drilling and blasting in rocks (sample)

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This work has been totally financed by the G eornining Technological Institute of S pain under contract with the E.F?M., S.A. Cornpany(Estudios y Proyectos Mineros, S.A.).

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Original text:Manual de perforacion y voladura de r ocas

O 1987 Instituto Geologico y Minero d e E spaila

Revised and updated ed ition in English:

O 1995 A.A. Balkerna, PO. Box 167 5,30 00 BR Rotterdarn, Netherland s (Fax: +3 1.10.4135947)

Distributed in USA & Canad a by:A.A. Balkema Publishers, Old Post Road, Brookfield, VT 05036, USA (Fax: 802.276.3837)

Printed in the Netherlands



Contents

FOREWORD

PREFACE

ACKNOWLEDGEMENTS

1 ROCK DRILLING METHODS1.1 Introduction1.2 Types of drilling operations used in rock

breakage1.3 Field s of applica tion for the different drilling

methods1.4 Classification of the rocks and their

principal physical propertiesReferences

2 ROTARY PERCUS SIVE DRILLING2.1 Introduction

2.2 Fundamentals of rotary percussive drilling2.3 Top hammer drilling2.4 Dri lling with down the hole harnmer2.5 Advance systems2.6 Mou nting systems2.7 Dust collectors2.8 Inclination instruments2.9 Penetration rate2.10 Average penetration rate2.1 1 Calculation of drilling costsReferences

3 ROTARY PERCUSS IVE DRILLING

ACCESSORIES3.1 Introduction3.2 Types of threads3.3 Shank adaptors3.4 Dnl l steel3.5 Couplings3.6 Dnl l bits3.7 Calculatio n of the necessary drilling

accessories3.8 Care and maintenance of the bits3.9 Care and maintenance of drill steel3.10 Guide for identifying accessory failure

and its causesReferences

4 ROTARY DRILLING WITH ROLLINGTRICONE BITS4.1 Introduction4.2 Mounting and propulsion systems4.3 Power sources4.4 Rotation systems

4.5 Pulldown /hoisting systems4.6 Mast and pipe changer4.7 Control cabin4.8 System for flushing drill cuttings4.9 Dnl l string4.10 Auxiliary elements4.1 1 Operative practice. Drilling parameters4.12 Penetration rate4.13 Calculation of drilling costsReferences

5 ROLLING CONE ROCK BITS

5.1 Rolling cone rock bits5.2 Major compo nents and design features5.3 The metallurgy of rolling cone rock bits5.4 Types of rolling cone bits5.5 Bit type selection5.6 Effects of the operating parameters on the

rolling cone bits5.7 Nozzle selection5.8 Evaluation of du11 rolling cones5.9 Example of roller iricone bit selection5.10 IDAC CodesReferences

6 ROTARY DRILLING WITH CUTTIN GACTION6.1 Introduction6.2 Fundam entals of drilling with cutting action6.3 Flushing of drill cuttings6.4 Cutting toolsReferences

7 SPECIAL DRILLING METHODS ANDMOLINTING SYSTEM S7.1 Introduction7.2 Drilling through overburden

7.3 Shaft sinking7.4 Raise driving



Contents

7.5 Jet piercing7.6 Water-jet drilling7.7 Drilling ornamental rockReferences

8 COMPRESSORS8.1 Introduction8.2 Types of compressors8.3 Drive8.4 Auxiliary elements8.5 Calculating pressure dropsReferences

9 THERMOCHEM ISTRY OF EXPLOSIVESAND THE DETONATION PROCESS9.1 Introduction9.2 Deflagration and detonation9.3 Detonation process of an explosive9.4 Thermochemistry of the explosives9.5 Heat of explosion9.6 Oxygen balance9.7 Volume of explosion9.8 Minimum energy available9.9 Temperature of the explosion9.10 Pressure of the explosionReferences

10 PROPER TIES OF EXPLOSIVES10.1 Introduction10.2 Strength and energy

10.3 Detonation velocity10.4 Density10.5 Detonation pressure10.6 Stability10.7 Water resistance10.8 Sensitivity10.9 Detonation transmission10.10 Desensitization10.11 Resistance to low temperatures10.12 FumesReferences

11 INDUSTRIAL EXPLOSIVES11.1 Introduction11.2 Dry blasting agents11.3 Slurries11.4 Emulsions11.5 Heavy ANFO11.6 Gelatin dynamites11.7 Granular dynamite11.8 Permissible explosives11.9 Blackpowders11.10 Two-component explosives11.11 Explosives cornmercialized in SpainReferences

12 EXPLOSIV E SELECTION CRITERIA12.1 Introduction

12.2 Explosive cost12.3 Charge diameter12.4 Rock characteristics12.5 Volume of rock to be blasted12.6 Atmospheric conditions

12.7 Presence of water12.8 Environmental problems12.9 Fumes12.10 Safety conditions12.11 Explosive atmospheres12.12 Supply problemsReferences

13 BLASTING ACCESSORIES 12392 13.1 Introduction 12392 13.2 Nonelectric initiation systems 12392 13.3 Electric initiation systems 12793 13.4 Sources of energy 13094 13.5 Other accessories 13294 References 1359595 14 INITIATION AND PRIMING SYSTEMS 13696 14.1 Introduction 13696 14.2 Priming and boostering bulk ANFO-type96 blasting agents 13697 14.3 Priming cartridge ANF O type blasting

agents 13898 14.4 Priming pumped or poured slurry and98 emulsion blasting agents 13998 14.5 Priming cartridged watergel and emulsion

101 blasting agents 140102 14.6 Location of primers 140102 14.7 Priming conventional cartridged102 explosives 143102 References 143102103 15 MECHANIZED SYSTEMS FOR CHARGING104 AND DEWATERING BLASTHOLES 144104 15.1 Introduction 144104 15.2 Mecha nized blasthole charging Systems 144105 15.3 Blasthole dewatenng Systems 152

References 153

106106 16 MECHANISMS OF ROCK BREAKAGE 154106 16.1 Introduction 154110 16.2 Rock breakage mechanisms 154111 16.3 Transmission of the strain wave through113 the rock mass 156115 16.4 Energetic yield of the blastings 157115 References 159116116 17 ROCK AND ROCK MASS PROPERTIES117 AND THEIR INFLLTENCE ON TH E117 RESULTS O F BLASTING117 17.1 Introduction

17.2 Rock properties119 17.3 Properties of the rock mass119 References



Contents V11

18 CHARACTERIZATION OF THE ROCKMASSES FOR BLAST DESIGNING18.1 Introduction18.2 Diamond drilling with core recovery and

geomechanic testing

18.3. Characteristics of the joint systems18.4 Seismic survey18.5 Geophysical techniques to obtain rock

mass data18.6 Logging of production blastholes18.7 Characterization of the rock mass during

blasthole drilling18.8 The attempt to correlate drilling indexes

with the blasting design parameters18.9 System of drilling data management in

actual timeReferences

19 CONTROLLABLE PARAMETERS OFBLASTING19.1 Introduction19.2 Blasthole diameter19.3 Height of bench19.4 Blasthole inclination19.5 Sternrning length19.6 Subdrilling19.7 Burden and spacing19.8 Blastho le patterns19.9 Geometry of the free face19.10 Sizean dshap eof the blast

