Drilling for Excavation by Blasting

This reference edition deals with surface rock drilling used for the purpose of excavating rock by means of blasting. Other types of drilling, such as drilling for oil and water, mineral exploration, and grouting are excluded.

The reader is given a brief explanation of the various prevail­ing drilling methods, as well as an introduction to blasting tech­nique, and the interrelation of drilling and blasting. Also dis­cussed are the main parameters involved when planning and exe­cuting blasthole drilling at quar­ries, open pit mines and various types of civil engineering projects.

The range of Atlas Copco products, with references to the Atlas Copco internet home pages, are presented and dis­cussed by comparing their suit­ability and expected productivity related to various applications. Updated case stories from differ­ent worksites in the world should prove interesting and beneficial, when planning and selecting methods and equipment for blasthole drilling applications.

Blastholes have certain unique and important characteristics. These are: hole diameter, hole depth, direction and straightness. Drilling produces a circular hole n the rock, whose strength must be overcome by the drilling tool. Depending on rock properties, there are several ways to accomplish this, as shown in the following article.

 

Rotary Drilling

Rotary drilling can be subdivided into rotary cutting and rotary crushing.

Rotary cutting creates the hole by shear forces, breaking the rock's ten sile strength. The drillbit isfurnished with cutter inserts of hard metal alloys and the energy for reaking rock is provided by rotation torque in the drillrod. This technique is limited to rock with low tensile strength, such as salt, silt, and soft limestone not containing abrasive quartz minerals.

Rotary crushing breaks the rock by high point load, accomplished by a toothed drillbit, which is pushed downwards with high force. The bit, being of tricone roller type fitted with tungsten carbide buttons, is simultane­ously rotated, and drill cuttings are removed from the hole bottom by blowing compressed air through the bit.

Drillrigs used for rotary drilling are large and heavy. The downwards thrust is achieved by utilizing the weight of the drillrig itself, and the rotation, via a hydraulic or electric motor, applied at the end of the drill pipe. Common hole diameters range from 8 to 17.5 in (200-440 mm) and, because adding the heavy drill pipes is cumbersome, most blasthole drillrigs use long masts and pipes to accommo­date single-pass drilling of maximum 20 m (65 ft). Electric power is usually chosen for the large rigs, whereas smaller rigs are often powered by diesel engines.

Rotation rates vary from 50 to 120 rev/min, and the weight applied to the bit varies from 0.5 t/in of bit diameter in soft rock, to as much as 4 t/in of bit diameter in hard rock.

Recent technical advances include: improved operator cab comfort; automatic control and adjustment of optimum feed force and rotation speed to prevailing geology and bit type and diameter; and incorporation of the latest technology in electric and hydraulic drive systems.

Rotary drilling, which is still the dominant method in large open pits, has limitations in that the rigs are not suited to drilling holes off the vertical line. As blasting theories and practice have proved, it is generally beneficial to design, drill and blast the bench slopes at an angle of approximately 18 degrees off vertical.

Many rotary rig masts have pinning capabilities permitting drilling at angles as much

as 30 degrees out of the vertical. However, the inclined hole drilling capabilities in rotary drilling are limited by the heavy feed force required, since part of this force is directed backwards. This causes rig stability problems, reduced penetra­tion, and shorter life of drilling con­sumables. Consequently, most blast hole drilling using rotary drillrigs is for vertical holes.

Percussive Drilling

Percussive drilling breaks the rock by hammering impacts transferred from the rock drill to the drillbit at the hole bottom. The energy required to break the rock is generated by a pneumatic or hydraulic rock drill. A pressure is built up, which, when released, drives the piston forwards. Figure 1 illustrates the principle of top hammer percussive drilling.

The piston strikes on the shank adapter, and the kinetic energy of the piston is converted into a stress wave travelling through the drill string to the hole bottom. In order to obtain the best drilling economy, the entire system, rock drill to drillsteel to rock, must harmonize.

Stress Wave

Theoretically, the stress wave has a rectangular shape, the length of which is twice that of the piston, while the height depends on the speed of the piston at the moment of impact, and on the relationship between the cross-sectional area of the piston and that of the drillsteel.

The total energy that the wave contains is indicated diagramatically in Figure 2. To calculate the output power obtained from a rock drill, the wave energy is multiplied by the impact frequency of the piston, and is usually stated in kW. Rock drill designers seek to find the best com­binations of various parameters, such as the piston geometry, the impact rate and the frequency. Two rock drills having the same nominal power rating might therefore have quite different properties.

