................................................................. 163 Bum or Parallel Hole Cuts ........................... 165 Design of Cut Holes ............................... 167 Calculations of Bum Cut Dimensions ........... 167 Empty Hole(s) (DH) ............................... 167 Calculation of 81 for Square 1 ............... 169 Simplified Bum Cut Calculations ........... 170 Depth of Blast Hole (H) .......................... 170 Depth of Advance (L) (Expected) ........... 171 Stoping Holes ........................................ 171 Lifter Holes ............................................ 171 Contour Holes, (Rib & Back Holes) ........ 172 Blasthole Timing .................................... 172 Initiator .................................................. 172 V-Cut ........................................................... 174 V-Cut Design ............................................... 176 Determination of Burden ........................ 176 Spacing Between Holes (Vertically) ....... 177 V-Angle ........... ,..................................... 177

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Depth of Cut or Advance (L) .................. 177 Stemming Distance ............................... 178 Lifter and Stoping Holes ........................ 178 Contour (Rib & Back) Holes ................... 178 Look Out ............................................... 178 Blasthole Loading .................................. 178 Timing Sequence .................................. 178 Fan Cuts ...................................................... 180 Heading and Bench Methods ....................... 181 10 VIBRATION AND SEISMIC WAVES .............................. 183

Seismic Waves ........................................................ Wave Parameters ........................................ Vibration Parameters ................................... Understanding Vibration Instrumentation .................. Seismic Sensor............................................ Seismograph Systems ................................. Vibration Records and Interpretation ........................ Seismograph Record Content.. .................... Field Procedures and Operational Guides .... Practical Interpretations ............................... Factors Affecting Vibration ....................................... Principal Factors .......................................... Charge - Distance Relationship .................... Estimating Particle Velocity ......................... Vibration Control... ....................................... Delay Blasting ....................................... Propagation Velocity vs. Particle Velocity ..................................... Scaled Distance .................................... Adjusted Scaled Distance ...................... Particle Velocity - Scaled Distance Graph ............ Ground Calibration ................................ Factors Effecting Vibration ....................

183 183 184 185 185 186 188 188 190 191 192 192 192 194 194 195 196 197 200 200 201 202

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11 BLAST VIBRATION STANDARDS ...••....••...•..•......•....•.• 204

Standards Development ........................................... 204 Recent Damage Criteria .............................. 206 Alternative Blasting Criteria ......................... 207 The Office of Surface Mining Regulations .... 209 Characteristic Vibration Frequencies ............ 212 Spectral Analysis ......................................... 213 Response Spectra ....................................... 214 Long Term Vibration and Fatigue ................. 214 Walter's Test ......................................... 215 CERL Tests ........................................... 215 Koerner Tests ........................................ 216 Vibration Effects .......................................... 216 Directional Vibrational Effects ................ 216 Non-Damage Effects ............................. 217 Causes for Cracks Other Than Blasting .................................... 218 Sensitivity to Vibration .............................................. 219 Effects of Blasting on Water Wells & Aquifers .......... 221 Aquifers ....................................................... 221 Vibration Effects .......................................... 221 Open Cut ..................................................... 222 12 AIR BLAST MONITORING AND CONTROL. ................. 223

Air Blast ................................................................... 223 Overpressure and Decibels ...................................... 223 Glass Breakage ........................................................ 224 Scaled Distance for Air Blast.. .................................. 225 Regions of Potential Damage for Air Blast.. .............. 226 Near Field .................................................... 226 Far Field and Air Blast Focusing .................. 226 Atmospheric Inversion ................................. 227 Wind Effect.. ................................................ 228 Procedures to Avoid Air Blast Focusing ....... 230

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Preface

The purpose of this book is to familiarize mining and civil engineers, contractors, and blasters with the basic fundamentals of blast design. Blasting has advanced from an art to a science, whereby, many of the blasting variables can be calculated using simple design formulas. This text is not meant to be a handbook or encyclopedia on blasting, rather it is meant to show a method of design which is rational and follows scientific principles. The step-by-step design methods described in this book will carry the reader from basic knowledge on explosives through considerations for proper blast design. The book concentrates on the fundamentals of blast design rather than details which can be learned from other texts or from field experience. Little time is spent discussing basic tie-ins of initiation systems and information of this type since it is readily available in other sources. This book will serve the beginner and the professional alike since it sorts though the vast amount of information available and puts forth a logical design procedure. The book backs up the design with some of the basic principles and theories necessary to have an understanding why things work as they do. The blasting industry is rapidly changing with new theories, product and techniques. It is the goal of the author to provide the reader with a better understanding of technology as it is today. It also point out method of overcoming common blasting problems . The techniques, formulas, and opinions expressed in this book are based on the experience of the author. They should aid the reader in assessing blast designs to determine whether they are reasonable and whether they should work under average blasting conditions .

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One area related to blasting which remains an art is the proper assessment of the geologic conditions at hand. Improper assessment may product poor results in the blast. Complex geology and other factors may require changes in the design from those shown in the book, however, the methods presented would be the first step to calculate blast design dimensions which then may have to be modified to accommodate unusual local geologic conditions . Calvin J. Kanya

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EXPLOSIVES ENGINEERING

1.1 INTRODUCTION Most raw materials, from which our modern society is built, are produced by the use of explosives in mines throughout the world. The construction of highways, canals and buildings are aided by the use of explosives. The plentiful food, which is available in this country, would not exist without explosives to produce the fertilizers and the metallic ores, which ultimately become tractors and other equipment The use of explosives in mining and construction applications dates back to 1627. From 1627 through 1865, the explosive used was black powder. Black powder was a different type of explosive than the explosives used today. In 1865, Nobel invented nitroglycerin dynamite in Sweden. He invented gelatin dynamites in 1866. These new products were more energetic than black powder and performed differently since confinement of the explosive was not necessary to produce good results, as was the case with black powder. From 1867 through the mid-1950's, dynamite was the workhorse of the explosive industry . In the mid-1950's, a new product appeared which was called ANFO, ammonium nitrate and fuel oil. This explosive was more economical to use than dynamite. During the decades of the 1970's and the 1980's, ANFO has become the workhorse of the industry and approximately 80% of all explosives used in the United States was ammonium nitrate and fuel oil. Other new explosive products appeared on the scene in the 1960's and 1970's. Explosives, which were called slurries or water gels, have replaced dynamite in many applications. In the late 1970's, a modification of the water gels called emulsions appeared on the scene . The emulsions were simple to manufacture and could be used in similar applications as dynamites and water gels. Commerciul explosives fall into three major generic categories, dynamites, blasting agents and slurries (commonly called water gels or emulsions) .

Blasting problems generally result from poor blast design, poor execution in drilling and loading the proposed design and because the rock mass was improperly evaluated. Blast design parameters such as burden, stemming, subdrilling, spacing and initiation timing must be carefully determined in order to have a blast function efficiently, safely and within reasonable vibration and air blast levels. Controlled blasting along highways must be done to reduce maintenance costs and produce stable safe contours. Those responsible for the execution and evaluation of controlled blasting must be aware of the procedures used to produce acceptable results and must understand how geologic factors can change the appearance of the final contour. Rock strengths change over both small and large scale. Geologic structures such as joints, bedding planes, faults and mud seams cause problems. These variations in structure require the blaster to change his patterns and methods to obtain reasonable results. Therefore, one must assume, from surface indicators, what the rock mass will be at depth. The drilling of blastholes provides information as to what type of structure intersects those holes. To enable the blaster to make enlightened judgments, when adjusting his blasting pattern to compensate for rock structure, he must have a thorough understanding of exactly how the explosive functions during blasting. Without that understanding, blasting is just a random trial-and-error process. This book was designed to provide a systematic approach to blast design. The information is presented in a practical manner. The book provides the reader with information to promote an understanding of the phenomenon and the anticipated results. The formulas presented are empirical and should provide reasonable values for general job conditions. However, unusual geologic conditions can require adjustments to calculated values.

1.1.1 SOURCES OF EXPLOSIVE'S ENERGY Two basic forms of energy are released when high explosives react. The first type of energy will be called shock energy. The second type will be called gas energy. Although both types of energy are released during the detonation process, the blaster can select explosives with different proportions of shock or gas energy to suit a particular application. If explosives are used in an unconfined manner, such as mud capping boulders (commonly called plaster shooting) or for shearing structural members in demolition, the selection of an explosive with a high shock energy would be advantageous. On the other hand, if explosives are being used in boreholes and are confined with stemming materials, an explosive with a high gas energy output would be beneficial.

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To help form a mental picture of the difference between the two types of energy, compare the difference in reaction of a low and high explosives. Low explosives are those which deflagrate or burn very rapidly. These explosives may have reaction velocities of 600 to 1500 meter per second and produce no shock energy. They produce work only from gas expansion. A very typical example of a low explosive would be black powder. High explosives detonate and produce not only gas pressure, but also another energy or pressure which is called shock pressure. Figure 1.1 shows a diagram of a reacting cartridge of low explosive. If the reaction is stopped when the cartridge has been partially consumed and the pressure profile is examined, one can see a steady rise in pressure at the reaction until the maximum pressure is reached. Low explosives only produce gas pressure during the combustion process. A high explosive detonates and exhibits a totally different pressure profile (Figure 1.1 ). Reaction front]

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High exploslve

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Figure 1-1 Pressure Profiles for Low and High Explosives

During a detonation in high explosives, the shock pressure at the reaction front travels through the explosive before the gas energy is released. This shock energy, normally is of higher pressure than the gas pressure. After the shock energy passes, gas energy is released. The gas energy in detonating explosives is much greater than the gas energy released in low explosives. In a high explosive, there are two distinct and separate pressures. The shock pressure is a transient pressure that travels at the explosives rate of detonation. This pressure is estimated to account for only 10% to 15% of the total available useful work energy in the explosion. The gas pressure accounts for 85% to 90% of the useful work energy and follows thereafter. However, the gas energy produces a force that is constantly maintained until the confining vessel, the borehole, ruptures.

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1.1.2 SHOCK ENERGY In high explosives, a shock pressure spike at the reaction front travels through the explosive before the gas energy is released. There are, therefore, two distinct separate pressures resulting from a high explosive and only one from a low explosive. The shock pressure is a transient pressure that travels at the explosives rate of detonation. The gas pressure follows thereafter. The shock energy is commonly believed to result from the detonation pressure of the explosion. The detonation pressure is a function of the explosive density times the explosion detonation velocity squared and is a form of kinetic energy. Determination of the detonation pressure is very complex. There are a number of different computer codes written to approximate this pressure. Unfortunately, the computer codes come up with widely varying answers. Until recently, no method existed to measure the detonation pressure. Now that methods exist to produce accurate measurements, one would hope that the computer codes would be corrected. Until that time occurs, one could use one of a number of approximations to achieve a number that may approximate the detonation pressure. As an example, one could use:

4.5x 10-6 Ve 2 d

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Detonation pressure Density of the explosive Detonation velocity

(Kbar) (g/cm 3 ) (m/s)

The detonation pressure or shock energy can be considered similar to kinetic energy and is maximum in the direction of travel, which would mean that the detonation pressure would be maximum in the explosive cartridge at the end opposite that where initiation occurred. It is generally believed that the detonation pressure on the sides of the cartridge are virtually zero, since the detonation wave does not extend to the edges of the cartridge. To get maximum detonation pressure effects from an explosive, it is necessary to place the explosives on the material to be broken and initiate it from the end opposite that in contact with the material. Laying the cartridge over on its side and firing in a manner where detonation is parallel to the surface of the material to be broken reduces the effects of the detonation pressure. Instead, the material is subjected to the pressure caused by the radial expansion of the gases after the detonation wave has passed. Detonation pressure can be effectively used in blasting when shooting with external charges or

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charges which are not in boreholes. This application can be seen in mud capping or plaster shooting of boulders or in the placement of external charges on structural members during demolition (Figure 1.2).

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Figure 1-2 Mud Cap Blasting

To maximize the use of detonation pressure one would want the maximum contact area between the explosive and the structure. The explosive should be initiated on the end opposite that in contact with the structure. An explosive should be selected which has a high detonation velocity and a high density. A combination of high density and high detonation velocity results in a high detonation pressure.

