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TALKING TECHNICALLY
Principles of Rock Drilling
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 prevail-
ing drilling methods, as well as an
introduction to blasting technique, Fig 1 Principle of tophammer drilling.
and the interrelation of drilling
and blasting. Also discussed are
the main parameters involved
when planning and executing blast Rotary crushing breaks the rock by has limitations in that the rigs are not
hole drilling at quarries, open pit high point load, accomplished by a suited to drill holes off the vertical line.
mines and various types of civil toothed drill bit, which is pushed As blasting theories and practice have
engineering projects. downwards with high force. The bit, proved, it is generally beneficial to
The range of Atlas Copco prod- being of tricone roller type fitted with design, drill and blast the bench
ucts, where references can be tungsten carbide buttons, is simultane- slopes at an angle of approximately 18
found to the Atlas Copco internet ously rotated, and drill cuttings are degrees off vertical.
home pages, are presented and removed from the hole bottom by Many rotary rig masts have pinning
discussed by comparing their suit- blowing compressed air through the capabilities permitting drilling at
ability and expected productivity bit. Drill rigs used for rotary drilling are angles as much as 30 degrees out of
related to various applications. Up- large and heavy. The downwards the vertical. However, the inclined hole
dated case stories from different thrust is achieved by utilising the drilling capabilities in rotary drilling are
work-sites in the world should weight of the drill rig itself, and limited by the heavy feed force
prove interesting and beneficial, the rotation, via a hydraulic or required, since part of this force is
when planning and selecting electric motor, applied at the end directed backwards. This causes rig
methods and equipment for blast of the drill pipe. Common hole dia- stability problems, reduced penetra-
hole drilling applications. meters range from 8 to 17.5 in tion, and shorter life of drilling con-
Blast holes have certain unique
(200-440 mm) and, because adding sumables. Consequently, most blast
and important characteristics. These
the heavy drill pipes is cumbersome, hole drilling using rotary drillrigs is for
are: hole diameter, hole depth,
direction and straightness. Drilling most blasthole drillrigs use long vertical holes.
produces a circular hole in the masts and pipes to accommodate
rock, whose strength must be over- single-pass drilling of maximum 20 m Percussive Drilling
come by the drilling tool. Depend- (65 ft). Electric power is usually
ing on rock properties, there are chosen for the large rigs, whereas Percussive drilling breaks the rock by
several ways to accomplish this. smaller rigs are often powered by hammering impacts transferred from
diesel engines. the rock drill to the drill bit at the hole
Rotation rates vary from 50 to bottom. The energy required to break
Rotary Drilling 120 rev/min, and the weight applied the rock is generated by a pneumatic
to the bit varies from 0.5 t/in of bit or hydraulic rock drill. A pressure is
Rotary drilling can be subdivided into diameter in soft rock, to as much as built up, which, when released, drives
rotary cutting and rotary crushing. 4 t/in of bit diameter in hard rock. the piston forwards. Figure 1 illustrates
Rotary cutting creates the hole by Recent technical advances include: the principle of top hammer percussive
shear forces, breaking the rock’s tensile improved operator cab comfort; auto- drilling.
strength. The drill bit is furnished with matic control and adjustment of opti- The piston strikes on the shank
cutter inserts of hard metal alloys, and mum feed force and rotation speed to adapter, and the kinetic energy of the
the energy for breaking rock is provid- prevailing geology and bit type and piston is converted into a stress wave
ed by rotation torque in the drill rod. diameter; and incorporation of the travelling through the drill string to
This technique is limited to rock with latest technology in electric and the hole bottom. In order to obtain
low tensile strength, such as salt, silt, hydraulic drive systems. the best drilling economy, the entire
and soft limestone not containing Rotary drilling, which is still the system, rock drill to drill steel to rock,
abrasive quartz minerals. dominant method in large open pits, must harmonise.
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TALKING TECHNICALLY
Stress Wave Piston 1
Piston 1
Fig 5 Stress level
– 0,8 MPa generated by different
Theoretically, the stress wave has a rec- – 8 bar pistons of same
tangular shape, the length of which is weight.
twice that of the piston, while the
Piston 2 ––12 MPa
Piston 2 120 bar
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 Piston 3 ––20 MPa
Piston 3 200 bar
the drill steel.