19.1 1 Available expansion volume19.12 Charge configuration19.13 Decoupling of the charges19.14 Explosives19.15 Distribution of explosives in the

blastholes19.16 Powder factor19.17 Initiation and priming19.18 Delay timing and initiation sequences19.19 Influence of loadinieq uipment on the

design of the blasts19.20 Specific dtilling19.21 Blasthole deviationReferences

20 BENCH BLASTING20.1 Introduction20.2 Small diameter bench blasting20.3 Large diameter blasting20.4 Bench blasting with horizontal blastholes20.5 Rip-rap production blasting20.6 Cast blastingAppendix 1 Formulas to calculate benchblasting patternsReferences

21 BLASTING IN OTHER SURFACEOPERATIONS2 1.1 Introduction2 1.2 Excavations for highways and railways

21.3 Trench blasting 208167 2 1.4 Ramp blasting (sinking cut) 210167 2 1.5 Blasting for ground leveling 212

21.6 Blastings for foundations 213167 21.7 Mini-hole blasting 2 14

167 2 1.8 Preblastings 215170 References 216

170 22 BLA STING FOR TUNNELS AND DRIFTS 217170 22.1 Introduction 217

22.2 Advance systems 217171 22.3 Blasting Patterns for tunnels 218

22.4 Types of cuts and calculation of the blasts 219174 22.5 Equipment for marking out dtilling

patterns 230177 References 230178

23 SHAFT SINKIN G AND RAISE DRIVING 23 123.1 Introduction 23 1

179 23.2 Shaft sinking 23 1179 23.3 Raise driving 232179 References 237181181 24 UNDER GROUND PRODUCTION BLASTiNG182 IN MININ G AND CIVIL ENGINEERING 239182 24.1 Introduction 239183 24.2 Crater retreat method 239183 24.3 Longho le method 243184 24.4 Sublevel stoping with blastholes in fan185 pattern 245

186 24.5 Room and pillar mining 248186 24.6 Cut and fill mining 248186 24.7 Underground chambers in civil187 engineering projects 249

References 25 1187188 25 CONTOUR BLASTiNG 252188 25.1 Introduction 252188 25.2 Mechan isms responsable for overbreak 252

25.3 The theory of contour blasting 253189 25.4 Types of contour blasts 254189 25.5 Th e parameters that intervene in a190 contour blasting 256190 25.6 Tendencies in the field of contour blasting 264

25.7 Evaluation of the results 267191 25.8 Exarnple 268191 25.9 Extrac tion of ornamental rock with191 contour blasting 268193 References 270195195 26 UNDERWATER BLASTiNG 272196 26.1 Introduction 272

26.2 Methods of execution 272199 26.3 Calculations for charges and drilling203 patterns 247

26.4 Charg ing the blastholes and primingsystems 275

205 26.5 Types of explosives 276205 26.6 Env ironmental effects associated with205 underwater blastings 276



Contents V11

18 CHARACTERIZATION OF TH E ROCKMASSES FOR BLAST DESIGNIN G18.1 Introduction18.2 Diamond drilling with core recovery and

geomechanic testing18.3-

Characteristics of the joint system s18.4 Seismic survey18.5 Geophysical techniques to obtain rock

mass data18.6 Logging of production blastholes18.7 Characterization of the rock mass during

blasthole drilling18.8 The attempt to correlate drilling indexes

with the blasting design parameters18.9 System of drilling data management in

actual timeReferences

19 CONTROLLABLE PARAMETERS OFBLASTING19.1 Introduction19.2 Blasthole diameter19.3 Height of bench19.4 Blasthole inclination19.5 Sternming length19.6 Subdrilling19.7 Burden and spacing19.8 Blasthole patterns19.9 Geometry of the free face19.10 Size and shape of the blast

19.11 Available expansion volume19.12 Charge configuration19.13 Decoupling of the charges19.14 Explosives19.15 Distribution of explosives in the

blastholes19.16 Powder factor19.17 Initiation and prirning19.18 Delay timing and initiation sequences19.19 Influence of loadingequipment on the

design of the blasts19.20 Specific drilling19.21 Blasthole deviation

References

20 BENCH BLASTING20.1 Introduction20.2 Small diameter bench blasting20.3 Large diameter blasting20.4 Bench blasting with horizontal blastholes20.5 Rip-rap production blasting20.6 Cast blastingAppendix 1 Formulas to calculate benchblasting patternsReferences

21 BLASTING IN OTHER SURFACEOPERATIONS21.1 Introduction21.2 Excavations for highways and railways

21.3 Trench blasting 208167 21.4 Ramp blasting (sinking cut) 210167 21.5 Blasting for ground leveling 212

21.6 Blastings for foundations 213167 21.7 Mini-ho le blasting 214

167 2 1.8 Preblastings 215170 References 216

170 22 BLASTIN G FOR TUNNELS AND DRIFTS 217170 22.1 Introduction 217

22.2 Advance systems 217171 22.3 Blas ting Patterns for tunnels 218

22.4 Types of cuts and calculation of the blasts 219174 22.5 Equipm ent for marking out drilling

patterns 230177 References 230178

23 SHAFT SINKING AND M I S E DRIVING 23 1

23.1 Introduction 23 1179 23.2 Shaft sinking 23 1179 23.3 Raise driving 232179 References 237181181 24 UNDERGROU ND PRODUCTION BLASTING182 IN MIN ING AND CIVIL ENGINEERING 239182 24.1 Introduction 239183 24.2 Crater retreat method 239183 24.3 Longhole method 243184 24.4 Sublevel stoping with blastholes in fan185 pattern 245

186 24.5 Room and pillar mining 248186 24.6 Cut and fill mining 248186 24.7 Underground chambers in civil187 engineering projects 249

References 25 1187188 25 CONTOUR BLASTING 252188 25.1 Introduction 252188 25.2 Mech anisms responsable for overbreak 252

25.3 The theory of contour blasting 253189 25.4 Types of conto ur blasts 254189 25.5 Th e parameters that intervene in a190 contour blasting 256

190 25.6 Tendencies in the field of contour blasting 26425.7 Eva luatio n of the results 267

191 25.8 Exam ple 268191 25.9 Extraction of ornamental rock with191 contour blasting 268193 References 270195195 26 UNDERW ATER BLASTING 272196 26.1 Introduction 272

26.2 Metho ds of execution 272199 26.3 Calculations for charges and drilling203 patterns 247

26.4 Charging the blastholes and primingsystems 275

205 26.5 Types of explosives 276205 26.6 Environm ental effects associated with205 underwater blastings 276



Forew ord

During the past two decades, there have been numeroustechnical contributions which have brought a better un-derstanding of rock fragmentation with explosives, animprovement in drilling equipment and a noticeable evo-lution in the development of new explosives and blastingaccessones. The Geomining Technological Institute ofSpain (ITGE), aware of this Progress and of the impor-tance which the breakage process has acquired in miningand civil engineering projects, has considered the publi-cation of a 'Rock Drilling and Blasting Handbook' ofgreat interest.