1.1.3 GAS ENERGY The gas energy released during the detonation process causes the majority of rock breakage in rock blasting with charges confined in boreholes. The gas pressure, often called explosion pressure, is the pressure that is exerted on the borehole walls by the expanding gases after the chemical reaction has been completed. Explosion pressure results from the amount of gases liberated per unit weight of explosive and the amount of heat liberated during the reaction. The higher the temperature produced, the higher the gas pressure. If more gas volume is liberated at the same temperature, the pressure will also increase. For a quick approximation, it is often assumed that explosion pressure is approximately one-half of the detonation pressure (Figure 1.3). It should be pointed out that this is only an approximation and conditions can exist where the explosion pressure exceeds the detonation pressure. This explains the success of ANFO which yields a relatively low detonation pressure, but relatively high explosion pressure. Explosion pressures are calculated from computer codes or measured using underwater tests. Explosion pressures can also be measured directly in boreholes, however, few of the explosive manufacturers use the new technique in rating their explosives. A review of some very basic explosives chemistry helps one to understand how powdered metals and other substances effect explosion pressures.

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Detonation Velocity m/s Detonation Explosion 7500 Pressure Pressure Kbar 6000 300 150 100 200 150 Density 100 50 4500 gI 50 20 40 30 0.8 20 3000 0.6 15 10 5

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Figure 1-3 Nomograph of Detonation & Explosion Pressure

1.1.4 CHEMICAL EXPLOSIVES Chemical explosives are materials which undergo rapid chemical reactions to release gaseous products and energy. These gases under high pressure exert forces against borehole walls which causes rock to fracture. The elements, which comprise explosives, are generally considered either fuel elements or oxidizer elements (Table 1. 1). Explosives use oxygen as the oxidizer element. Nitrogen is also a common element in explosives and is in either a liquid or solid state, but once it reacts it forms gaseous nitrogen. Explosives sometimes contain ingredients other than fuels and oxidizers. Powdered metals such as powdered aluminum are used in explosives. The reason for the use of the powder metals is that, upon reaction, powdered metals give off heat. The heat increases the temperature of the gases, which result from the other ingredients, causing a higher explosion pressure. Explosives may contain other elements and ingredients which really add nothing to the explosives energy. These other ingredients are put into explosives to decrease sensitivity or increase surface area. Certain ingredients such as chalk or zinc oxide serve as an antacid to increase the storage life of the explosive. Common table salt actually makes an explosive less efficient because it functions as a flame depressant and cools the reaction. On the other hand, the addition of

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table salt allows the explosive to be used in explosive methane atmospheres because the cooler flame and shorter flame duration makes it less likely that a gas explosion would occur. This is the reason that permissible explosives are used in coal mines or in tunneling operations in sedimentary rock where methane is encountered. Table 1-1 Explosive Ingredients

INGREDIENT

FUNCTION

CHEMICAL FORMULA

Nitroglycerin

C::1Hc;OQN::1

Explosive Base

Nitrocellulose

CRH7011N::1

Explosive Base

Trinitrotoluene (TNT) Ammonium Nitrate Sodium Nitrate Fuel Oil

C7Hc;ORN::1

Explosive Base

H 4 0::1N?

Oxygen Carrier

NaN0::1

Oxygen Carrier

CH?

Fuel

CRHrnOc;

Fuel

Carbon

c

Fuel

Powdered Aluminum

Al

Sensitizer-Fuel

Wood Pulp

CaC0::1

Antacid

Zinc Oxide

ZnO

Antacid

Sodium Chloride

NaCl

Flame Depressant

Chalk

The basic elements or ingredients which directly produce work in blasting are those elements which form gases when they react, such as carbon, hydrogen, oxygen, and nitrogen. When carbon reacts with oxygen, it can either form carbon monoxide or carbon dioxide. In order to extract the maximum heat from the reaction, we want all elements to be completely oxidized or in other words for carbon dioxide to form rather than carbon monoxide. Table 1.2 shows the difference in heat released when one carbon atom forms carbon monoxide versus the case where one carbon atom forms carbon dioxide. In orde~ to release the maximum energy from the explosive reaction, the elements should react and form the following products: 1. 2. 3.

Carbon reacts to form carbon dioxide. (Figure 1.4) Hydrogen reacts to form water. (Figure 1.5) Liquid or solid nitrogen reacts to form gaseous nitrogen. (Figure 1.6)

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Table 1-2 Heats Of Formation For Selected Chemical Compounds COMPOUND

FORMULA

Al?O~

Corundum

MOL

On or Qr

WEIGHT

(Kcal/Mole)

102.0

-399.1

CH?

14.0

- 7.0

Nitromethane

CH~O?N

61.0

- 21.3

Nitroglycerin

C~H!iO!)N~

227.1

- 82.7

C!iHR01?N4

316.1

-123.0

C7H!iORN~

227.1

- 13.0

Fuel Oil

PETN TNT Carbon monoxide

co

28.0

- 26.4

Carbon dioxide

co?

44.0

- 94.1

H?O

18.0

- 57.8

N?H 4 0~

80.1

- 87.3

Aluminum

Al

27.0

0.0

Carbon

c

12.0

0.0

Nitrogen

N

14.0

0.0

Nitrogen oxide

NO

30.0

+ 21.6

Nitrogen dioxide

NO?

46.0

+ 8.1

Water Ammonium nitrate

INGREDIENTS

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Figure 1-4 Carbon-Oxygen Ideal Reaction

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Figure 1-5 Hydrogen-Oxygen Ideal Reaction INGREDIENTS

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NITROGEN GAS Ni

PROOUCT

Figure 1-6 Nitrogen-Nitrogen Ideal Reaction

If only the ideal reactions occur from the carbon, hydrogen, oxygen, and nitrogen, there is no oxygen left over or any additional oxygen needed. The explosive is oxygen balanced and produces the maximum amount of energy . If two ingredients are mixed together, such as ammonium nitrate and fuel oil, and an excess amount of fuel oil is put into the mixture, the explosive reaction is said to be oxygen negative. This means that there is not enough oxygen to fully combine with the carbon and hydrogen to form the desired end products. Instead, what occurs is that free carbon (soot) and carbon monoxide will be liberated (Figure 1. 7) .

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INGREDIENlS

INGREDIENlS

CA.'mON MONOXIO

CARBON

c

co PROOUCT

PRODUCT

Figure 1-7 Non-Ideal Carbon-Oxygen Reaction

If too little fuel is added to a mixture of ammonium nitrate and fuel oil, then the mixture has excess oxygen which cannot react with carbon or hydrogen. This is called an oxygen positive reaction. What occurs is that the nitrogen which is normally an inert gas will be changed from nitrogen gas to an oxide of nitrogen (Figure 1.8). If oxides of nitrogen are formed, they will form rust colored fumes and reduce the energy of the reaction. INGREDIENTS

INGREDIENTS

NllROGEN OXIDE NO

PROOUCT

PRODUCT

Figure 1-8 Non-Ideal Nitrogen-Oxygen Reaction

The energy is reduced because other ideal gases liberate heat when they form, nitrogen oxides absorb heat in order for them to form. This can be seen in Table 1.2. Water and carbon dioxide have a

10

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•

I~ h,.

e.

ov. e,

•• • •

..

~

ltJ

I

negative sign which means they give off heat when they form. The nitrogen oxides near the bottom of Table 1.2 have a plus sign meaning that they take in heat when they form. The net result is that the reaction will occur at a lower temperature. The gas pressure is lowered if the reaction temperature is lowered. Figure 1.9 shows the reaction products which form if the reaction is oxygen positive. ELEMEll.1l2 CARBON H'lllROGEN OXYGEN NllROGEN I OXYGEN BA!ANCEQ PRODUCT GASES

~

I

''-------~

OXYGEN NEGATIVE PRODUCT GASES

OXYGEN POSITIVE PRODUCT GASES

l CARBON (BLACK SOLID) CARBON MONOXIDE (COLORLESS) WATER VAPOR (LIGHT GREY) N!lROGEN GAS (COLORLESS)

CARBON DIOXIDE (COLORLESS) WATER VAPOR (LIGHT GREY) NllROGEN GAS (COLORLESS)

J CARBON DIOXIDE (COLORLESS) WATER VAPOR (LIGHT GREY) NllROGEN OXIDES (RUST-YELLOW GAS)

(co2 ) (fl:iO) (NO)

(NOz)

I

RESULTANT COLOR

LIGHT GREY FUMES

DARK GREY FUMES -'NO I OR CARBON ON BOREHOLE WALL

RUST OR YEU.OW FUMES

Figure 1-9 Identification of Problem Mixtures

1.2 IDENTIFICATION OF PROBLEM MIXTURES There are visual signs of proper and improper energy release. Gas colors are indicators of reaction efficiency and associated energy release. When light gray colored steam is present, oxygen balance is near ideal and maximum energy is released. When gases are either yellow or rust colored, they indicate an inefficient reaction that may be due to an oxygen positive mixture. Oxygen negative mixtures produce dark gray gases and can leave carbon on borehole walls (Figure 1.9). In order to demonstrate the importance of oxygen balance to energy release, one can explore the example of ammonium nitrate and fuel oil which is a very common explosive. If either too little or too much fuel oil is added to ammonium nitrate, non-ideal chemical reactions occur which cause an energy loss .

••

• 0

11

••

•.• •• •• •.;••

Figure 1.10 shows energy loss versus the percent of fuel oil in the mixture. It can be seen that the ideal amount of fuel oil is near 6%. When insufficient oil is added and too much oxygen remains in the mixture, oxides of nitrogen are produced and large energy losses occur. At 1% fuel oil the energy loss is approximately 42%. If too much fuel is added, the energy losses are not as severe as in the case where too little fuel is added. When fuel oil is greater than 6%, free carbon and carbon monoxide will form. These visual signs can give the operator an indication as to whether or not the explosives are functioning properly. so:ii:

.l.I .I.(•: .l.t.(•: '

E -40ll: N E R

G JOll:

y

L 0

s s

20%

(

10ll: Carbon Carbon Monoic1de

0

2ll:

4ll:

6ll:

ex

10ll:

Oil CONTENT

Figure 1-10 Energy Loss in ANFO

.!.1·~

•I

.:•·

•I ...

•

12

•• •• •

!

i

•

~ r~

2.

1:

•

~o

·.

•.

MECHANICS OF ROCK BREAKAGE



ns

ift:e

•• •

• lO

Figure 2-6 Cantilever Bending Diagram

Two general modes of flexural failure of the burden exist. In one case, the burden bends outward or bulges in the center more quickly than it does on the top or bottom. In the second case, the top or the bottom of the burden moves at a higher rate than the center. When the burden

17

rock bulges at its center, tensile stresses result at the face and compression results near the charge. Under this type of bending condition, the rock will break from the face back toward the hole (Figure 2.5). This mode of failure generally leads to desirable breakage. In the second case, the rock is cantilevered outward (Figure 2.6) and the face is put into compression and the borehole walls are in tension. This second case is undesirable. This mechanism occurs when cracks between blastholes link before the burden is broken and is normally caused by insufficient blasthole spacings. When the cracks between holes reach the surface, gases can be prematurely vented before they have accomplished all potential work. Air blast and flyrock can result along with potential bottom problems. The bending mechanism or flexural failure is controlled by selecting the proper blasthole spacing and initiation time of adjacent holes. When blasthole timing results in charges being delayed from one another along a row of holes, the spacing must be less than that required if all the holes in a row were fired simultaneously. The selection of the proper spacing is further complicated by the stiffness ratio. As bench heights are reduced compared to the burden, one must also reduce the spacing between holes to overcome the problems of stiffness.

2.4 BREAKAGE PROCESS The rock breakage process occurs in four distinctive steps. As the explosives detonates, a stress wave moves through the rock uniformly in all directions around the charge. Radial cracks then propagate predominantly toward the free face. After the radial cracking process is finished, high pressure gases penetrate into the cracks approximately two-thirds of the distance from the hole to the face throughout the radial crack system. Only after the gas has time to penetrate into the crack system are the stresses on the face of sufficient magnitude, to cause the face to move outward. Before the face begins to move and bend outward, fractures are created in the third dimension as a result of the flexural failure or bending.