 

 

 

The shock waves that are generated by hydraulic (Figure 3) and pneumatic (Figure 4) rock drills are significantly different in shape. Drillrods used with hydraulic rock drills will normally show substantially longer service life, compared with pneumatic rock drills, because of the higher stress level obtained with the pneumatic driven piston.

The reason is the larger cross-section needed when operating at substantially lower pressure, which is 6-8 bars, compared to the 150-250 bars used with hydraulic systems. The slim­mer the piston shape, the lower the stress level.

Figure 5 compares the stress level generated by three different pistons having the same weight, but with dif­ferent shapes and working at different pressures. The lowest stress, or shock wave amplitude, is obtained with the long slender piston working at high pressure.

Efficiency and Losses

The wave loses some6-10% of its energy for even, additional coupling, as it travels along the drillstring. This loss is partly due to the difference in cross-sectional area between the rod and the sleeve, and partly due to imperfect contact between the rod faces. The poorer the contact, the greater the energy loss.

When the shock wave reaches the bit, it is forced against the rock, there­by crushing it. The efficiency at the bit never reaches 100%, because some of the energy is reflected as a tensile pulse. The poorer the contact between the bit and the rock, the poorer the efficiency (Figure 6).

To optimize drilling economy, the drilling parameters for percussion pressure, feed force, and rotation must harmonize.

Percussion Pressure

The higher the pressure, the higher will be the speed of the piston, and consequently, the energy. Where the bit is in good contact with hard and competent rock, the shock wave energy can be utilized to its maximum. Conversely, when the bit has poor contact, the energy cannot leave the drillstring, and reverses up the drill-string as a tensile wave.

It is only when drilling in suffi­ciently hard rock that the maximum energy per blow can be utilized. In soft rock, to reduce the reflected energy, the percussion pressure, and thus the energy, will have to be lowered (Figure 7).

For any given percussion pressure, the amplitude, and hence the stress in the drillsteel, will be higher with reduced cross-section of the drillrods.

 

 

To get the longest possible service life from shank adapters and rods, it is important to ensure that the working pressure is matched to the drillstring at all times.

Feed Force

The purpose of the feed is to maintain the drillbit in close contact against the rock. However, the bit must still be able to rotate. The feed force must always be matched to the percussion pressure. Figure 8 illustrates this relationship.

Rotation

The purpose of rotation is to turn the drillbit to a suitable new position for the next blow. Using button bits, the periphery is turned about 10 mm between blows. Consequently, the rotation rate is increased using higher impact frequency and reduced bit diameter. Using insert bits, the recommended rotation rate is 25% higher.

Setting Parameters

In practice, the driller sets the percussion pressure that the rock can cope with, and then sets the rev/min with regard to the percussive frequency and the bit diameter.

When drilling starts, the feed is adjusted to get even and smooth rota­tion. In case this is not achieved, which will show up in low shank adapter life, the percussion pressure can be progressively reduced, until even and smooth rotation is reached.

The temperature of the adapter sleeve can be checked to ensure that the drilling parameters are correctly set. Immediately after drilling, the temperature should be 60-70 degrees for dry drilling, and approximately 40 degrees for wet drilling.

Drilling problems, mainly related to loose couplings, may arise whatever parameters are used. In order to tight­en the couplings during drilling, the friction of the bit against the hole bottom has to be increased. This can be done by increasing the feed, increasing the rotation rate, or chang­ing the bit.

Flushing

Drill cuttings are removed from the hole bottom to the surface by air blow­ing or water flushing. As the power output from rock drills increases, accompanied by increased penetration rate, efficient flushing becomes gradu­ally more important. The flushing medium is normally air for surface drilling, and water for underground drilling.

The required flushing speed will depend on: specific gravity - material having a density of 2 t/cu m requires at least 10 m/sec, whereas iron ore, for example, having a density of 4 t/cu m, requires an air speed of 25-30 m/sec; particle size - the larger the particles, the higher flushing speed required; particle shape -- spherical particles require more speed than flaky, leaf shaped particles.

Productivity and Methods

During the past century there has been a rapid and impressive increase in efficiency and productivity related to top hammer drilling. Starting from hitting a steel manually by a sledge hammer 100 years ago, today's hydraulically powered rock drills utilize the latest state-of-the-art technology.

Every drilling method has its pros and cons, making an objective com­parison quite cumbersome. In view of this, the table in Figure 9 can serve as a guideline when comparing the various percussion drilling alternatives which Atlas Copco can offer. The choice of best drilling method to apply depends on hole size and type of application.