This handbook was conceived with integration inmind, as the Systems and machines of drilling, the typesand characteristics of explosives and the methods forcalculating the blasts are treated together, without everforgetting that these breakage operations form part of a

macrosystem and that the results obtained by them in-fluence the production and economy of the whole exploi-tationor construction process. At the Same time, theobjectives and contents of this handbook contribute toimproved safety in mining.

There are very few similar works in other languages,and certainly none other in Spanish.

We sincerely hope that this handbook, which bringstogether practical and theoretical aspects, will be of use toall engineers who work with drilling and blasting as arock breakage method.

Camilo Caride deLiiianDirector of the Geomining Technological Institute ofSpain



Preface

Rock breakage with explosives has existed since theXVII cen tury when black powder came into use in min-ing, rapidly becoming one of the most popular methods.The important historical events which have m arked an erawere the invention of dynamite by Alfred Nobel in 1867,

the use ofANFO

starting in 1955, the developm ent ofslu mes from the late fifties on and, lastly, the preparationof blasting agents such as emulsions, heavy ANFO, etc.,which are still in evolution.

At the Same time, blasthole drilling progressed withsuch decisive events as the the use of com pressed air asthe source of energy in rotary percussive rigs in 1861, theuse of larg e rotary drills and of down-the-h ole hammersin the fifties and the development of hydraulic hammersin the late seven ties.

However, rock blasting was always considered, untilrecently, as an ar t bom from the skill and experience ofthe blasters. Now it has become a technique based on

scientific principles derived from k nowledge of the ac-tion of explosives, the mechanisms of breakage and thegeomech anic properties of the rock m asses.

The purpose of this handbook is to give basic knowl-edge of the drilling Systems, the types of av ailab le explo-sives and accessaries and the Parameters that intervene inblast designing , whether controllable or not.

The handbook is primarily meant for studen ts of theTechnical Schools, to be useq as a textbook, and for allprofessionals who are involved with explosives in miningoperations and civil engineenng projects.

'

This handbook was written by the following engineers:

Carlos Lopez Jimeno, Doctor of Mining Engineering,Project Director for EPM., S.A.

Emilio Lopez Jim eno, Doctor of M ining Engineering.

Francisco Javier Ayala Carcedo, Doctor of Mining Engi-neering, Project Director for ITGE.

Translated by: Yvonne Visser de Ramiro

This work has been totally financed by the GeominingTechnological Institute of Spain under contract with theEPM, S.A. Company (Estudios y Proyectos Mineros,S.A.).

Carlos and Emilio Lopez Jimeno



Acknowledgements

The authors wish to exp ress their mo st sincere gratitude to the following experts, com panies and official organisms fortheir collaboration and release of tech nical material as well as perm ission to reproduce certain data and figures.

Am erind-M ackissic, Inc.: G. J. Kno ttsAmos L . Dolby Co.: J. PetrunyakApp1ex:S.O. OlofssonAtlas Copco S.A.E.: E MenendezAtlas Powder Company: VA. Sterner, L. Osen &

PM . MillerAtlas Powd er International: J. Garcia M illaBauer, Calder & Workman, Inc.: J.L. Workman &

A. Bauer (T)

Bill Lane Inc.: W.C. LaneBlasting &Minin g Consultants, Inc.: J. Ludw iczakBucy rys Erie Co.: J.D. Nelma rk & G. RekoskeBendesanstalt für Geowissenschaften und Rohstdffe:

R. LüdelingCanmet: G. Larocque

Ci1 Inc.: S. Chung, B. Mohanty, K.C. Joyce, PR. Day,W.K. Webster, D. Dayphina is, I. Huss & K.R. Sharpe

Com inco Ltd.: W Russe11Crow snest Resources Ltd.: R.A. R eipasDavid , S. Robertson & Assoc iates Inc.: C. DavenportDupo nt Canada: D. TanseyE. I. Du Pont De Nem ours& Co.: P D. Porter, B. L. Glenn ,

J. R. Knudson & A. B. AndrewsEntrecan ales y Tavora, S.A.: J. AznarGardner Denver Mining and konstruction GroupGeovanca: R. UcarGolde r Associates: T.N. Hagan, E. Hoek & Guy Le BellHullera Vasco Leonesa: E. C astellsHydro-Quebec: F LacomteIberd uero , S.A.: J. ForaICI Australia Operations Pty Ltd.: G. Harries,

J. K. Mercer& G.G. PaineIlmeg: S. Johanss onIngersoll RandInstituto Tecnologico Geominero de Espaiia: E J. Ayala& M. Abad

Instituto Superior Tecnico de Lisboa: C . Dinis Da GamaIreco Can ada Inc.: L. de CouteurIrish Industrial Explosives, Ltd.: J. P Higg insJulius Kruttschnitt Mineral Research Centre, University

of Quee nsland: C. K. Mc kenzie& K. E. MathewsKaiser Eng ineers, Inc.: G.V. Borqu ez

Kemira OyKenneth Mede aris Associates: K. Med earisKometa Oy: R. IkolaKontinitro A.G.L.C. Lang & Assoc iates, Inc.: L.C.LangLew is L. Oriard, Inc.: L .L.OriardLKAB: L. HermanssonMartin Marietta Laboratories: D.A. Anderson &

S. R. WinzerMcGill University: R.E Favreau, R.R. MacLachlan,

W. Comeau & J.C. Leighto nMich igan Techno logical Unive rsity: F.O. O tuonyeNew Jersey Institu te of Technology: W. KononNitro Consu lt, A.B.: I. HanssonNitro Nobel AB: B. Larsson, PA . Persson, M. Landberg

& G. LandeNobe l's Explo sives Company Lim ited: M. J. BallThe Norwegian Institute of T echnology: K. NielsenThe Ohio State University: R.G. LundquistOy ForcitPalabora Mining Co.: G . P FauquierPetromin: \!CobeiiaPrecision Blasting S ervices: C.J. KonyaQue en's U niversity: P N. Calde rReed M ining Tools, Inc.: M. SuiirezRichard L. Ash & Assoc iates: R. L. AshRietspru it Mining Co.: K. I. Macdo naldSocieta Esplosivi Industriali S.PA.: G. Calarco &

G. BertaStrornrne : A. M. HeltzenThermex Energy Corporation: R.C. Paddock

T Peal, S.A.: J. Alonso & R.ArnaizUnion Espaiiola de Explosivos: R. BlancoUniv ersity of Misso uri Rolla: P N. Worsey, R. R. Rollins& N.S. Smith

U.S. Bureau of Mines Twin Cities. Research Center:L. R. Fletcher

At the Same time we would also like to acknow ledge thedrawings and photography done by Jose Maria de Salas

and the corrections mad e by Carlos Ram iro Visser.