18

-

•

3. EXPLOSIVE PRODUCTS

3.1 ENVIRONMENTAL CHARACTERISTICS OF EXPLOSIVES The selection of the type of explosive to be used for a particular task is based on two primary criteria. The explosive must be able to function safely and reliably under the environmental conditions of the proposed use, and the explosive must be the most economical to use to produce the desired end result. Before any blaster selects an explosive to be used for a particular task, he must determine which explosives would best suit the particular environment and the performance Five characteristics which will suit the conditions of the job. characteristics are considered in the selection of explosives which concern environmental factors, sensitiveness, water resistance, fumes, flammability and temperature resistance.

3.1.1 SENSITIVENESS Sensitiveness is the characteristic of an explosive which defines its ability to propagate through the entire length of the column charge and controls the minimum diameter for practical use. Sensitiveness is measured by determining the explosive's critical diameter. The term critical diameter is commonly used in the industry to define the minimum diameter in which a particular explosive compound will detonate reliably. All explosive compounds have a critical diameter. For some explosive compounds, the critical diameter may be as little as a millimeter. On the other hand, another compound may have a critical diameter of 100 millimeters. The diameter of the proposed borehole on a particular job will determine the maximum diameter of explosive column. This explosive diameter must be greater than the critical diameter of the explosive to be used in that borehole. Therefore, by pre-selecting certain borehole sizes, one may eliminate certain explosive products from use on that particular job (Table 3.1 ).

19

Sensitiveness is also a measure of the explosive's ability to propagate from cartridge-to-cartridge, assuming the diameter is above critical. It can be expressed as the maximum separation distance (in centimeters) between a primed donor cartridge and an unprimed receptor cartridge, where detonation transfer will occur. Table 3-1 Sensitiveness (Critical Diameter)

CRITICAL DIAMETER

TYPE

SO mm

x x

x x

x x x

Poured ANFO PackaQed ANFO HeawANFO

x x

3.1.2 WATER RESISTANCE Water resistance is the ability of an explosive to withstand exposure to water without it suffering detrimental effects in performance_ Explosive products have two types of water resistance, internal and external. 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. 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_ As an example, ANFO has no internal water resistance yet, if it is placed in a sleeve or in a cartridge within a borehole, it can be kept dry and will perform satisfactorily_ The sleeve or cartridge provides the external water resistance for this particular product The effect which water has on explosives is that it can dissolve or leach some of the ingredients, or cool the reaction to such a degree that the ideal products of detonation will not form even though the

20

.!

•:•'

•'

•.

,(

•~ ·~:

•

,

~I

.t,•p: ~

~<

~~

• .h •• •••• •

-~~

product is oxygen-balanced. The emission of reddish-brown or yellow fumes from a blast often indicates ine(ficient detonation reactions frequently caused by water deterioration of the explosive. This condition can be remedied if a more water resistant explosive or better external packaging is used. Manufacturers can describe the water resistance of a product in two different ways. One way would be using terms such as excellent, good, fair, or poor (Table 3.2). When water is encountered in blasting operations, the explosive with at least a fair water resistance rating should be selected and this explosive should be detonated as soon as possible after loading. If the explosive is to be in water for an appreciable amount of time, it is advisable to select an explosive with at least a good water resistance rating. If water conditions are severe and the exposure time is significant, the prudent blaster may select an explosive with an excellent water resistance rating. Explosives with a poor water resistance rating should not be used in wet blastholes. Table 3-2 Water Resistance

TYPE

RESISTANCE

Granular Dynamite

Poor to qood

Gelatin Dynamite

Good to excellent

Cartridged Slurry

Very good

Bulk Slurry

Very good

Air Emplaced ANFO

Poor

Poured ANFO

Poor

Packaged ANFO

Very good*

Heavy ANFO Poor to very good * Becomes poor if package is broken Water resistance ratings have also been given numbers, such as a Class 1 water resistance would indicate 72 hours of exposure to water with no detrimental effects; Class 2 - 48 hours, Class 3 - 24 hours, The descriptive method of rating water and Class 4 - 12 hours. resistance is the one commonly seen on explosive data sheets. In general, producf price is related to water resistance. The more water resistant the product, the higher the cost The ability to remain unaffected by high static pressures is defined as water pressure tolerance. Some explosive compounds are densified an~ desensitized by hydrostatic pressures which result in deep

21

boreholes. Combinations of factors such as cold weather and small primers will contribute to failure. Under these conditions, energy release may be minimal. Problems with water pressure tolerance most often occur with slurry and heavy ANFO.

3.1.3 FUMES 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. 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. It should be pointed out that in surface operations, especially in deep cuts or trenches, fume production and retention can be hazardous to the personnel on the job. Certain blasting conditions may produce toxic fumes even when the explosive is oxygen balanced. Some conditions which can cause toxic fume production are insufficient charge diameter, inadequate water resistance, inadequate priming and premature loss of confinement. The Institute of Makers of Explosives have adopted a method of rating fumes. The test is conducted by the Bichel Gauge method. The volume of poisonous gases released per 200 grams of explosives is measured. If less than 4530 cm 3 of toxic fumes are produced per 200 grams of explosives, the fume class rating would be 1. If 4530 cm 3 to 9344 cm 3 of poisonous gases are produced, the fume class rating is 2, and if 9344 cm 3 to 18972 cm 3 of poisonous gases are produced, the fume class rating is 3. Typical products are qualitatively rated in Table 3.3. Strictly speaking, carbon dioxide is not a fume since it is not a toxic gas in its own right, i1owever, many deaths have occurred over the years due to the generation of large amounts of carbon dioxide during blasting in confined areas. Although carbon dioxide is not poisonous, it is produced in large quantities in most blasts and it has the effect of causing the involuntary muscles of the body to stop working. In other words, the heart and lungs would stop working in high concentrations of carbon dioxide. If concentrations are 18% or higher in volume, death can occur by suffocation. An additional problem with carbon dioxide is that it has a density of 1.53 as compared to air and it would tend to pocket in low places in the excavation or where there is little air movement. A simple solution to the problem is to use compressed air to dilute any possible high concentrations in depressions of trenches.

22

••

Table 3-3 Fume Quality

QUALITY

TYPE Granular Dynamite

Poor to aood

Gelatin Dvnamite

Fair to verv aood

Cartridaed Slurry

Good to verv aood

Bulk Slurry

Fair to verv aood

Air Emolaced ANFO

Good*

Poured ANFO

Good*

Packaaed ANFO

Good to verv aood

Heavy ANFO Good* *Can be poor under adverse conditions

3.1.4 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. Flammability is important in storage, transportation and use of explosives. Some explosives, although very economical, have lost their marketability due to flammability. A good example is LOX, liquid oxygen and carbon, which was used in the 1950's as a blasting agent. Its flammability and inherent safety problems caused its demise. Most explosive compounds used today are not anywhere near as flammable as LOX, however, accidents still occur due to flammability. Over the past two decades, explosive products, in general, have Some manufacturers indicate that certain become less flammable. products can be burned without detonation in quantities as large as 20,000 kilograms. The problem results because many blasters are given a false sense of security. Some believe that all products today are relatively inflammable. This false sense of security has led to the death of people who have been careless with explosives and have assumed that flammability is not a problem. All explosive compounds should be treated as highly flammable. There should not be smoking during the loading process, and if the explosives are to be destroyed by burning, the guidelines produced by the IME should be followed regardless of the type of explosive involved.

23

3.1.5 TEMPERATURE RESISTANCE Explosive compounds can suffer in performance if stored under Under hot storage extremely hot or cold conditions (Table 3.4). conditions, above 32.2 degrees Celsius, many compounds will slowly decompose or change properties and shelf life will be decreased. Storage of ammonium nitrate blasting agents in temperatures above 32.2 degrees Celsius can result in cycling, which will effect the performance and safety of the product. Table 3-4 Temperature Resistance

TYPE Granular Dvnamite Gelatin Dynamite CartridQed Slurry Bulk Slurry Air Emplaced ANFO Poured ANFO PackaQed ANFO Heavv ANFO

BETWEEN -18°C - 38°C Good Good Poor below 4.5°C Poor below 4.5°C Poor above 32.2°C Poor above 32.2°C Poor above 32.2°C Poor below 4.5°C

3.1.5.1 THE CYCLING OF AMMONIUM NITRATE The chemical formula for ammonium nitrate is NH4N03 or more simply written N2H403. For its weight, it supplies more gas volume upon detonation than any other explosive. In pure form, ammonium nitrate (AN) is almost inert and is composed of 60% oxygen by weight, 33% nitrogen, and 7% hydrogen. With the addition of fuel oil, the ideal oxygen balanced reactions for NH4N03 is:

3N2H403 + CH2

=>

3N2 + 7H20 + co2

Two characteristics make this compound both unpredictable and dangerous. Ammonium nitrate is water soluble and if uncoated can attract water from the atmosphere and slowly dissolve itself. For this reason, the spherical particles, prills, have a protective coating of tacl, zelite, etc., which offers some amount of water resistance. The second and most important characteristic is a phenomena called cycling. Cycling is the ability of a material to change its crystal form with temperature. Ammonium nitrate will have one of the five crystal forms depending on temperature.

24

1. 2. 3. 4. 5.

Above 125°C cubic crystals exist. Above 84.4°C & below 125°C tetragonal crystals exist. Above 32.2°C & below 84.4°C orthorhombic crystals exist. Above -18°C & below 32.2°C pseudotetragonal crystals exist. Below-18°C tetragonal crystals exist.

The cycling phenomena can seriously effect both the storage and performance of any explosive which contains ammonium nitrate. Most dynamites, both regular NG or permissibles, contain some percentages of AN while blasting agents are composed almost totally of this compound. The two temperatures at which cycling will occur under normal conditions are -18°C and 32.2"C. This is to say that products which are stored over the winter or for a period of time during the summer most likely will undergo some amount of cycling. During the summer in a poorly ventilated powder magazine or storage bin located in the sun, the cycling temperature may be reached daily. The effect of cycling on AN when isolated from the humidity in the air is that the prills break down into finer and finer particles. The prills are made up of pseudotetragonal crystals. When the temperature exceeds 32.2°C, each crystal breaks into smaller crystals of orthorhombic structure. When the temperature again falls below 32.2°C, the small crystals break into even finer crystals of the pseudotetragonal form. This process can continue until the density is no longer near 0.8 g/cm', but can reach a density near 1.2 g/cm3 . The density increase can make the product more sensitive and contain more energy per unit 3 volume at a density above 1.2g/cm the ANFO will not detonate. To further complicate the situation, some cartridged blasting agents or those stored in bins may not efficiently exclude humidity. After the AN has undergone cycling, the water-resistant coating is broken and the water vapor in the air condenses on the particles. As cycling continues water collects on the particles and the mass starts to dissolve (Figure 3.1 ). Recrystalizing into large crystals can occur with a reduction of temperature. Therefore, it is evident that a !age mass of AN after cycling may have very dense areas and areas of large crystals. The performance of this product may range from that of a very powerful explosive to one that deflagrates or one that will not shoot at all.

3.1.5.2 COLD RESISTANCE

•• •

• 0

Extreme cold conditions can also effect the performance of products. Most dynamites and blasting agents will not freeze under ordinary exposure under the lowest temperature encountered in the

25

country. This is because the manufacturers have added ingredients to these products which allow them to perform properly, in spite of the cold weather. Some products may tend to stiffen and become firm after prolonged exposure to low temperatures and may become more difficult to use in the field.

Figure 3-1 Cycled Prill

Slurry explosives, which include water gel and emulsions. can have serious detonation problems if stored in cold temperatures and not allowed to warm up before they are detonated. Slurries are quite different from the other products previously mentioned, such as dynamite and blasting agents. The problem comes about because in the past the blaster has been accustomed to using blasting agents from any manufacturer without having any problems due to cold weather. The blaster also has become accustomed to using dynamites from any manufacturer with good results. Today the slurry explosives do not all perform identically. Some can be used immediately if stored at temperatures of -18°C where others will not detonate if stored at temperatures below 4.5°C. The sensitivity of the product can become affected. The priming procedure, which was employed when the produce was stored at 20°C, may cause a misfire if the product is stored at 6°C. It is a good practice to consult the manufacturer's data sheet whenever any new product is introduced on the job, but it is absolutely essential to consult that data sheet if any new slurry explosives are introduced, since their properties and performance with temperatures can vary greatly {Figure 3.2).