 

Lists of words

 

1. drilling – свердлування, буріння

2. rotary – ротаційний

3. rock – скеля, гірська порода

4. percussive –ударний

5. stress – тиск, напруження

6. rod – штанга

7. rotation –обертання

8. flushing - змивання

9. specifiс gravity – питома вага

10. pressure – тиск

11. drillstring – буровий снаряд

12. approximately - приблизно

13. drill cuttings – буровий дрібняк

14. hydraulically - гідравлічно

15. density – щільність, густина

16. penetration - проникнення

17. frequency - частота

18. relationship - взаємовідношення

19. accomplish – здійснювати

Exercises:

1) Give the definitions of the following terms:

Drilling, excavation, blast hole characteristics.

2) Read and give Ukrainian equivalents of the following international words:

Reference, excavating, method, interrelation, technique, planning, engineering, present, productivity, rotary, diameter, pneumatic, hydraulic.

3) Match a word in column

А with a definition in column В:

1. drillrigs 1. буровибухова свердловина

2.blasting technique 2. діаметр свердловини

3. blasthole 3. 6ypoвий станок

4. hole diameter 4. бурова коронка

5. rotary drilling 5. методи буро-вибухових робіт

6. bit 6. похиле буріння

7. technical advances 7. роторне буріння

8. inclined hole drilling 8. nepepiз

9. crosscection 9. новi технічні рішення

10. drilling parameters 10. параметри буріння

4) Complete the following sentences:

1) Rotary drilling can be subdivided into.......

2) Drillrigs used for rotary drilling are........

3) Rotation rates vary from.........

4) Percussive drilling breaks the rock.........

5) The higher the pressure, the higher..........

5) Tell what you know about:

1) Rotary Drilling

2) Percussive Drilling

3) Efficiency and Losses

4) Rotation

5) Productivity and Methods

Ergonomics and Safety

 

 

Drilling Efficiency

Efficient drilling is a function of many individual aspects, both mechanical and human. While it is necessary to have a powerful and easily-maneuverable drillrig, it is also important to take good cary of the operator's needs, and those of the external environment. What value is a machine that is too noisy, or cannot be operated in harsh weather? Operator comfort and safety has to be planned into the modern drilling at the drawing board stage of design. To get maximum return on investment, the owner needs to be sure that the drillrig is designed to work in all conditions, without the operator getting unduly tired, and with minimum disturbance of the environment in the neigh bourhood of the site. All Atlas Copco drill-rigs are designed with these parameters in mind.

In addition, a whole array of options is available to control the inclination, alignment and depth of drillholes, in order to get the maximum rock breaking effect from the minimum drilling effort, reducing noise and dust, and saving time, money, and materials.

Operator's Cabin

The interplay between man and machine starts in the operator's cab, and Atlas Copco has expended great effort in the development of a well conceived, purpose-built cab. Even the smallest details have been considered to produce a modern workstation, not only' in terms of mechanical function, but also paying attention to the requirements of ergonomics, safety and the environment. The cabin offers a superb view of the drillhole from the operator's seat, facilitating accurate collaring and control of the drilling operation. The angled, laminated glass windscreen is equipped with a large wiper, as are the right hand side and roof windows, and improved air ventilation effectively clears condensation mist from all of these windows. A jumbo-sized rear-view mirror gives good visibility and safer rig moving. In addition, the spacious cab gives good moveability and comfort for the operator, with easy-to-read instrumentation that is simple to learn and to use. A mechanized tube-permutation system enables any damaged drill rods to be identified from the operator's cab. The service hatches are easily accessed, but are also lockable, as a good security precaution at isolated work­sites. The cabins are even equipped with a 12-volt outlet for mobile telephone.

Ergonomics and Environment

The cabin is laid out so that the opera­tor can monitor and control the entire drilling process without changing body position. This relieves neck, shoulders and back from strain. The ergonomically-designed seat is vertical­ly and laterally adjustable, with con­trol levers and control panel located in, or in close proximity to, the col­lapsible arm rests. The seat is slewable for easy entry into the cab, and the door is fitted with a safety stop that prevents crushed fingers.

Good insulation affords a noise level below 80 dB, and rubber-damp­ing of the cab mounting reduces vibra­tion and gives greater comfort during tramming. The cabin has an efficient CFC-free air conditioning system for both cooling and heating, and many surfaces are textile covered for greater comfort.

The cab complies with the European and international safety demands for Roll-Over Protective Structure

(ROPS) and Falling Object Protective Structure (FOPS).