CHAPTER 1

Rock drilling methods

1.1 INTRODUCTION

Rock drilling, in the field of blasting, is the first operation

carried out and its purpose is to Open holes, with the

adequate geometry and distribution within the rock mas-

ses, where the explosive charges will be placed alongwith their initiating devices.

The systems of rock drilling that have been developed

and classified according to their order of present day

applicability are:- Mechanical: Percussion, rotary, rotary-percussion.- Themzal: Flame, plasma, hot fluid, Freezing.- Hydraulic: Jet, erosion, cavitation.- Sonic: High frequency vibration.- Chemical: microblast, dissolution.- Electrical: Electric arc, magnetic induction.- Seismic: Laser ray.- Nuclear: Fusion, fission.

Even though there is an enormous variety of possible

rock drilling systems, in mining and civil engineering

drilling is presently canied out, almost exclusively, by

mechanical energy. Therefore, in this handbook only the

mechanical means will be discussed, reviewing the fun-

dalmentals, tools and equipment for each of them.

The main components of a drilling system of this type

are: the drilling rig which is the source of mechanical

energy, the drill steel which is,the means of transmitting

that energy, the bit which is the tool that exercises that

energy upon the rock, and the flushing air that cleans out

and evacuates the drilling cuttings and waste produced.

1.2 TYPES OF DRILLING OPERATIONS USED IN

ROCK BREAKAGE

Within the large variety of excavations using explosives,

numerous machines have been developed which can be

classified in two types of drilling procedures:- Manual drillin g. This is canied out with light equip-

ment that is hand held by the drillers. It is used in small

operations where, due to the size, other machinery cannot

be used or its cost is not justified.- Mechanized drilling. The drilling equipment is

mounted upon rigs with which the Operator can control all

drilling Parameters from a comfortable position. These

structures or chasis can themselves be mounted on

wheels or tracks and either be self-propelled or towable.

On the other hand, the types of work, in surface as well

as in underground operations, can be classified in the

following groups:- Bench drilling. This is the best method for rock

blasting as a free face is available for the projection of

material and it allows work tobe systemized.

It is used in surface projects as well as in undergroundoperations, usually with vertical blastholes, although

horizontal holes can be drilled on occasion.- Drilling fordrifting and tunnelling. An initial cavity

or cut must be opened towards which the rest of the

fragmented rock from the other charges is directed. Blast-

hole drilling can be carried out with hand held drills, but

the trend is towards total mechanization, using jumbos

with one or various booms.- Production drilling. This term is used in rnining

operations, fundamentally underground, to describe the

labors of ore extraction. The equipment and methods

used vaq with the exploitation systems, having the com-

mon factor of little available space in the drifts for blast-

hole drilling.- Drilling for raises. In many underground and civil

engineering projects it is necessary to Open raises. Al-

though there is a tendency to apply the Raise Bonng

method, still today the long blasthole method is used as

well as other special drilling systems combined with

blasting.- Drilling rocks with overburden. The drilling of rock

masses which are covered with beds of unconsolidated

materials calls for special drilling methods with casing.

This method is also used in underwater operations.

- Rock supports. In many underground operations andsometimes in surface ones it is necessary to support the

rocks by means of bolting or cementing cables, in which

drilling is the first phase.

1.3 FiELDS OF APPLICATION FOR THEDIFFERENT DRILLING METHODS

The two most used mechanical drilling methods are

rotary-percussion and rotary.- Rotary-percussive methods. These are the most fre-

quently used in all types of rocks, the top harnrner as well

as the down-the-hole hammer.- Rotary methods. These are subdivided into two

groups, depending upon if the penetration is canied out

by crushing, with tricones or by cut with drag bits. The

first system is used in medium to hard rocks, and the

second in soft rocks.



2 Drilling and blas ing of rocks

By taking into account the compressive strength of therock s and the drilling diam eter, the fields of application of

the different methods can be defined as refiected in F ig.

1 . 1 .

On the other hand, depending upon the type of m ining

or civil engineenng surface project, the most comrnonequipment and diameters for bench blastings are indi-

cated in Fig. 1.2.

In the Same manner, the most frequently used equip-

ment for the different underground mining method s andthe characten stic drilling data are indicated in Fig. 1.3.

Other criteria to be accounted for in the selection of

drilling equipm ent are: cost, mechanical design, m ainte-

nance and semice, operative capacity, adaptability toequipment of the exploitation, and the work area condi-

tions (accessability, type of rock, sources of energy , etc.).

DOWN TUE HOLE

HUO HEL0 DRLLS

APLlCATlONRANGE

DIAMETERI "OLE

(Inch)

mOOUCTIOH.-1HO N 1mGE SCUE. W A C EWFig. 1 . 1 Fields of application for drilling

methods as function of the compressive

I " 11/2" 2" 3" 3 1/2" 5" 6" 9,' 12" 15" I

METHODS OF

BENCHBLASTING

L

1 ROTARY PERCUSSIVE DRILLING I ROTARY DRILLING I

L W E HEAW B

4 Wstrength of the m cks and the diameters of

the blastholes.

DOWN THE1 1 HOLE HAMMER I CRUSHING 1 CUTTMG I

CONSTRUCTION

WORK

SURFACEMlNlNG

I

180-200 mm

no t "."aI)

(80-200 mm .

no t "sunll

Fig. 1.2 Drilling methods for surface ope-rations (Atlas Copc o).



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- s h 3 a 8 ~e l 3 q ~! 'hrqAdiod paieu!wouap s! ase3 aiw p-a w al u! u v 'aiqd e palle3 s! i! puo3as aqi U! p m ' p a w o js! al!ieui8ad palp3 3301 e a3mlsu! i s ~ y ql UI .p!de~s!l! J! lleuis pue '~ 0 1 s! q j! a8 nl fs le~ aup I az!~le]shr~qijo azrs u ~ 1 8ql eui8e w aql jo 8u!loo3jo aleJ aqL

-330~noau813!uoinldJO aA!sniiu! s m o j 11 V a q o a ~ e p n s qi qieawapuns loo3 eu i8eu i aq i u a m .y3o ~ noau81 ~ F E ~ I O AO aA!s

-rulxa se pay!ssep s! q a ~ e p n s,q m a aq l ur013 JaleM JOJ? qirm I ~ E I U O ~! paloo 3 si3q 3301aq ljI -1euaieuisi38 pue3301paAloss!pjo a ~ n l x y s! q 3 !q ~ eui8eui palle3 'ssewuaqoui e w o g uo!le3g!p!los Aq p a w o j a n s y 3 o ~noau81

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.sa!vado~d3!seq qaqlpue s leualeu i jo sadAl ~ua~a j j !p q ~ o u y1 'uo!le~ado

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SBILXEldOXd 7V3ISA Hd 7 Vd I3N Iüd M BHLCINV SX 30X BH L d0 NOILV3HISSV73 9'1

5 1 - 5 1 S Z - S I C L - B i 6 0 - 1 0 6 0 - 1 0 P I - 0 1 L I - 6 0 0 9 - o c o ~ - ~ i 1 3 ~ ~ ~ ~ a 3 1 1 1 8 a ~

N 0 1 1 3 ~ O O t

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4 Drilling and blasting of rocks

the same sense that igneous rocks are poorer in silica,they are richer in ferromagnesian silicates. The acids aremore abrasive and harder than the basic ones, but they arealso more dense and resistant to impact.