26

-2Y

s

-20·

'C

T A

R -15"

T

IN 13 "C WATER

I

N -10·

G T

-s·

E M p

o· 1 HR

2 HR

TIME TO MINIMUM RECOMMENDED USE

Figure 3-2 Slurry Wann Up Chart

3.2 PERFORMANCE CHARACTERISTICS 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.

3.2.1 SENSITIVITY The sensitivity of an explosive product is defined by the amount of input energy necessary to cause the product to detonate reliably. This is sometimes called the minimum booster rating or minimum priming requirements. Some explosives require little energy to detonate reliably. The standard number 8 blasting cap will initiate dynamite and some of the cap sensitive slurry explosives. On the other hand, a blasting cap For reliable alone will not initiate bulk loaded ANFO and slurry. detonation, one would have to use a booster or primer in conjunction with the blasting cap. Many factors can influence the sensitivity of a product. As an example, the sensitivity can be reduced by the effect of water in the blasthole, inadequate charge diameter and temperature extremes. Sensitivity of a product defines its priming requirements, the primer size and energy output. If reliable detonation of the main charge does not occur, fumes can increase, ground vibration levels can rise, blastholes can geyser and flyrock can be thrown. Hazard sensitivity defines an explosive's response to the accidental addition of energy, such as bullet impact (Table 3.5).

27

0

•

Table 3-5 Sensitivity

TYPE

HAZARD SENSITIVITY

PERFORMANCE SENSITIVITY

Granular Dynamite

Moderate to high

Excellent

Gelatin Dynamite

Moderate

Excellent

Cartridged Slurry

Low

Good to very good

Bulk Slurry

Low

Good to very good

Air Emplaced ANFO

Low

Poor to good *

Poured ANFO

Low

Poor to good *

Packaged ANFO

Low

Good to very good

Heavy ANFO

Low

Poor to good*

* Heavily dependent on field condition

·-(.··~

.:.a ~

3.2.2 VELOCITY The detonation velocity is the speed at which the reaction moves through the column of explosive It ranges from 1,524 to 7,620 m/s for commercially used products. Detonation velocity is an important consideration for applications outside a borehole, such as plaster shooting, mud capping or shearing structural members. Detonation velocity has significantly less importance if the explosives are used in the borehole. Detonation velocity can be used as a tool to determine the efficiency of the explosive reaction in field use. If a question arises as to performance of an explosive compound during actual field use, velocity probes can be inserted in the product When the product is detonated, the reaction rate of the product can be measured and its performance judged by the recorded velocity. If the product is shooting at a velocity significantly lower than its rated velocity, it is an indication that its performance is not up to standard expectations. Typical explosive detonation velocities are given in Table 3.6.

28

-~ ·~ "•t

•1 •:i

= .i

; ••

-

•• •

Table 3-6 Detonation Velocity (mis)

DIAMETER

TYPE

76mm

32mm Granular Dynamite

2100 - 5800

Gelatin Dynamite

3600 - 7600

Cartridge Slurry

4000 - 4600

4300 - 4900 4300 - 4900

Bulk Slurry

229mm

3700- 5800

Air Emplaced ANFO

2100 - 3000

3700 -4300

4300 - 4600

Poured ANFO

1800-2100

3000 - 3400

4300 - 4600

3000 - 3700

Packaged ANFO Heavy ANFO

4300 - 4600 3400 - 5800

3.2.3 DETONATION PRESSURE The detonation pressure is the near instantaneous pressure derived from the shock wave moving through the explosive compound (Table 3.7). When initiating one explosive with another, the shock pressure from the primary explosive is used to cause initiation in the secondary explosive. Detonation pressure can be related to borehole pressure but it is not necessarily a linear relationship. Two explosives with similar detonation pressures will not necessarily have equal borehole Detonation pressure is calculated pressure or gas pressure. mathematically. Table 3-7 Detonation Pressure

KBARS

TYPE

Granular Dvnamite

20 - 70

Gelatin Dvnamite

70 - 140

Cartridaed Slurrv

20 - 100

Bulk Slurrv

20 - 100

Poured ANFO

t

••

7 - 45

Packaaed ANFO

20- 60

Heavv ANFO

20- 90

! ..

••

j.

1' ~

29

The detonation pressure is related to the density of the explosive and the reaction velocity. When selecting explosives for primers, detonation pressure is an important consideration. Methods to estimate detonation pressure and their relationship to priming will be discussed in Chapter 5, Primer and Booster Selection.

3.2.4 DENSITY The density of an explosive is important because explosives are purchased, stored and used on a weight basis. Density is commonly expressed in terms of specific gravity, which is the ratio of explosive density to water density. The density of an explosive determines the weight of explosive that can be loaded into a specific borehole diameter. On a weight basis, there is not a great deal of difference in energy between various explosives. The difference in energy on a unit weight basis is nowhere near as great as the difference in energy on a volume basis. When hard rock is encountered and drilling is expensive, a denser product of higher cost is often justified. The density for some explosive products are given in Table 3.8. Table 3-8 Density

TYPE

DENSITY (g/cm 3 )

Granular Dynamite

0.8 - 1.4

Gelatin Dynamite

1.0 -1.7

Cartridged Slurry

1.1-1.3

Bulk Slurry

1.1-1.6

Air Emplaced ANFO

0.8 - 1.0

Poured ANFO

0.8 - 0.9

Packaged ANFO

1.1 - 1.2

Heavy ANFO

1.1-1.4

The density of the explosive is commonly used as a tool to approximate strength and design parameters between explosives of different manufacturers and different generic families. In general terms, the higher the explosive density, the more energetic the product. A useful expression of density is what is commonly called loading density or the weight of explosive per length of charge at specified diameter. Loading

•

4'e

• fja

•• •• i •• .h •• ••• •

• •:i err

·~ 0c· .p

.~a

·~

·i i-~tl ~I

.~

••

30

•• •1

•

density is used to determine the total kilograms of explosive which will be used per borehole and per blast.· The density of commercial products range from about 0.8 to 1.6 g/cm3 . An easy method to calculate loading density is: ?..

SG e x De x 1t

...

•

·~

'•• •

cfl

...

•••• .

de=

(3.1)

4000

where:

= = =

Loading density Density of the explosive Diameter of the explosive

(Kg Im) (g I cm 3 ) (mm)

Determine the loading density of an explosive which has a charge diameter of 76.2 mm and a density of 1.2. 2

1.2 X76.2 X1t

de==-----4000

5.47 Kg

m

.,;.

t~

•~· •..... 5,

•

•••. ~f

~~

••

~

1•

~

3.2.5 STRENGTH Strength refers to the energy content of an explosive which in turn is the measure of the force it can develop and its ability to do work . Strength has been rated by various manufacturers, both on a equal weight and an equal volume basis, and are commonly called weight strength and cartridge or bulk strength There is no standard strength measurement method universally used by the explosives manufacturers . Instead many different strength measurement methods exist such as the ballistic mortar test, seismic execution values, strain pulse measurement, cratering, calculation of detonation pressures, calculation of borehole pressures, and determination of heat release . However, none of these methods can be used satisfactorily for blast design purposes. Strength ratings are misleading and do not accurately compare rock fragmentation effectiveness with explosive type . In general, one can say that strength ratings are only a tool used to identify the end results and associate them with a specific product. One type of strength rating, the underwater shock and bubble energy test used to determine the shock energy and the expanding gas energy, is used by some for design purposes. As an example, the bubble energy test used does produce reliable results which can be used for approximating blast design dimensions .

31

3.2.6 COHESIVENESS Cohesiveness is defined as the ability of the explosive to maintain its original shape. There are times when explosive must maintain its original shape and others when it should flow freely. For example, when blasting in cracked or broken ground, one definitely wants to use an explosive which will not flow into the cracked area causing holes to be overloaded. Conversely, in other applications such as in bulk loading, explosives should flow freely and not bridge the borehole nor form gaps in the explosive column.

••

•

3.3 COMMERCIAL EXPLOSIVES The products used as the main borehole charge can be broken into three generic categories, dynamite, slurries, and blasting agents (Figure 3.3). A fourth, very minor, category will be added to the discussion which is the binary or two component explosives. Although the volume of binary explosives sold annually is insignificant when compared to the other major generic categories, its unique properties warrants its mention. All the generic categories discussed in this section are high explosives from the standpoint that they will all detonate. On the other hand, one commonly hears some of these high explosives called by other terms such as blasting agents. The term blasting agent does not detract from an explosive's ability to detonate or function as a high explosive. The term blasting agent is a classification considered from the standpoint of storage and transportation. Explosives which are blasting agents are less sensitive to initiation and therefore can be stored and transported under different regulations than what would normally be used for more sensitive high explosives. The term high explosive refers to any product used in blasting that is cap sensitive and that reacts at a speed faster than the speed of sound in the explosive media. The reaction must be accompanied by a shock wave for it to be considered a high explosive. Blasting agents, a subclass of high explosives, is a material or mixture which consists of a fuel and an oxidizer. The finished product, as mixed and package for shipping, cannot be detonated by a number 8 blasting cap in a specific test described by the Bureau of Mines. Normally, blasting agents do not contain ingredients which in themselves are high explosives. Some slurries containing TNT, smokeless powder or other high explosive ingredients can be classed as a blasting agent if they are insensitive to initiation by a number 8 blasting cap (Figure 3.3).

32

•

•:-

I

DYNAMllE

I

St.URRY

.---

GRANULAR DYNAMITE

SlRAIGHT DYNAMITE HIGH...;.OENSITY DYNAMITE LOW-DENSITY DYNAMITE

-

GELATIN DYNAMITE

SlRAIGHT GELATIN DYNAMITE AMMONIA Gil.ATIN O'INAMITE SEMIGELATIN DYNAMllE

-

SLURRY, WATER GEL, EMULSION, CARTRIDGED

-

BULK

I~

MONOME1HYLAMINENl1RATE ALUMINIZEO AIR SENS111ZEO

AIR SENS111ZED ALUM INI ZED EXPLOSIVE SENSITIZED

HEAVY ANFO

I

ANFO

~

-

ANFO ALUMINIZED ANFO

BULK

DRY

ANFO ALUMINIZED ANFO DENS1FlED ANFO

.__ BLASTING

AGENTS, CARTRIDGED

ICOM~~ENT~ ------------0•-il J-

T\W)

I

COMPONENT EXPLOSIVES

Figure 3-3 Types of Explosives

3.3.1 DYNAMITE Most dynamites are nitroglycerin based products. A few manufacturers of dynamite have products in which they substituted nonheadache producing high explosives such as nitrostarch for the nitroglycerin. Dynamites are the most sensitive of all the generic classes of explosives used today. Because of the sensitivity, they offer an extra margin of dependability in the blasthole since gaps in loading within the explosive column and many other environmental factors which cause

33

other explosives to malfunction would not affect dynamite. Of course, it is true that dynamite is somewhat more susceptible to accidental initiation because of the sensitivity. Operators must decide which of these properties is most important to them when making their explosive selections. Nitroglycerin was the first high explosive used in commercial blasting. It has a density of 1.6 and detonation velocity of approximately 7,600 mis. Nitroglycerin is extremely sensitive to shock, friction and heat, which makes its use in liquid form extremely hazardous. In Sweden in 1865, Nobel found that if this hazardous liquid was absorbed into an inert material, the resulting product would be safe to handle and would be much less sensitive to shock, friction and heat. This product was called dynamite. Within the dynamite family, there are two major subclassifications, granular dynamite and gelatin dynamite. Granular dynamite is a compound which uses nitroglycerin as the explosive base. Gelatin dynamite is a mixture of nitroglycerin and nitrocellulose which produces a rubbery waterproof compound.

3.3.2 GRANULAR DYNAMITE Under the granular dynamites, there are three subclassifications which are straight dynamite, high density extra dynamite and low density extra dynamite (Figure 3.4 ). lngredlents

Granular dynamite

Gelatin dynamite

Straight dynamite

Choractedstlcs

Straight gelatin

High density Ammonia dynamite

Ammonia gelatin

Low density Ammonia dynamite

Semigelotin

Figure 3-4 Characteristics of Dynamite

3.3.2.1 STRAIGHT DYNAMITE Straight dynamite consists of nitroglycerin, sodium nitrate, carbonaceous fuels, sulfur and antacids. The term straight means that a dynamite contains no ammonium nitrate. Straight dynamite is the most sensitive commercial high explosive in use today. It should not be used for construction applications since its sensitivity to shock could result in sympathetic detonation from adjacent holes, firing on an earlier delay.