With regards to the environment, exhaust emission values of all new Atlas Copco drillrigs are lower by a good margin than those stipulated in international standards, as are the noise values. There is efficient dust collection and coarse separation, the feeds are fitted with collectors for lubricating oil, and biologically degradable hydraulic oils are available as options.

Hole Alignment

Regulations governing rock blasting in open-air excavations are found in countries all over the world. Rock excavation by drilling and blasting is a particular menace in quiet, urban areas, and it is strictly governed with respect to allowable levels of pressure shocks and ground vibrations.

Productivity, and the need to control ground vibrations, means that capable and precise instruments are required to guide the driller. Faults in blasthole direction, hole deviations, and variations in hole depths must be minimized, if efficient production is to be obtained. Instruments for angle set-ting and hole depth control increase drilling accuracy and reduce human error.

There are various ways for a con-tractor to comply with the blasting rules, without increasing costs for rock excavation. Several benefits can be obtained from improved blasthole alignment and drill depth control. Improved accuracy in the application of the drilling pattern saves time, drill-metres, and explosives. At the same time, less boulders need to be broken, and less crushing of oversize is required. All this means reduced oper­ating cost, and money saved. A modern hole alignment instrument offers: more precise drilling with a reduction in cost/hole; automatic depth control; better fragmentation; automatic drill feed stop; flat benches after blasting; and less overdrilling. By guaranteeing a perfect blast result, there is no need for the contractor to return and clean up rough surfaces, or condition overbreak.

Much blasting inconvenience can be reduced by special drilling patterns, which distribute the explosive charges evenly, and limit the level of ground vibrations, so as not to harm property in the vicinity. To achieve this, it is crucial to collar the hole on the right spot, maintain the correct alignment, and drill to a predetermined level.

Depth Control

The alignment instrument confirms the hole angle, while a laser sensor monitors drilling to the proper depth, regardless of the surface conditions. All this comes together for a successful blast, breaking rock to a level floor, with minimum ground vibration, and even fragmentation.

The HQS and the HEC3 angle set-ting instruments, used with the aiming device, helps set the correct angle of inclined holes.

The angle and hole depth reader function controls the length of a blast hole, from the start of the hole on surface, to the preset bottom elevation. The instrument does not require any assistance from the operator, and comes with an automatic drill feed stop function for optimum drilling accuracy.

The instruments feature automatic compensation, regardless of the drilling surface ground conditions. A beacon, mounted on a tripod, generates a horizontal reference plane on a 350 m radius across the worksite by a rotating laser beam. A sensor, fitted to the rock drill cradle, reacts to the signal generated by the rotating laser beam. The HQS 12 instrument pro-vides a reference plane for all holes, automatically adjusting hole depth to match the plan, with an accuracy within 5 cm.

 

Lists of words

 

1. efficiency –продуктивність

2. operator - машиніст

3. environment – навколишнє середовище

4. alignment – вирівнювання, вивіряння

5. cradle - рама

6. vibration - вібрація

7. margin –край

8. degrade - руйнувати

9. blast – вибух

10. adjusting – пристрій

 

 

11.collaring - забурювання

12. boulder - негабарит

13. condition - умова

14. pattern - контур

15. hole deviation – відхилення свердловини

16. worksite - робоче місце

17. insulation - ізоляція

18. fragmentation - фрагментація

19. dust collection - пиловловлювання

20. drill feed – подача бура

Exercises:

1) Give the definitions of the following terms:

Ergonomics, drilling efficiency, operator comfort

2) Read and give Ukrainian equivalents of the following international words:

Ergonomics, efficiency, function, individual aspect, mechanical, human, comfort, modern, maximum, conditions.

Match a word in column

A with a definition in column В:

1. both mechanical and human aspect 1. зменшення шуму та пилу

2. to get maximum return 2. увесь процес буріння

3. reducing noise and dust 3. i механічний i людський аспект

4. entire drilling process 4. вщповщати:шжнародним вимогам з охорони

праці

5. air conditioning system 5. точні інструменти

6. compile with international 6. напрям бурової свердловини
safety demands

7. precise instruments 7. лазерний сенсор

8. blasthole direction 8.витрати на видобуток руди

9. costs for ore excavation 9. система кондищювання повітря

10. laser sensor 10. отримати максимальну віддачу

4) Complete the following sentences:

1) Operator comfort and safety.....

2) To get maximum return on.......

3) The cabin offers..........................................................

4) The cabin is laid out so that.....

5) Productivity, and the need to control..........

 

5) Tell what you know about:

1) Drilling efficiency

2) Operator's cabin

3) Ergonomics and Environment

4) Hole alignment

5) Depth control