1.4.1.2 Metamorphic rocksMetamorphic rocks are derived from other pre-existingendogenic or exogenic rocks through important transfor-mations of their mineral components. These markedchanges are produced by the necessity of stabilizing theirminerals under the new conditions of temperature, pres-sure and chemism.

These rocks are intermediate in physicai and chemicalcharacteristics, between the igneous and the sedimentary,because they have associations of minerais that pertain tothe two types. Thus, minerals such as quartz, feldspars,rnicas, amphiboles, and olivines, essential in igneous

rocks, are also found in metamorphic rocks; howeverthey do not contain aikali feldspars. As in sedimentaryrocks, they can have calcite, dolomite, silica and he-matites; but they do not contain evaporites. Mineralscomrnon to the two other types also appear such astourmaline, zircon, magnetite, topaz and corundum; all ofwhich are very stable in any exogenous or endogenousmedium.

There is a series of minerals that are very specific tometamorphic rocks, which can form part of the grains ofdetrital rocks, owing to their stability in exogenous me-dium~,nd others are at the same time products of meteo-ric alteration of the minerals in endogenic rocks. Actually,

meteorization is a mineralogical transformation that isboth a physical and chemical process, but at low tempera-ture and pressure.

1.4.1.3 Sedimentary rocksSedimentary rocks are formed by accumulation of brokenand decomposed rock material, by chemical precipitationof solubilized minerals or by accumulation of shells orother organic material: animal or vegetable.

In the first case, detritic sediments are produced such asgravels, conglomerates or sands in which gravity hasplayed a role in their precipitation. In the second case one

Fig. 1.4. Geological cycle of rocks.

Table 1.1

Classification Mohs' scale of Compressivehardness strength(MPa)

Very hard +7 +200Hard 6-7 120-200

Medium hard 4.5-6 60- 120Medium soft 3-4.5 30-60Soft 2-3 10-30

Very soft 1-2 -10

can find, as an example, the evaporites or saline rocksprecipitated by over-saturation of a brine that is subjectedto intense evaporation. The third type are accumulationsof shells, skeletons of animals or remains of plants, suchas the conchiferous limestones. This last group is subdi-vided into organogenous biochemistry and mineralbiochemistry depending upon whether their components

are of organic or inorganic nature. For the first we havecoal and petroleum, and for the second the limestones,dolornites and phosphatic rocks.

For an initial classification of sedimentary rocks, theirformation process is taken into account, later the grainsize, the characteristics of their bonding, apart from thetypes and quantities of their rninerai components.

1.4.2 Rock properties that affect drilling

The principal physical rock properties that have influenceupon penetration mechanisms and, as a consequence, onchoice of the drilling method are: hardness, strength,

elasticity, plasticity, abrasiveness, texture, structure, char-acteristics of breakage.

1.4.2.1 HardnessHardness is considered to be the resistance of a surfacelayer to be penetrated by another body of harder con-sistency.

In rock, it is a function of the hardness and compositionof its mineral grains, the porosity, degree of humidity,etc. E

The hardness of rocks is the principal type of resistancethat must be overcome during drilling, because once the

bit has penetrated, the rest of the operation is easier.Rocks are classified as to their hardness by usingFriedrich von Mohs' Scale of Hardness (1882), in whichthe concept is that any mineral can scratch anything thathas a lower or equai number to it, numbering from 1 to 10.As can be seen from Table 1.1, there is a certain correla-tion between hardness and compressive strength of therocks.

1.4.2.2 StrengthMechanical strength of a rock is the property of opposingdestruction by an extemal force, either static or dynarnic.

The rocks give maximum resistance to compression,

normally, as the tensile strength is not more than 10 or15% of the compressive strength. This is due to thefragility of rocks, to the large quantity of local defects andirregularities that exist and to the small cohesion betweenthe particles of which they are constituted.



Rock drilling rnethods 5

The rock strength fundam entally depends on its mine-ralogical composition. Among the integrating minerals,quartz is the mo st solid with a strength that goes over 500MPa, w hile that of the ferromagnesian silicates and thealuminosilicates vary between 20 0 and 500 MP a, and thatof calcite from 10 to 20 M Pa. Th erefore, the higher the

quartz content, the mo re the strength increases.The mineral strength depends upon the size of the

crystals and diminishes with their increase. Thisinfluence is significative when the cry stal size is under 0.5mm .

In rocks, the size factor has less influence on strengthas the intercry stalline cohesio n force also intervenes. Forexample, the compressive strength of a fine grainedarkose sandstone is almost double that of a coarsegrained; that of marble composed of 1 rnrn graines isequal to 100 MP a, whereas a fine grained limes tone- to4 mm - as a strength of 200 to 250 M Pa.

Amo ngst the sedim entary rocks the ones with higheststrength are those that contain silica cement. With thepresence of clay cement, the strength is drasticallyreduced.

Porosity in roc ks with the Same lithology also reducesstrength proportionately, more po rosity- ess streng th; asit simultaneously reduces the number of contacts of themineral particles and the force of reciprocal action be-tween them.

The depth at w hich rocks were forme d arid the degreeof metam orphism also have influence upon their strength.Therefore, the strength of clay beddings near the groun dsurface can be of 2 to 10 MP a, whereas in clay rocks that

went through a certain metamorphism the strengths canreach 50 to 100 MPa.

On the other hand, the strength of ansiotropic rocksdepends upon the sense of action of the force. The com-pressive strength of rocks in the perpendicular to

stratification sense or schistosity is larger than in a paral-lel sense. The q uotient that is usually ob tained betweenboth strength values varies between 0.3 and 0.8, and it isequal to 1 only for isotropic rocks.

In Fig. 1.5, the most frequent compressive strengths

for different types of rock are indicated.

1.4.2.3 Elasticity

The majonty of rock minerals have an elastic-fragilebehavior, which obeys the Law of Hooke, and are des-troyed w hen the strains exceed the limit of elasticity.