34

on the other hand, straight dynamite is an extremely valuable product for dirt ditching. The sympathetic detonation previously discussed is an attribute in ditching because it eliminates the need for a detonator in each and every hole. In ditching applications, normally one detonator is used in the first hole and all other holes fired by sympathetic detonation. Although ditching dynamite is more costly than other types of dynamite, for ditching applications it can save a considerable amount of money and time since the charges need no initiator and no hookup of initiation system is required.

3.3.2.2 HIGH DENSITY EXTRA DYNAMITE This product is the most widely used dynamite. It is similar to straight dynamite except that some of the nitroglycerin and sodium nitrate is replaced with ammonium nitrate. The ammonia or extra dynamite is less sensitive to shock and friction than the straight dynamite. It has found broad use in all applications, quarries, underground mines and construction.

3.3.2.3 LOW DENSITY EXTRA DYNAMITE Low density extra dynamites are similar in composition to the high density products except that more nitroglycerin and sodium nitrate is replaced with ammonium nitrate. Since the cartridge contains a large proportion of ammonium nitrate, its bulk or volume strength is relatively low. This product is useful in soft rock or where a deliberate attempt is made to limit the energy placed into the blasthole.

3.3.3 GELATIN DYNAMITE Gelatin dynamite, used in commercial applications, can be broken into three subclasses, straight gelatin, ammonia gelatin and semigelatin dynamites.

3.3.3.1 STRAIGHT GELATIN DYNAMITE Straight gelatins are basically blasting gels with additional sodium nitrate, carbonaceous fuel and sometimes sulfur added. In strength, it is the gelatinous equivalent of straight dynamite. A straight blasting gelatin is the most powerful nitroglycerin-based explosive. A straight gel, because of its composition, would also be the most waterproof dynamite.

35

3.3.3.2 AMMONIA GELATIN DYNAMITE Ammonia gelatin is sometimes called special or extra gelatin. It is a mixture of straight gelatin with additional ammonium nitrate added to replace some of the nitroglycerin and sodium nitrate. Ammonia gels are suitable for wet conditions and are primarily used as bottom loads in small diameter blastholes. Ammonia gelatins do not have the water resistance of a straight gel. Ammonia gels are often used as primers for blasting agents.

3.3.3.3 SEMIGELATIN DYNAMITE Semigelatin dynamites are similar in some respects to ammonia gels except that more of the nitroglycerin, nitrocellulose mixture and sodium nitrate is replaced by ammonium nitrate. Semigelatin dynamites are less water resistant than the ammonium gels and more economical because of their lower costs. Because of their gelatinous nature, they do have more water resistance than many of the granular dynamites and are often used under wet conditions and sometimes used as primers for blasting agents.

3.3.4 SLURRY EXPLOSIVES A slurry explosive is a mixture of ammonium nitrate or other nitrates and a fuel sensitizer which can either be a hydrocarbon or hydrocarbons and aluminum. In some cases explosive sensitizers, such as TNT or nitrocellulose, along with varying amounts of water are used (Figure 3.5) An emulsion is somewhat different from a water gel or slurry in characteristics, but the composition contains similar ingredients and functions similarly in the blasthole (Figure 3.6). In general, emulsions have a h.gher detonation velocity, are not electrically conductive and are not affected by low temperature. For discussion purposes, emulsions an-i water gels will be treated under the generic family of slurries. Slurries, in general, contain larger amounts of ammonium nitrate and are made water-resistant through the use of gum, waxes, cross linking agents or emulsifiers. A number of varieties of slurries exist, and it must be remembered that different slurries will exhibit different characteristics in the field. Some slurries may be classified as high explosives while others are classified as blasting agents since they are not sensitive to a number 8 blasting cap. This difference in classification is important from the standpoint of magazine storage. An added advantage to slurries over dynamites is that they can be delivered as separate ingredients for on-site mixing. The separate ingredients brought to the job site in large tank trucks is non-explosive until mixed at

36

•

•

the blasthole. The bulk loading of slurries can greatly reduce the time and cost of loading large quantities of explosives (Figure 3. 7). Slurries can be broken down into two general classifications, cartridge and bulk.

INGREDIENTS

WA~ ~ M'.:00$, .. oacANIC MTilAltS, l"OtOil 181.GQ

MM

. . l;\ 1s.ee .. l.7.9Q

.1.99 ..

M

..'ii .............. .

Figure 7-5 Data for Pattern Number 1

1.831

i• ••. •• ~

••

•• ••• •• -·•• .,

•• •• I

....

110

•• •• • •

i

• I

·~

•• •• ... • •• •• •• •• •• ...• •• •• •••• •• •• l

i

~

Precision Blasting Services, Inc.

B R E A K E R

V4.00 Page: 1 Date: OS-24-1994

Entry: 2 3 METER BENCH HEIGHT File: MB76 ENTRY 2 ·cal. based on Volume Strength, Staggered pattern Number of rows . . . . . . . . . . . . . . . . . . . . . . s Total number of holes . . . . . . . . . . . . . . . so Diameter of blasthole . . . . . . . . . . . . . . . 101.60 mm Burden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.SO m Spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.SO m Stemming/. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. so m Bench height . . . . . . . . . . . . . . . . . . . . . . . . 3.00 m Subdrill . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.00 m Hole depth ..... .' . . . . . . . . . . . . . . . . . . . . 4.00 m Drilling angle from vertical ....... . 0.00 Type of rock . . . . . . . . . . . . . . . . . . . . . . . . Specific gravity of rock . . . . . . . . . . . . Rock strength (weak=l, strong=lO)

LIMESTONE 2.60 g/cm' 6.34

\

Type (brand) of explosive . . . . . . . . . . . Specific gravity of explosive ...... . Explosive strength (ANFO=lOO) Diameter of explosive . . . . . . . . . . . . . . . Length of explosive charge Weight of explosive charge ......... . Total explosive weight . . . . . . . . . . . . . . Total blasted rock

ANFO 0.8S 100.00 101.60 --./ 2.SO 17.23

g/cm' nun m

Kg

861. so Kg 937.50 m3 2,437.SO t

Powder factor

0.92 Kg/m 3 1. 09 m 3 /Kg

Powder factor . . . . . . . . . . . . . . . . . . . . . . .

0.35 Kg/t 2.83 t i Kg

111

Precision Blasting Services, Inc.

BREAKER

Entry: 2

V4.00 Page: 2 Date: 05-24-1994

3 METER BENCH HEIGHT

File: MB76

------------------------------------------------------------SCREEN +--- p A SIZE cm Entry m>

s s I

N

G

---+

t

%

+---

I

F

R

A

c T I

0 N ----+

t

%

m'

I

--------------------------- ----------------------------------

2

.5

2

12

32

1. 30

1

2

25

64

2.64

2

2

50

130

5.32

4

2

99

257

10.56

8

2

191

497

20.37

16

2

349

907

37.20

32

2

575

1,

495

61. 32

64

2

803

2,087

85.62

128

2

920

2,391

98.09

Average size=

HB76

<

2

DiaMeter

23.56 cm

25

64

2.64

25

65

2.68

49

128

5.24

92

239

9.82

158

410

16.82

226

588

24.12

228

592

24.30

117

304

12.47

Fragmentation index =

1. 030

)

ie1.69 MM

3.0Q

...

4.09 ..

2.se .. 1.0Q "

__y_ -- ---- -- -- _____y_

Figure 7-6 Data for Pattern Number 2

•• •• •• • •• •• •·• •• • I

~

•~

• ·•• •• ••

~.

112.

•~

-~ I

~

~­

••

•• •• •• • ·-• •• ••· •• •

Precision Blasting Services, Inc. Entry: 1 Entry: 2 ENTRY 1 ENTRY 2 ENTRY 1

B R E A K E R

18 METER BENCH HEIGHT 3 METER BENCH HEIGHT

File: MB75 File: MB76

Cal. based on Volume Strength, Staggered pattern Cal. based on Volume Strength, Staggered pattern ENTRY 2

Number of rows . . . . . . . . . . . . . . . . . . . . . . Total number of holes . . . . . . . . . . . . . . . Diameter of blasthole . . . . . . . . . . . . . . . Burden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spacing ... _. . . . . . . . . . . . . . . . . . . . . . . . . . Stemming . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bench height . . . . . . . . . . . . . . . . . . . . . . . . Subdrill . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hole depth . . . . . . . . . . . . . . . . . . . . . . . . . . Drilling angle from vertical ....... . Type of rock . . . . . . . . . . . . . . . . . . . . . . . . Specific gravity of rock . . . . . . . . . . . . Rock strength (weak=l, strong=lO)

50 50 101. 60 101. 60 mm 2.50 2.50 m 2.50 m 2.50 2.00 1.50 m 18.00 3.00 m 1. 00 m 1. 00 19.00 4.00 m 0.00 0.00 ° LIMESTONE LIMESTONE 2.60 2.60 g/cm 3 6.34 6.34

Type (brand) of explosive . . . . . . . . . . Specific gravity of explosive ...... Explosive strength (ANFO=lOO) Diameter of explosive . . . . . . . . . . . . . . Length of explosive charge ....... Weight of explosive charge .........

ANFO 0.85 100.00 101.60 17.00 117.15

. . . . .

5

Total explosive weight . . . . . . . . . . . . . . 5,857.50

5

ANFO 0.85 g/cm 3 100.00 101.60 mm 2.50 m 17.23 Kg 861. 50 Kg

Total blasted rock . . . . . . . . . . . . . _ ... 5,625.00 937.50 m3 14,625.00 2,437.50 t Powder factor 1. 04 0.92 Kg/m 3 0. 96 1.09 m3 /Kg Powder factor ... ____ ...... _ ... _ ... .

••

•••• . "

V4.00 Page: 1 Date: 05-24-1994

113

0.40 2.50

0.35 Kg/t 2.83 t/Kg

Precision Blasting Services, Inc. Entry: 1 Entry: 2

1

m'

t

1

4

2

12

10 32

I I

0.07 l. 30 13

25

35 64

0.24 2.64

34 25

89 65

0.61 2.68

121 49

313 128

2.14 5.24

408 92

1,061 239

7.26 9.82

1,220 158

3,171 410

21.68 16.82

2,397 226

6,232 588

42.61 24.12

1,388 228

3,610 592

24.68 24.30

43

112

117

304

0.77 12.47

35 64

13 25

0.24 2.64

1

2 124

48 50

1 2

0.85 5 .32

130

1 2

168 99

1 2

438 257

2.99 10.56

1 2

577 191

1 2

8

1,499 497

10.25 20.37

1 2

16

4,670 907

1, 796

1

349

2

31. 93

37.20

1

2

32

4,193 575

1 2

10,903 1,495

74.55 61. 32

1 2

64

5,582 803

1 2

14,513 2,087

99.23 85.62

l

2

128 1 2

1 2

~• ••

I

2 1

4

t

m'

1

2

2

File: MB75 File: MB76

P A S S I N G ---+ +--- F R A C T I 0 N ---+

+---

I

~

V4.00 Page: 2 Date: 05-24-1994_

18 METER BENCH HEIGHT ~ METER BENCH HEIGHT

SCREEN SIZE cm Entry .5

BREAKER

5,625 920

Average size Average size =

14,625 2,391

100.00 98.09

22.07 cm 23.56 cm

Fragmentation index 1.831 Fragmentation index= 1.030

Figure 7 -7 Summary of Fragmentation Data

•• •• •• •• •• ·•• .,.,•

.I.1 •..,.j , I

114

• •

j

••

,.,.•

P

A $

S

I

H C

/CJ

59 Y.

25 y,

i

1·:• •• •

•• •• •

.•

e

2'5 Y.

e

::r.

FRACTIOH

109 Y.

75 Y.

25

x

25 Y.

e.s

1

~

2

4

16

HB75

32

IZZZZ.a

64



128 MB? 6

Figure 7-9 Cumulative Distribution of Fragmentation Data

!f)

'••.. •

:• ~

59 Y.

Figure 7 -8 Comparison of Sizes From Both Blasts

•• •• •• ••

1"•

75 Y.

x-t=-~~===---r"-'=-~-r-~~~==;=:::::::=--~-r~~~-;-~~~-;-~~~-;-~~~-+ 0.5 1 2 4 Hi 8 32 64 l.28 Cl.> "B75 HB76

•

:•

19& Y.

.......,:--

75 Y.

115

~ CD®@©®®4

Figure 7-10 Single Row Progressive Delays, S = B

116

•

,e ,

1

49

•~

•• •• •• •• ef •

-

•41

••• • ·~

S

Conditions:

1.4 B

Drill pattern:

Initiation Timing: Vibration Level: Fragmentation: High Bench:

First hole, burden distance from face. Spacing is 1 A B for LIB-, 4 Progressive MS Delays

, I

r

-." •

LIB> 4

Figure 7-11 Single Row Progressive Delays, S

,

•• •••

Low, each hole on a separate delay Uniform, one tight distribution

117

= 1.4 B

s

Conditions:

= 1.4 8

Drill pattern:

Initiation Timing: Vibration Level: Fragmentation:

First hole, burden distance from face. Spacing is 1.4 B for LIB-, 4 Alternating MS Delays

High Bench:

High, since one-half of total holes firing on one delay Majority of rock in two different size distribution. L/ B > 4

Figure 7-12 Single Row Alternating Delays, S

=1.4 B

•• ••• •• •• •• •• •• ••er •• •• ••1 e: •• •• • •

.i I

118

•

I

•

I

;

1:

'

...

·~

~

••

..• • •• ••• •

•.

S = 1.4 B

I

••

Conditions:

Drill pattern:

I

• • !'\

•• •• ••

Initiation Timing: Vibration Level: Fragmentation:

Hiqh Bench:

High, since all holes firing on one delay Majority of rock in two different size distribution (fines and boulders) . LIB> 4

Figure 7-13 Single Row Instantaneous, S = 1.4 B

..,.

•.

First hole, burden distance from face. Spacing is 1.4 B for LIB Instantaneous.

119

s

Conditions:

= 2 8

Drill pattern: Initiation Timing: Vibration Level: Fragmentation: High Bench:

First hole, burden distance from face. Spacing is 28. Progressive Delays. Low, each hole on separate delay. Average, some boulders, rough face. L/ B > 4

Figure 7-14 Progressive Delays, S =2 B

120

•• •••• •• •• •• •• •• ••• •• •• •• •• •• •• •• • •

I

•

l~ • I

I

~

•• i

•••

•• • •• • •

-

s

COARSE

2 B

w

•

;

Conditions:

....• ~·

•• ~

Drill pattern:

Initiation Timing: Vibration Level: Fragmentation: Hiqh Bench:

.. ""'

·~

High Uniform, coarse. LIB> 4

Figure 7 -15 Single Row Instantaneous, S

• ••

•• • ••

First hole, burden distance from face. Spacing is 28 for UB-A. Instantaneous row.

121

=2 B

•, ~ • ~

717//J///H///ff/Y/n»Y//////////IY/ffn»Y/7//ff///////14', B

© @ @ @ @ @ ©f

1.4 B

©

@

@

@

@

©

@+

®

@...1.

1.4 B

® ® ©

©

l-s-l-s-J

= • I

Figure 7-16 V-Cut (Square Corner), Progressive Delays, S = 1.4 B

7/)//J/ff/J,7/////////7//7///.17/////////)//)//)//////.17.ff//4), B

©

@

@

@

@

@

©

@

@

@

©

©f 1.4 B

+

1.4 B

©

I- s-1-- s-l

S

=

@

©

...1.

1.4 B

Figure 7-17 V-Cut (Angle Corner), Progressive Delays, S

122

=1.4 B

••• •• •• •• •• •• •• •• •• ,..• •• •• • •

l

••

..• I

©@ ®®

I

••

® 0

•• •• •• •• •'ti • •

.

•• •• •• •• •• l •• I •• I •

e-J

© ®

l-s~

©

®

®

S = 1.4 B

Figure 7-18 Box Cut, Progressive Delays, S

=1.4 B

'7//7/7//m//7/7/7//7/7/7/7/7/////7/7/7/7/7/7/7//w///7/74//,

@

Q)

@-+ ©-t

B

Q)

Q)

B

©@

©

@

©

@

® ®

®

®

®

® ®__i_

s-l

l-s~

B

~

S - 1.4 B

Figure 7-19 Box Cut, Alternating Delays, S = 1.4 B

,..,

I

123

B

(D

@

@

@

@

©

@

©

@

@

@-+ 1.4 B

0-t1.4 B

@

©

@

~s~

8

@-1.

0

@

•, •• .: •!

•• •

S = 1.4 B

Figure 7-20 Square Corner, Cut Fired on Echelon, S = 1.4 B

1.4 B

©--t

@

@

@

©-----'--

1.4 8

/

0

/

/

/

/ /

/

/

/

/

0

/ /

/ /

/

/

/

/

m © /

/

/

m

--05- --Q}- ---V

/

= 1.4 8

•• •• •• •• •• •• •• •• •1 •!

el

••: e: •'•1 • ~

Figure 7-21 Angle Corner, Fired on Echelon, S = 1.4 B

124

I

•• ~

B

I.• ~

B

@

@1---~

••

• ~

,...

••

....

•• • ,._• •-

@--+ B

I

•• •• • •• •"'• •• ....• •• •

-JL

s >-JL

s >-JL

s >-JL

2

c -

c -

3

c -

c-

9.3.2.4 DEPTH OF BLAST HOLE (H) The depth of the blasthole which will break to 95% or more of their depth can be found from the following equation:

•• •• •• •• •• •• •• •• •• •• ••

.;•:•1 I

H

DH +16.51

= ---'-'---

(9.14)

••

41.67

• ! ~l

where:

=

=

Depth Hole diameter

170

(m) (mm)

•1 •• •:J

•• ••. •• • ....... • ••" • I

9.3.2.5 DEPTH OF ADVANCE (L) (EXPECTED)

'~~

• "••• tt

•• ..... •• ••

Check if charge can break burdens in each square. Use burden formula .

2 SG. )D• B=0.012 ( --+1.5 SGR

9.3.2.6 STOPING HOLES 2 SG. ) D. B=0.012 ( --+1.5 SGR

··~ ••

•-~

s 8 T

= = =

Spacing Burden Stemming

1!t

~

·~

(m) (m) (m)

9.3.2.7 LIFTER HOLES ) D. 2 SG. B=0.012 ( --+1.5 SGR

S=l.IB T=0.2 B

'"'

I

!

(9.17)

where:

I

IJ!t

(9.16)

S=l.lB T = 0.5 B

tit

..._I

(9.15)

L =0.95 H

171

(9.18) (9.19)

9.3.2.8 CONTOUR HOLES (RIB & BACK HOLES) Commonly trim blasted with holes on 0.045 m to 0.6 m centers, otherwise:

2 SG B = 0.012 - • + 15 ) D. ( SGR

j

(9.20)

S=l.IB T=B

(9.21)

9.3.2.9 BLASTHOLE TIMING Cut holes fired with delays at least 50 ms between periods. Stoping holes delayed at least 100 ms or LP delays. Contour holes (trim blasted) fired on same delay. Lifters shot last.

9.3.2.10 INITIATOR Always placed on bottom of blastholes.

Example 9.3 An 8 meter high by 10 meter wide rectangular tunnel will be blasted using a large hole burn cut. The cut will be placed near the center and bottom of the tunnel. The center empty hole will be 102 mm and the loaded holes will be 28 mm in diameter. The emulsion with a density of 1.2 g!cm' will be loaded into all cut holes. Emulsion in 25 mm. 29 mm and 32 mm cartridges are available. Presplit explosive will be used on the ribs and back and trim hole spacing will be 0. 6 m. The rock is granite with a density of 2. 8 gfcm•. The 103 mm diameter hole was chosen to enable an advance of at least 95% on a drill depth of 3.8 m. Design the blast. CALCULATIONS OF INDIVIDUAL PARAMETERS:

Sc~ .JL

1 0.153 0.153 0.216 0.153 Sc~

1.9 m

2 0.216 0.324 0.459 0.108 Sc~

172

1.9 m

I

•• ••

•• •• •• ~ • ••

.

•• !• j

F111 out table using formulas given in Table 1. Square No. B= R= Sc= T= Check

~ ••= •

3 0.459 0.688 0.973 0.230 Sc~

1.9 m

4 0.973 1.459 2.063 0.487 Sc~

1.9 m

:

••

=

1. Depth (HJ given as 3. 8 m 2. Advance (L) given as 0. 95 x 3. 8 m = 3. 61 m

JI,= .J3.61=1.9 3. Burden calculation:

J

2 SG B = 0.012 - • + 1.5 D. ( SGR

2xl.2 ) B 25 =0.012 ( --+I.5 25=0.71 m 2.8 2xI.2 ) B 29 = 0.012 ( - - + 1.5 29 = 0.82 m 2.8 2x 1.2 ) B 38 = 0.012 ( - - + 1.5 38 = 1.07 m 2.8 4. Stoping Holes:

2 2 B=o.012( xl. +l.5)38=I.07m 2.8 S = 1.183m=:1.2 m T= 0.215 m 5. Utters Same Burden and Spacing as stoping holes

T= 0.215m I

""Ji

I

tl1 tl1

6. Contour (Trim Holes) Use o_ 6 m Spacing

d

tl1

;9,

600 ] = 115 .E_ (~J =IO ( 177 _177 m B = l.3x0.6 = 0.78 m =0.8 rn

=IO ec

1 ••

:.'"' I

!

!•'

I"''11

,J ""!

173

ASSEMBLING PLAN

1. utters

10 12=8.33

NOTE: Must round to whole numbers

10 -8= l .25m=S

IF

10 9=1.11 m = S use 9 spaces or 10 holes

OR

2. Look Out

0.1+H(tan2°) =0.1 +3.8(tan2°) =0.23 m 3. Holes

= 10 = 46 16

Lifters Stoping Cut

Controlled Blasting Walls Roof

=

68

26 15 41

holes

.. .. .. .. .. .. .. .. ........

D

E

....

10 m 2.065

9.3.3 V-CUT The most common cut used in underground work with angle drill holes is the V-cut. The V-cut differs from the bum cut in that less holes are drilled and less advance can be made per round with a V-cut when compared to a burn cut. The advance per round is also limited by tunnel width. In general, the advance per round increases with width, and an advance of up to 50% of the tunnel width is attainable. The angle of the

174

•• •• •• •• •• •• •• •• •• •• •• •• •• •• •• •• •

•

.,...."' 41111,

.,

V must not be acute and should not be less than 60°. More acute angles require higher energy charges for the amount of burden used. A cut normally consists of two V's, but in deeper rounds, a cut may consist of as many as four (Figure 9.15).

~I

000 000 000

\Iii

tli

-~ tl1

"'11 ~I

"11

•1

.. "'

·lb

Figure 9-15 Basic V-Cut

Each V in the cut should be fired with the same delay period using millisecond detonators to ensure minimal cap tolerance between each leg of the V as it fires. Delay time between adjacent V's should be at least 75 milliseconds (minimum). Basic layout of the V's are shown in Figure 9.16.

'tit~

••

•• tll

•.

Wii--MS - 175

F.'mt::n:JE&~- MS

- 250

-f-::ll'lft'""Tl'~mf

:

.,i.,,

I ,. ~I

1,. I

'.I•

I

~h

Figure 9-16 Timing for V-Cut

175

Two burdens are expressed in Figure 9.15. The burden at the back of all holes and the burden between theVs. The distance indicated as B-1 (Figure 9.15) which is located between V's is twice the normal burden if a 60° angle is used in the apex of the V. In some cases, an additional blasthole is drilled perpendicular to the face following the line of B-1, which is called the "Breaker hole". This is used if the fragmentation originating within the V is too large.

/

Figure 9-17 V-Cut Dimensions

Figure 9.17 indicates the dimension needed to drill a proper Vcut. Three sets of specific dimensions are needed for each hole. These are, (1) the distance at which the hole is collared from the center of the entry, (2) the angle at which the hole enters the rock mass, and (3) the length of the particular blasthole. In order to get the proper dimensions we will discuss the design calculations for a V-cut.

9.3.4 V-CUT DESIGN 9.3.4.1 DETERMINATION OF BURDEN The burden is always measured at the very bottom of the blasthole and is placed as shown in Figure 9.15. It is realized that this is not the exact true burden and holes with greater angles (those that approach V) have a smaller true burden. This, however, is done to simplify design. When one considers drilling error and other factors, the reduction in true burden can actually be beneficial. The burden can be determined by using the same equation that we have used before.