Depending upon the nature of de forma tion, as functionof the Stresses produ ced by sta tic charges, three groups ofrocks are taken into con sideration: 1) The elastic-fragileor those which obey the Law of Hooke, 2) The plastic-fragile, that hav e plastic deformation before destruction,3) The highly plastic or very po rous, in which the elastic

deformation is insignificant.The elas tic properties of rocks are characte nzed by theelasticity m odule 'E' and the Poisson coefficient ' V ' . Th eelasticity modu le is the proportionality factor between thenormal Stress in the rock and the relative correspondantdeform ation, its value in most rocks varies between 0 .03X 104 and 1.7 X 1o5 MP a, basically depending upon themineralogical comp osition, porosity, type of deformationand magnitud of the applied force.

The values of the elasticity modules in the majority ofsedimentary rocks are lower than those corresponding tothe minerals in their com position. The texture of the rockalso has influ ence on this Parameter, as the elasticity

module in the direction of the bedding or schistosity isusually larger than when perpendicular.

Poisson's coefficient is the factor of proportionalitybetween the relative longitudinal deformations and thetransversal deformations. For m ost rocks and minerals itis between 0.2 and 0.4, and only in quartz is it abnon nallylow, aroun d 0.07.

0 I 0 20 30 40 60 SO

DEFORMATION (mm x 108)

Fig. 1.6.Curves of stress-deformation or different types of rocks.



6 Drilling und blasting of rocks

1.4.2.4 PlasticityAs indicated before, in some rocks the plastic deforma-

tion preceeds destruction. This begins when the Stresses

exceed the limit of elasticity. In the case of an ideally

plastic body, that deformation is developed with an inva-

riable stress. Real rocks are deformed and consolidated at

the Same time: in order to increase the plastic deformation

it is necessary to increase the effort.

The plasticity depends upon the mineral composition

of the rocks and diminishes with an increase in quartz

content, feldspar and other hard minerals. The humid

clays and some homogeneous rocks have plastic proper-

ties.

The plasticity of the stony rocks (granites, schistoses,

crystallines and sandstones) becomes noticeable

especially at high temperatures.

1.4.2.5 AbrasivenessAbrasiveness is the capacity of the rocks to wear away the

contact surface of another body that is harder, in the

rubbing or abrasive process during movement.

The factors that enhance abrasive capacities of rocks

are the following:

- The hardness of the grains of the rock. The rocks

that contain quartz grains are highly abrasive.

- The shape of the grains. Those that are angular are

more abrasive than the round ones.

- The size of the grains.

- The porosity of the rock. It gives rough contact

surfaces with local stress concentrations.- The heterogeneity. Polymineral rocks, although

these are equally hard, are more abrasive because they

leave rough surfaces with hard grains as, for exarnple,

quartz grains in a granite.

This property has great influence upon the life of drill

steel and bits.

In Table 1.2, the mean arnounts of quartz for different

types of rock are indicated.

Table 1.2

Rock ype Quartz content %

Amphibolite

AnorthositeDiabase

DioriteGabbroGneiss

Granite

Greywacke

Limestone

MarbleMica gneissMica schist

Nori te

Pegmatite

PhylliteQuartzite

Sandstone

Shale

SlateTaconi e

1.4.2.6 TextureThe texture of a rock refers to the structure of the grains of

minerals that constitute it. The size of the grains are an

indication, as well as their shape, porosity etc. All these

aspects have significative influence on drilling perfor-

mance.

When the grains have a lenticular shape, as in a schist,

drilling is more difficult than when they are round, as in a

sandstone.

The type of material that makes up the rock matrix and

unites the mineral grains also has an important influence.

As to porosity, those rocks that have low density and,

consequently, are more porous, have low crushing

strength and are easier to drill.

In Table 1.3 the classification of some types of rocks is

shown, with their silica content and grain size.

Table 1.3. Cornmon rock names and their geological definitions (based on Dearm an, 1974; SRM, 198la).

W k rw p

m n

s.dh.mY Wtamorphk I Ign-

Ba4d.d F U i M

C ~ I U I -

mcLs

Sdlne

h.lll&~hydrlta.

c n ~

Chml

nlnt

OnlnQralm srr oi ra k qurm..In' T*minIaldipu. and sl.y mlmu

mm

V W Gnlna arr oi mcL ir.pmnti

M

M8uhrJOhl l .d

G l o r l

60* anIn* anO' uttonar*

rudlls

2 .

&M. ~ d i p . nmw .dcuhrdrrk mlmnh

Omln:

. n - T w ~oi p r u u l u a d

n i i ; y d m

Fr.pmnid (d.mW ~ n l n i )

60% onlns ui 1fintgrrlnedIpneau. rock

G n b a ZJ~ . 15%n~a d k i id1midap.r mpmu

om

S(itstrns:50* nncpnlind0

f i r n z o CI. o n s o r ~ n ~praind *drnd"

QR~&

Cryniillnr or o l y ~ l l l n )

Volunk 0lP.r:ob.idin. pltchslo(~. rhyutr

D9ndiP'mt -

o-

C i 2

lall*(üi.L)

m " ~ p n i m :bmala

pndnon:gnh r inmJnPlm l w d h.prnrm

fl0und.dprslna:T

~ Z a rgralm:

voIc.nIs

I b'"'

-.un

AJk .7HlWWU,UPl02YS

~ p l r , r o ~ r n p t y a s m ~

uorit-mhd mim rb inq u ak l d s w .da.

w d

B-

M a

~ m i i b

in d Iak&mdIkm

lnt.m.dim

E

Mlcmdlcdiok-lt.

m l n n h

8.lk

DOM.



Rock drilling methods 7

Table 1.4. Properties of rock types according to origin-based

classification.

Rock type Specific Grain Swell Compressivegravity size factor strength

(m3) (mm) (MPa*)

Tntru- Diorite 2.65-2.85 1.5-3 1.5 170-300.-.-

sive Gabbro 2.85-3.2 2

INGENOUS Granite 2.7 0.1-2

Andesite 2.7 0.1

Extrusive Basalt 2.8 0.1

Rhyolite 2.7 0.1

Trachyte 2.7 0.1

Congomerate 2.6 2

Sandstone 2.5 0.1-1

SEDIMEN- Shale 2.7 1

TARYDolomite 2.7 1-2

Limestone 2.6 1-2

Limerock 1.5-2.6 1-2

Gneiss 2.7 2

Marble 2.7 0.1-2

METAMOR- Quartzite 2.7 0.1-1

PHICSchist 2.7 0.1-L

Serpentine 2.6 -Slate 2.7 0. L

* 1 MPa = 1 MN/^^ = 10 kg/cm2 = 142.2 psi

In Table 1.4, the cha racteristic properties of differenttypes of rocks are indic ated, according to their origin.

1.4.2.7 Structure

The stmctural properties of the rock masses, such asschistosity, bedding planes, joints, diabases and faults, aswell as their dip and strike affect the allignment of theblastholes, the drilling performance and the stability ofthe blastho le walls.

SPACING OF JOINTS lOOOcm 105 10 1 0.1

A) STRONG ROCKB) MEDIUM ROCKC) WEAK ROCKD) VERY WEAK ROCK

Fig. 1.7. Classification of the rock masses.

In Fig. 1.7, the rock masses are classified from thespacing between joints and the strength of the r o c hmaterial.

REFERENCES

Atlas Copco: Manual atlas copco, 4th edition. 1984.

Heinz, W.F.: Diamond drilling handboo k. 1989.

Hunt, R.E.: Geotechnical engineeßng techniques und pracdces.

McGraw Hill. 1986.

Sandvik-Coromant:Manual de perforacibn de ro cas. Teoriay tkcnica.

1983.

Tamrock: Handbook of surface drilling. 1989.



CHAPTER 2

Rotary percussive diilling

2.1 INTRODUCTION

Drilling by rotary percussion is the most classic system

for drilling blastholes, and its chronological appearance

coincides with the industrial development of the nin-

teenth century. The first Prototype machines made bySinger (1838) and Couch (1848) were run by steam, but it

was when compressed air was used as the source of

energy, in the execution of the tunnel of Mont Cenis in

1861, that this system evolved and was put into extensive

use. This event, along with the arrival of dynamite, was

decisive in the rapid development of rock breakage in

mining and civil engineering at the end of the last cen-

t ury.

The drilling pnnciple of these rigs is based upon the

impact of a steel piece (piston) that hits a utensil which

transrnits at the Same time that energy to the bottom of the

blasthole by means of the final element called the bit. The

rotary percussive rigs are classified in two large groups,

depending upon where the hammer is located:- Top hammer.In these drills, two of the basic actions,

rotation and percussion, are produced outside the blast-

hole, and are transmitted by the shank adaptor and the

dnll steel to the dnll bit. The hamrners can be driven

hydraulically or pneumatically.- Down the hole hammer.The percussion is delivered

directly to the drill bit, whereas the rotation is performed

outside the hole. The piston is driven pneumatically,

while the rotation can be hydraulic or pneumatic.

Depending upon the fields of application of these

drilling ngs, surface or underground, the most comrnonrange of diameters are shown in Table 2.1.

The main advantages of rotary percussive dnlling are:- It can be applied to any type of rock, from soft to

hard.- Wide range of diameters;- Versatile equipment, it adapts well to different ope-

rations and is very Mobile;- Only requires one Operator;- Easy, quick maintenance, and- The capital cost is not high.

In view of these advantages and characteristics, the

type of operations where it is used are:- Underground civil engineering; tunnels, under-

ground hydraulic plants, residual deposits, etc., and in

surface operations; roads, highways, indusirial excava-

tions, etc.

- In underground mines and in small to medium sized

surface operations.

2.2 FUNDAMENTALS OF ROTARY PERCUSSIVE

DRILLING

Rotary percussion drilling is based upon the combination

of the following:- Percussion. The impacts produced by repeated

blows of the piston generate shock waves that are tran-

smitted to the bit through the drill steel (in top harnmer) or

directly upon it (down the hole).- Rotation. With this movement, the bit is turned so

that the impacts are produced on the rock in different

positions.- Feed, or thrust load. In order to maintain the contact

of the dnll bit with the rock, a thrust load or feed force is

applied to the drill siring.- Flushing. aushing removes the drill cuttings from

the blasthole.

The indentation forming process with which penetra-

tion is achieved in this drilling system is divided into five

times, as indicated in Fig. 2.2.

a) Crushing of the rough edges of the rock upon bit

contact.

b) Radial cracks appear from the points of Stress con-

centration and aV shaped wedge is formed.

C) The rock of the wedge is pulverized.

d) The larger fragments are chipped in the zones next

to the wedge.e) The drill cuttings are flushed away.

This sequence repeats itself with the Same impact

rhythrn of the piston upon the system of energy transmis-

sion to the bit.

The yield of this process increases proportionally with

the size of the rock chippings.

2.2.1 Percussion

The kinetic energy E, of the piston is transmitted from the

hammer to the drill bit, through the dnll steel, in the form

of a shock wave. The wave travels at high speed and its

shape depends basically on the design of the piston.

When the shock wave reaches the drill bit, part of the

energy is transformed into work, causing the bit to pene-

trate, and the rest is reflected and returns through the drill

steel. The efficiency of this transmission is difficult to



Rotary percussive drilling

Tahle 2.1 .

Drilling method Drilling diameter (mm)Surface Underground

Top hammer 5 0- 127 38- 65Down the hole 75 -2 00 100- 165

ROTATION

FEED FORCE

Fig. 2. 1. Basic actions in rotary percussive drilling.

Fig. 2.2. Sequence of rock failure during Center formation (Hartman,

1959).

evaluate as it depends upon many factors such as: type ofrock, shape and size of piston, drill steel characteristics,bit design, etc. Another thing to take into account is thatenergy is lost through the sleeves of the rod couplings,due to reflection and fricton which is converted into heatand wear on the drill steel threads. In the first coupling thelosses oscillate between 8 and 10%of the shock wave

energy.In down the hole drilling the piston energy is tran-

srnitted directly to the bit, giving greater performance.In these drilling Systems, percussion force is the pa-

rameter that most influences the penetration rate.The energy freed per hammer stroke can be estimated

from the following equations:

where: m, = Mass of the piston, V = Maximum pistonspeed,p, = Pressure of the work ffuid (oil or air) insidethe cylinder, A, = Surface area of the piston face,I, = Stroke of the piston.

In the majority of hydraulic hammers, the manufac-turers indicate the impact energy value, but this is not thecase with the pneumatic hammers. Special care should betaken in estimating the p, for these, as it is 30 to 40%

lower in the cylinder than in the compressor, owing tocharging and expansion losses of air with each stroke ofthe piston.

Thus, the hamrner power is the energy per strokemultiplied by the frequency of strokesn,:

. .and taking into account the previous equations, the fol-

lowing can be stated:

The percussion mechanism consumes from 80 to 85%of

the total power of the equipment.

2.2.2 Rotation

Rotation, which tums the dnll bit between consecutiveblows, has the function of making the bit stnke upondifferent points of the rock in the bottom of the blasthole.In each type of rock there is an optimum rotation speedwhich produces larger sized cuttings taking advantage of

the free area of the hole created with each impact.When drilling with insert bits, the most common rota-

tion speeds oscillate between 80 and 150 r.p.m. withangles between indentations of 10 to 20°, Fig. 2.3. For

button bits from51 to 89mm, the speeds should be lower,between 40 and 60 r.p.m., that bring turning angles be-tween 5 and 7". Bits of larger diameters require evenlower speeds.