176

•• •• •• •• •

•• •• •

•• •• •• ••

.,., .1

•••' •• .,j••I • I

•• :···• ••

•... l

2 SG ) B =0.012 _ _ e +15 D. ( SGR The distance between V's is shown in Figure 9.15 as distance 8 1 is calculated as follows. (9.22)

B 1 =2B where:

8 B1

= =

Burden Burden

(m) (m)

··1'

..'

The vertical spacing between V's is:

. • •• -~

·-••

•• ·•• • I

where: S B

=

=

Spacing Burden

(m) (m)

9.3.4.3 V-ANGLE The normal angle in the apex of the V is about 60°. For small narrow tunnels, V angles of less than 60° have been used. However, the explosive loading density in each hole must be increased.

9.3.4.4 DEPTH OF CUT OR ADVANCE (L) In general, the depth of the cut will vary from 28 to at maximum Blastholes normally will not break to the bottoms and advance can be assured to be between 90%-95% of drill depth.

50% of the tunnel width.

•

•• I

•,. I

;".J

-~~

.....

(9.23)

S=l.2B

~ ".')

"'I

9.3.4.2 SPACING BETWEEN HOLES (VERTICALLY)

177

••

el

9.3.4.5 STEMMING DISTANCE 81astholes are normally loaded to within 0.38 - 0.58 to the collar depending on the strength of the materials to be blasted. Collars are either left open or clay stemming plugs are sometimes used.

9.3.4.6 LIFTER AND STOPING HOLES The same design procedure is used as previously discussed with the burn cut.

9.3.4.7 CONTOUR (RIB & BACK) HOLES The same procedure is used as previously discussed with the burn cut.

9.3.4.8 LOOK OUT Same procedure is used as in design of burn cut.

9.3.4.9 BLASTHOLE LOADING It is important to have initiators placed at the bottom of the blastholes. The loading density can be reduced near the collar when explosive cartridges are used, rather than pneumatically loaded ANFO. The loading density reductions can begin after 1/3 of the hole is loaded with the designated amount to achieve proper burdens.

9.3.4.10 TIMING SEQUENCE The timing in V-cuts should be at least 50 ms between V's where multiple V's occur one behind the other. The timing must be so designed to allow movement of rock to begin before subsequent holes fire. For this reason, minimum delays should be 75 to 100 ms as shown in Figure 9.16.

•'.! e: •: •• •• •• •• •• I

•• •• •• ••

• •• ••• •• • •

Example 9.4 The tunnel will be in limestone (density=2.6 glcm and is designed to be 6 meters wide and 4 meters high. A Semigel dynamite with a density of 1.3 glcm• in 32 mm diameters will be the explosive charge. Design the V-cut. 3 )

• !

...

I

178

•• ...• ••

1.) Burden Calculation:

I

•

B = 0.012 (

-

2.) Spacing between V's (vertically)

S = 1.2 B = 1.2 x0,96 = 1.152 m 3.) Depth of Cut (L).

~

"'

--

15

2 B = 0.012( x1.3 + l.5)32 = 0.96 m 2.6

~

.....

8~~e + )ne

2

L = 2B=2x0.96=1.92 111 4.) Drill Depth (H):

1.92 H =--=2.13111 0.9

~

5.) Calculation of stemming

.,.11>'

• •• • •" •• ••

• •• • •• •

T=0.5B=0.48111 6.) Look out

LO= 0.1 + H(tan 2°)

=

0.1+2. l 7(tan 2°) = 0.18 m

7.) See figure for calculations of individual holes. Angles for Holes Hole 1 30° Hole2 21° Hole 3 10.89° Hole 4 -4.47°

E

,...,

,.;

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179

Depth of Hole 1

2.13

x = cos

30

= 2.46 m

Depth of Hole 2

2.13 x=--=2.28m cos 21

Depth of Hole 3

2.13

x=

cos 10.89

=2.17 m

Depth of Hole 4

x=

2.13 cos 4.47

=2.14m

9.3.5 FAN CUTS The fan cut is similar in design and method of operation to the V-cut. Both the fan and V-cut must create relief as holes fire towards the one open face. There is no additional relief created by empty holes as is done in the burn cut. A typical fan cut is shown in figure 9.18. Dimensions are determined using the same methods and formulas as in the V-cut.

Figure 9-18 Fan Cut

180

•• •• •• •• •• •• •• •• •• •• •• •• •• • '•• •• ••

•

•• t•• !

•t• '

I

~

'

9.3.6 HEADING AND BENCH METHODS The Heading and Bench Method (Figure 9.19) is a combination of an underground tunnel round and a surface bench blast The top heading is driven ahead of the bench. Any of the cuts or tunnel rounds discussed could be used to develop the heading. The bench is designed using the same principles as previously discussed for bench blasting in Chapters 6 and 7.

.. • • •

4.9 m

~~

flt

... --~

..,.

·~· ..~

~­

••

... ~'

•• •• •• ••

Figure 9-19 Heading and Bench Method

Example 9.5 The bench blast for the tunnel shown in Figure 9. 19 will be designed. The blasthole will be loaded with 32 mm Semigel dynamite with a density of 1.3 gl cm'. The rock will be a limestone with the density of 2.6 glcm'. 1.) Burden:

2 SG ) B =0.012 ( SGRe +15 De

I

2xl.3 } B = 0.012 ( - - + 1.5 32 = 0.96 m 2.6

i

I!"-

i • 2.) Stemming·

T = 0.7 B = 0.7x0.96 = 0.67 m

181

3.) Subdrilling:

J = 0.3 B = 0.3x0.96 = 0.29 m 4.) Hole Depth:

H

= L+J = 7.6+0.29 = 7.89 m

5.) Timing: All holes delayed or V cut (bench blast). See chapter 7 for pattern dimensions. 6.) Spacing If delayed, then check:

L 7.6 -=-=7.9 B 0.96

S= l.4B= 1.34 m If bench blast, V cut spacing also= 1.48 or 1.34m 7.) Number of Rows The number of rows is normally 3-5 depending on the availability of delay blasting caps and vibration specifications on the project. 8.) Number of Holes per Row The width of the tunnel is divided by the spacing:

15.2 m - - + I = 11.34+1 l.34

Twelve holes will be used. The actual spacing is:

15.2

- - = 1.38 m

11

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e:

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VIBRATION AND SEISMIC WAVES 10.1 SEISMIC WAVES Seismic waves are waves that travel through the earth, These waves represent the transmission of energy through the solid earth, Other types of wave transmission of energy are sound waves, light waves, and radio waves_ Earthquakes generate seismic waves_ The science that studies earthquakes is Seismology, the name being derived from the Greek word seismos meaning to shake. In addition to the naturally generated seismic waves, there are many man made sources of seismic waves_ When these man made seismic waves are sensible, that is when they can be felt, they are referred to as "vibration ...

,,. 10.1.1 WAVE PARAMETERS The fundamental properties that describe wave motion are called wave parameters_ These are measured and quantified when discussing wave motion or vibration, Consider the simple harmonic wave motion illustrated in Figure 10.1 and represented by the equation:

••

y =A sin(w t)

·~

~

where:

t;'~

• •

• •• I

• ••

~

i

~p

• ~)

•

y

=

t

= = = = =

A

w T f

Displacement at any time t, measured from the zero line or time axis Time Amplitude or maximum value of y 2 pf Period or time for one complete oscillation or cycle Frequency, the number of vibrations or oscillations occurring in one second, designated Hertz, Hz

183

length, L

j.

--t Time axis

Trough Figure 10.1 Wave Motion And Parameters

Period and frequency are reciprocals so that:

f =

l T

or

T =

l

( 10.1)

f

Wave length L is the distance from crest to crest or trough to trough measured in meters and is equal to the wave period multiplied by the propagation velocity V

L=VT

(10.2)

10.1.2 VIBRATION PARAMETERS Wave parameters were discussed earlier. Vibration parameters are the fundamental properties of motion used to describe the character of the ground motion. These are displacement, velocity, acceleration and frequency. As a seismic wave passes through rock, the rock particles vibrate, or are moved from the rest position. This is displacement. When the particle is displaced and moves, it then has velocity and can exert force that is proportional to the particle's acceleration. These fundamental vibration parameters are defined here:

184

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...•

•• •• •• ~•• • ., • ••

.

:

•. .• •

:' •

•

• ....• ...

• I

•• ~­

•• • ti• .._ ·~~~ !• .,

~i.

1't 41•

Velocity - The speed at which the rock particle moves when it leaves its rest position. It starts at zero, rises _to a maximum, and returns to zero. Particle velocity is measured in millimeters per second. Acceleration - The rate at which particle velocity changes. Force exerted by the vibrating particle is proportional to the particle acceleration. Acceleration is measured in fractions of "g", the acceleration of gravity. (g = 2rcFV / 9810) Frequency - The number of vibrations or oscillations occurring in one second, designated Hertz (Hz).

1t

«•

Displacement - The distance that a rock particle moves from its rest position. It is measured in millimeters. (Displacement = V I 27iF)

..

•• • ...... •• •

••

.•• "' •

Vibration seismographs normally measure particle velocity since the standards of damage are based on particle velocity. There are, however, displacement seismographs and acceleration seismographs. Also, velocity seismographs can be equipped to electronically integrate or differentiate the velocity signals to produce a displacement or acceleration record.

10.2 UNDERSTANDING VIBRATION INSTRUMENTATION 10.2.1 SEISMIC SENSOR The function of vibration instrumentation is to measure and record the motion of the vibrating earth. In basic scientific terms, this is a seismograph comprised of a sensor and recorder . The sensor is in fact three independent sensor units placed at right angles to each other. One unit is set in the vertical plane, while the remaining two units lie in the horizontal plane at right angles to each other. Each sensor will respond to motion along its axis. Three are necessary to completely determine the ground motion. The three units are enclosed in a case as shown in Figure 10.2.

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•

'9

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..ii ••

nCD~ .. CJ --mot~

0

I

l

Olrectlon of

•• •

sensor motion

Figure 10.2 Seismograph Sensor

10.2.2 SEISMOGRAPH SYSTEMS There are many seismograph systems, or simply seismographs, available today, each of which performs the basic function of measuring ground motion. The many variations are a response to needs, constraints, and advancing technology. A brief description of the main types of seismographs will be helpful. Analog seismograph - a three component system that produces a record of the ground motion. It is called analog because the record is an exact reproduction of the ground motion only changed in size, amplified, or de-amplified. Tape seismograph - the same as the analog seismograph, except that it records on a magnetic tape cassette instead of producing a graphic record. A record of the ground motion is obtained by use of a playback systems and a chart recorder. Vector sum seismograph - the standard seismograph system consists of three mutually perpendicular components. The resultant ground motion can be determined by combining the components using the relationship:

R

=

.Jv

2

+ L2 + T 2

186

(10.3)

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•• •·• •1

•• •• •• •

•

• • r•• • l

I~

1,'"'

!'11

,w

i' l!'I) lll!>

1,j' 1'!)

'

~~

....

·"'"" "' ~

-

-~ "' '

'"" I

1"

•• i

• I I

•• ••

~

R V L T

= = =

Resultant motion Vertical component of motion Longitudinal component of motion Transverse component of motion

The vector sum seismograph performs this mathematical calculation electronically; that is, it squares the value of each of the components for each instant of time, adds them, and takes the square root of the sum. It then produces a record of the vector sum_ Bar graph seismograph - a three component system that differs in its recording system_ Instead of recording the wave form of the ground motion at each instant of time, only the maximum ground motion of three components is recorded as a single deflection or bar whose magnitude can be read from the record graph_ This is a very slow speed recording system which can be put in place and left to record for periods up to thirty or sixty days_ Triggered seismograph - an analog or tape seismograph which automatically starts to record when the ground vibration level reaches a predetermined set value, which triggers the system_

i"!)

1t

where:

'Ill

Computer Controlled Digital Seismograph - the digital seismograph begins to automatically record when either ground vibration levels or airblast levels reach a predetermined set value_ The information gathered can be transferred to a computer disk whereby it can be further analyzed on an IBM PC or compatible computers. The seismograph will normally electronically determine peak particle velocity on all traces, frequency of the peak and airblast levels. Some seismographs will also do fft's, response spectrum, comparison to known standards, display the output in different languages and function in either metric or US units.

i

!

I

i

~·t

• ....·~

187

Most seismographs are equipped with meters that register or liquid crystal displays that hold the maximum value of the vibration components and the sound level. Other seismographs are equipped to produce a printout which gives a variety of information such as maximum values for each vibration component, frequency of vibration for the maximum value, vector sum, and sound level. Blast information such as date, blast number, time, location, job designation, and other pertinent information can also be added to the printout.

10.3 VIBRATION RECORDS AND INTERPRETATION 10.3.1 SEISMOGRAPH RECORD CONTENT Normally a seismograph record will show the following: Four lines or traces running parallel to the length of the record. Three traces are the vibration traces, while the fourth trace is the acoustic or sound trace. (There may not be an acoustic trace.) Each of the four traces will have a calibration signal to show that the instrument is functioning properly. Timing lines will appear as vertical lines crossing the entire record or at the top only, the bottom, or both top and bottom. An example of a typical seismogram, or vibration record, is shown in Figure 10.3. One vibration trace or component is vertical, the other two horizontal. The components are usually specified as follows in Figure 10.4. Vertical Longitudinal or Radial

Transverse

motion up and down, designated V. motion along a line joining the source and the recording point, designated L or R. motion at right angles to a line joining the source and the recording point, designated T.

The sensor normally has an arrow inscribed on the top. By pointing the arrow toward the vibration source, the vibration traces will always occur in the same sequence, with the arrow indicating the L component also the direction of motion will be consistent from shot to shot. The instrument manufacturer will indicate the proper sequences.

188

..;•1!

•'•' •• •• •• •• •• •• •• •• •• •~ •

•••

= •=

4. 19

-.

'"....·· ·

-,

T

if}

"\

.,

··~

.

·~

., ., •i. ~

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Figure 10.3 Vibration Record

·1· .

Vertical /

Shot

Seismograph

I•

•• • •• •

Transverse

Figure 10.4 Vibration Components

~

••

.• l

"

Each trace represents how the ground is vibrating in that component. If the seismograph is measuring velocity, then each trace shows how the particle velocity is changing from instant to instant in that component. Similarly, if the seismograph is a displacement system or an acceleration system, the traces will show the instant to instant change in these parameters. The acoustic trace shows how the sound level changes with time.

_.

.

l I

~

I

~

~

189

10.3.2 FIELD PROCEDURE AND OPERATIONAL GUIDES Site selection is the first item of procedure. This is usually determined by a complaint or sensitive area. which needs to be checked. If there is no such problem, than place the seismograph at the nearest structure that is not owned by or connected with the operation. The seismograph distance should always be less than or at most equal to the distance to the structure. vyhen dealing with residents or persons in the vibration affected area, the engineer must be factual and direct. He must emphasize that the purpose of the seismograph measurement is to protect them and their property from vibration damage, and that standards have been developed by the Federal Government to do this. Place the sensor on solid ground. Do not place it on: • • • • • •

Grass Isolated slab on stone or concrete Loose earth Any soft material Inside a structure except on a basement floor Concrete or driveway connected to a blast area

Failure to observe these precautions will result in distorted readings that are not representative of the true ground vibration. Level the sensor, some sensors have bulls eye level on top for this purpose. Others can be leveled by eye. Make sure the sensor is solidly planted. In cases of large ground motion it may be necessary to cover the sensor with a sand bag, spike it down or dig a hole and cover it with earth otherwise the sensor may be decoupled from the earth and the vibration record will not represent the true ground motion. Remember that the ground displacement is usually only a few hundredths of a millimeter so do not expect to see the decoupling of the sensor. Most sound measurement is made with a hand held microphone. Hold the microphone at arm's length away from you to avoid reflection of the sound wave from your body. Regardless if the microphone is used set up on a stand or hand held, do not set it up in front of a wall. This will prevent sound reflection from the wall.

190

•• •• •• •• •• •• •• •• •• •• •• •• •• •fl •• •• ••

10.3.3 PRACTICAL INTERPRETATIONS

~

~

~

.._

.

~

~

._ ~ ~

.. "'~

The seismograph record can be used for much more than obtaining the peak particle velocity. It can be helpful in engineering the blast and provide information to the operator as to how to achieve the best vibration control as well as optimizing the use of the explosives energy to break rock. Assume one has a seismograph record that exhibits one large peak in the center of the wave trace That large peak has a particle velocity of 2 in/s or 50.8 mm/s. Also assume that no other peak on the record is larger than 1.0 in/s or 25.4 mm/s. That one large peak at 50.8 mm/s is controlling how we design and execute all our blast in the future. In blasting, it is not the average vibration value that counts, it is the maximum. Therefore, common sense would dictate that if one could reduce that 2 in/s or 50.8 mm/speak to 1.0 in/s or 25.4 mm/s, it would not only be better for the residence in the area but would be more economical for the operator. What does this large peak mean from a practical standpoint? If it occurred in the center of the record, it is indicating that something occurred that was of unusual nature approximately half way through the blast. These peaks in the vibration record indicate energy release over time. The record indicates that for some reason significantly more seismic energy was obtained approximately halfway through the blast. If all blastholes were loaded the same, this indicates there is inefficiency in the blasting process approximately half way through the blast. Now go back to the blasting pattern and determine approximately where the problem resulted. You might be able to find and correct the problem. A common problem which occurs is that if blastholes are wet, blasters commonly do not place as much energy in the wet portion of the hole as in a totally dry hole. This is because cartridged product is used instead of, for example, bulk ANFO. The smaller diameter cartridged product may not have as much energy as a larger diameter ANFO charge and therefore, the vibration level would increase. How you handle wet hoie situations can greatly effect the vibration generated from the blast. Another common problem is drilling inaccuracy. If a blasthole within the pattern has an excessive burden at the time it shoots, vibration levels go up. The seismograph record, therefore, can be used as a diagnostic tool to determine where within the blast the problem occurred which resulted in the higher vibration level.

191

Ideally, if one looks at vibration records and assumes that the peaks indicate energy release over time. Common sense would dictate that one would like to see all peaks near equal throughout the entire record. If this would occur, the explosives energy is being used efficiently and is reducing vibration to a minimum. In the past, to expect a vibration record to have near identical peaks would have been considered an academic solution which was not practical in the field, however, today with advanced technology this type of vibration record can be achieved on blasts that are well engineered.

10.4 FACTORS AFFECTING VIBRATION 10.4.1 PRINCIPAL FACTORS There are two principal factors that affect the vibration level that results from detonation of an explosive charge. These are distance and charge size. Common sense indicates that it is safer to be far away from a blast than to be near it. Common sense further indicates that a large explosive charge will be more hazardous than a small charge.

10.4.2 CHARGE - DISTANCE RELATIONSHIP Extensive research has been conducted to determine the mathematical relationship between vibration level, charge size, and distance. The U.S. Bureau of Mines Bulletin 656 (Nichols, Johnson and Duvall, 1971) states such a relationship. The relationship is: (10.4)

where:

v D

= = =

H a b

= = =

w

Predicted particle velocity (in/s) Maximum explosive charge weight per delay (lbs) Distance from shot to sensor measured in 100's of feet (e.g., for distance of 500 feet., 0=5) Particle velocity intercept Charge weight exponent Slope factor exponent

•1•·

= .,el

••1 •• •• •• •• •• •• •• ••

:

r•.

••

•: I

192

•

This is known as the Propagation Law because it shows how the particle velocity changes with distance and explosive charge weight The numerical values for H a and b are slightly different for each component. For the longitudinal or radial component, the law is numerically expressed as:

)·1.63

D V, = 0.052 (

(10.5)

Wo512

Introducing the following approximations: a= 0.512 or 0.5 b =-1.63 or-1.6 Expressing D in feet instead of hundreds of feet produces a simplified approximation for this relationship:

v

= 100 (

Jw

-1.6 )

(10.6)

where:

., '

..

V d W

=

Particle velocity in inches/second Distance from shot to sensor (ft) Maximum explosive charge weight per delay (lbs)

The Dupont Blaster's Handbook (E.I. Dupont de Nemours & Co., 1977) gives the following relationship: ·16 (10. 7)

v

=

160

(

Jw . )

If Metric (SI) units, the US Bureau of Mines equation becomes the following: -16

PV where:

I

"")

714.4

(~ J

.

PV d W

=

= =

millimeters/second meters kilograms

The Dupont equation can be expressed in metric (SI) units as follows:

PV

=

1143 (

,J;; J-1.

6

d

where: PV d W

= = =

millimeters/second meters kilograms

10.4.3 ESTIMATING PARTICLE VELOCITY The formulas enable one to estimate the particle velocity likely to result from the detonation of a given charge weight of explosive at a given distance. Obviously the Dupont formula will give a higher value for the expected particle velocity. From this, it can be seen that these formulas serve merely as guides, and are not meant to give exact numbers. The values of a, b and H are determined by conditions in the area, rock type, local geology, thickness of overburden and other factors. The values of a= 0.5 and b = 1.6 are fairly well fixed. The value of H is highly variable and is influenced by many factors.

10.4.4 VIBRATION CONTROL The operator would like to have a convenient, effective means of vibration control. The formulas just discussed are a means to such control, and have led to the development of other techniques.

194

•• •• •• ••

•• •• •• •• •• •• •• •• •• •• •• •• ••

10.4.4.1 DELAY BLASTING Before discussing these techniques, delay blasting should be considered. With the development of the delay cap; particularly millisecond delays, a method came into play by which a large explosive charge could be detonated as a series of small charges, rather than one large charge. Obviously, the reduction in charge size can be made by the use of multiple delays. For example, the use of ten delays would reduce the effective vibration generating charge to one tenth the original charge. Consider the following example:

~~ I

~ ~

'1

,...,

-

Example 10.1 A shot consists of 40 holes, 120 Kg of explosive per hole with a total charge of 4, 800 Kg and is fired instantaneously. The probable vibration

;

level can be calculated at a distance of 300 meters.

00000 00000 00000 00000 00000 00000 00000 00000 40 Holes Fired Instantaneously ~

v

~

• •""' . ~

-~

-"' ~

I·

30 714.4 ( ~ .y4800

)"1.

MS2 @@@@@ MS1

@@@@@

l

@@@@@

@@@@@

20 Holes Fired Per Delay

]

-I~

300 r;;--;;;;;;

V = 714.4 (

-v2,400

~

~

68.47 mm/ s

@@@@@ @@@@@ @@@@@ @@@@@

"p

'"")

=

This is a dangerously high particle velocity, two delays were introduced to reduce the vibration level. This divided the shot into two series or parts of 20 holes each, with 2,400 Kg. per delay.

~

-·

6

195

=

39.33 mm/ s

If two more delays MS3 and MS4 were introduced, reducing the number of holes per delay to 10 and the charge per delay to 1,200 Kg, the probable parlic/e velocity can be calculated.

MS3 @@@@@

MS1

@@@@@

CD-

0.50 in/sec, , / _____________ _, , ptoster

0: '')

., ·~

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The Office of Surface Mining, in preparing its regulations, modified the Bureau of Mines proposed criteria based on counter proposals that it received and came up with a less stringent standard similar to the Bureau of Mines alternative safe blasting criteria. Recognizing a frequency dependence for vibration associated with distance, the Office of Surface Mining Presented its regulation as follows: TABLE 11.2 OFFICE OF SURFACE MINING, REQUIRED GROUND VIBRATION LIMITS

DISTANCE FROM THE BLASTING SITE (m) 0 to 91 91to1524 1524 and beyond

MAXIMUM ALLOWABLE PEAK PARTICLE VELOCITY {mm/s) 31.8 25.4 19.0

SCALED DISTANCE FACTOR TO BE APPLIED WITHOUT SEISMIC MONITORING 22.7 25 29.5

This table combines the effects of distance and frequency. At short distances, high frequency vibration predominates. At larger distances, the high frequency vibration has attenuated or died out and low frequency vibration predominates. Buildings have low frequency response characteristics and will resonate and may sustain damage. Therefore, at large distances a lower peak particle velocity, 19 mm/s, and a larger scaled distance, Os = 29.5, are mandated. At the shorter distances, a higher peak particle velocity, 31.8 mm/s, and a smaller scaled distance, Os = 22. 7, are permitted. The displacement and velocity values and the frequency ranges over which each applies as specified by the Office of Surface Mining are shown in Figure 11.3. Figure 11.3 also compares the same vibration data to the British, German, and French Standards .

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