2.2.3 Thrust load

The energy generated by the mechanism of hammerblows should be transfered to the rock, for which it isnecessq to have the dnll bit in permanent contact withthe bottom of the hole. This is achieved with the thrustload or pull down, supplied by a pull down motor, which

should be adapted to rock type and drill bit.Insufficient thrust load has the following negative ef-

fects: lower penetration rates, greater wear of rods andsleeves, loosening of drill steel threads and heating of theSame. On the contrary, if the pull down is excessive thepenetration rate is also diminished, there is increased

INSERT BIT

BUTTON BIT

Fig. 2.3. Rotation speed between consecutive blows as a function ofpenetration rate and bit diameter.



10 Drilling und blasting of rocks

FEE0

Fig. 2.4.The effec t of thmst load upon penetration rate in top hammerdnlling.

rotation resistance, drill steel can become jammed, thewear on the bits increases as well as the rotation rate andequipment vibrations, and the blastholes can be de-viated.

As occurs with rotation, this Parameter does not havedecisive influence on the penetration rates, Fig. 2.4.

2.2.4 Flushing

FLUSHINGFLUID

In order to have efficient drilling, the bottoms of theblastholes must be maintained clean by evacuating drillcuttings as soon as they appear. If this is not done, a largequantity of energy will be consumed in regrinding withthe consequent wear on drill bits and decrease in penetra-tion, apart from the risk of jamming.

Blasthole flushing is carried out with a flow of air,water or foam that is injected by pressure to the bottomthrough an opening in the Center of the drill steel and

flushing holes in the dnll bits.The cuttings are removed up through the space be-

tween the rod and the blasthole walls, Fig. 2.5.Fiushing with air is used in surface operations, where

the dust produced can be eliminated by means of dustcollectors.

Water flushing is mostly used in underground drilling,which also keeps dust down, although it reduces perfor-mance by about 10 to 20%. ,

Foam is used as a complement to air as it helps bringlarge particles up to the surface and also acts as a seaierfor blasthole walls when drilling through loose material.

The velocity of air flow for efficient cleaning with airgoes from 15 to 30 mls. The minimum velocities for eachcase can be calculated from the following equation:

where: V , = Velocity of air flow (mls), pr = Rock density(g/cm3),d, = Diameter of the particles (mm).

Therefore, the flow that should be supplied by thecompressor is:

where: Qa = Fiow (m3/min), D = Blasthole diameter,d = Diameter of the rods (m).

When water is used for flushing, the velocity of air

Pnnciple of fiushing.

flow should be between 0.4 and 1 m/s. In these cases thepressures are maintained between 0.7 and 1MPa, to keepthe flow from entering into the hammer.

When using air with top hammers, it is not common tohave a high pressure compressor for flushing alone. Onlyin down the hole hammer drilling is a high pressurecompressor used (1 - 7 MPa) because the percussionpower is increased along with the flushing of cuttings.

An important factor to remember when estimating theflushing flow is that of charging losses produced due tothe narrow conducts through which the fluid must pass(flushing needle, drill steel holes) as well as along the dnllstnng.

In Table 2.2, the flushing velocities for top hammerdrilling are indicated as function of air compressor flowand drill steel diameter.

2.3 TOP HAMMER DRILLING

This drilling System can be qualified as the most conven-tional or classic, and although its use by pneumatic drivewas limited by the down the hole and rotary equipment,the appearance of the hydraulic hammers in the sixtieshas given a new boost to this method, complementing andwidening its field of application.

2.3.1 Pneumatic drilling rigs

Hammers driven by compressed air basicaily consist in:- A cylinder with a front Cover that has an axial

opening where the rotation chuck goes, as well as aretaining device for the drill rods.

- The piston that altemately strikes the dnll steelshank through which the shock wave is transmitted to therod.



Rotarypercussive drilling 11

- The valve that regulates the passage of compressedair in a pre-set volume and in alternating form to the front

and back of the piston.- A rotation mechanism, that can be a spirally fluted

nfle bar or of independent rotation.

- A flushing System that consists in a tube that allowsthe passage of air to the inside of the drill steel.

Thes e elements are cornrnon to all the types of ham-mers o n the market, with only a few design c haractensticsthat differ: diameter of the cylinder, length of the pistonstroke, distribution valves, etc. The following describes

the working pnnciple of a pneumatic harnrner, Figs. 2.6to 2.12.

1. The piston is at the end of its return stroke and isready to Start its work ing stroke. T he air, at line pressure,fills the backhead (1) and passes through th e back supplyport (2) into the cylinder (3). The air pushes the piston

fonvard, beginning the working stroke. Meanwhile, thecylinder front end (5) is at atm ospheric pressure since the

exhaust port (6) is Open.2. T he piston (4) continues to accelerate forward, dri-

ven by the line pressure, until the leading ed ge (7) of thepistons control head shuts off the e ntrance of compressedair. The air confined in the back e nd of the cylinder (3)starts to expand and contiunes to d rive the piston forward.Note that the piston flange (4) closes the exhau st port (6)and that the front end is still at atmosph eric pressure.

3. The air confined at the back of the piston (3) con-

tinues to expand until the back edge of the piston flangestarts to uncover the exhaust port (6). Rem ember that the

piston control head (7) has already shut off the com-pressed air entrance, so that no compressed air will bewasted w hen the exhaust port is opened.

Up front, the piston has trapped air that was a atmophe-ric pressure (5), and has now compressed it to slightlyabove atmospheric pressure.

4. Th e piston continues to move forward because of its

momentum until it strikes the drill shank steel. Now, theback edge of the piston flange (8) has uncovered theexhaust port (6) and the air in the back en d is exhaustedinto the atmosphere. While this was going On, the backedge (10 ) of the control head o pened the front supply port

adrnitting compressedair

to the front end (5) driving thepiston back o n the return stroke. Dur ing this Stage there iscompressed air pushing against the piston from the front

end (5 ) and also pushing against the back end (10). Thefront surface area is much larger than the back (10) so thepiston moves towards the rear.

5. T he piston is accelerated back on the return stroke,until the back edg e of the control head (10) Covers up thefront air supply port. The air up fron t then continues topush the piston back.

6. T he piston continues to accelerate backwards whilethe air in the front end (5) expan ds until the front end ofthe piston flange (11) uncovers the exhaust port, trapping

the air in the back end of the cylinder and com pressing itto a pressure slightly more than atmospheric. Note thanthe front edg e of the con trol head (7 ) is just about to Openthe back supply port.

WFig . 2 .6 . Piston at the end of its return stroke.I

ig. 2 .7 . The piston accelerates forward.2.

W -Fig. 2. 8. The backe dge of the piston flange uncovers the exhaust port.

w -Fig . 2 .9 . The piston com presses the air in front of it.I

ig. 2.1 0. The piston is accelerated back.

7Fig. 2.1 1 . The front edge of the piston flange uncovers the exhaustport.

Fig. 2.1 2. Return strokeof the piston finishes.

drilling and blasting in rocks (sample)

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