s
5200 m/s
+
–
2 x piston length
Fig 2 Stress wave energy. 3
2
The total energy that the wave
contains is indicated diagramatically in 1
Figure 2. To calculate the output
power obtained from a rock drill, the Shock-wave amplitude
wave energy is multiplied by the
impact frequency of the piston, and
is usually stated in kW. Rock drill The shock waves that are generated When the shock wave reaches the
designers seek to find the best com- by hydraulic (Figure 3) and pneumatic bit, it is forced against the rock, there-
binations of various parameters, such (Figure 4) rock drills are significantly by crushing it. The efficiency at the bit
as the piston geometry, the impact different in shape. Drill rods used with never reaches 100%, because some of
rate and the frequency. Two rock drills hydraulic rock drills will normally show the energy is reflected as a tensile
having the same nominal power rating substantially longer service life, com- pulse. The poorer the contact between
might therefore have quite different pared with pneumatic rock drills, the bit and the rock, the poorer the
properties. because of the higher stress level efficiency (Figure 6).
obtained with the pneumatic driven
piston. s
Reflecting wave
s 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 Primary wave
the stress level.
Figure 5 compares the stress level
generated by three different pistons Fig 6 Poor contact between bit and rock
having the same weight, but with dif- results in poor efficiency.
ferent shapes and working at different
Fig 3 Shock wave generated by
pressures. The lowest stress, or shock
hydraulic rock drill. wave amplitude, is obtained with the To optimise drilling economy, the
long slender piston working at high drilling parameters for percussion
s pressure. pressure, feed force, and rotation must
harmonise.
+ Efficiency and Losses
– Percussion Pressure
The wave loses some 6-10% of its
energy for every additional coupling, The higher the pressure, the higher
as it travels along the drill string. This will be the speed of the piston, and
loss is partly due to the difference in consequently, the energy. Where the
cross-sectional area between the rod bit is in good contact with hard and
and the sleeve, and partly due to competent rock, the shock wave
Fig 4 Shock wave generated by imperfect contact between the rod energy can be utilised to its maximum.
pneumatic rock drill.
faces. The poorer the contact, the Conversely, when the bit has poor
greater the energy loss. contact, the energy cannot leave the
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TALKING TECHNICALLY
Percussion pressure
Percussion pressure
Hydraulic
Drilling method Tophammer DTH COPROD
hole diameter, mm 76-127 85-165 105-165
penetration rate 2 1 3
hole straightness 1 3 3
Soft rock Hard rock hole depth 1 3 3
production capacity (tons rock/shift) 2 1 3
Fig 7 To reduce reflected energy, fuel consumption/drill metre 2 1 2
percussion pressure is lowered. service life of drill string 1 2 3
investment in drill string 2 2 1
drill string, and reverses up the drill
suitability for good drilling conditions 3 2 2
string as a tensile wave.
It is only when drilling in sufficiently suitability for difficult drilling conditions 1 3 3
hard rock that the maximum energy simplicity for operator 2 3 1
per blow can be utilised. In soft
rock, to reduce the reflected energy, adjustability of flushing capacity 1 2 3
the percussion pressure, and thus
the energy, will have to be lowered Fig 9 Ratings are: fair = 1, good = 2, and very good = 3
(Figure 7). Comparison for 20 m bench drilling in a limestone quarry
For any given percussion pressure,
the amplitude, and hence the stress in between blows. Consequently, the power output from rock drills in-
the drill steel, will be higher with rotation rate is increased using higher creases, accompanied by increased
reduced cross-section of the drill rods. impact frequency and reduced bit penetration rate, efficient flushing
To get the longest possible service life diameter. Using insert bits, the recom- becomes gradually more important.
from shank adapters and rods, it is mended rotation rate is 25% higher. The flushing medium is normally air
important to ensure that the working for surface drilling, and water for
pressure is matched to the drill string Setting Parameters underground drilling.
at all times. The required flushing speed will
In practice, the driller sets the percus- depend on: specific gravity – material
Feed Force sion pressure that the rock can cope having a density of 2 t/cu m requires
with, and then sets the rev/min with at least 10 m/sec, whereas iron ore, for
The purpose of the feed is to maintain regard to the percussive frequency and example, having a density of 4 t/cu m,
the drill bit in close contact against the the bit diameter. requires an air speed of 25-30 m/sec;
rock. However, the bit must still be When drilling starts, the feed is particle size – the larger the particles,
able to rotate. The feed force must adjusted to get even and smooth rota- the higher flushing speed required;
always be matched to the percussion tion. In case this is not achieved, which particle shape – spherical particles
pressure. Figure 8 illustrates this will show up in low shank adapter require more speed than flaky, leaf
relationship. life, the percussion pressure can be shaped particles.
progressively reduced, until even and
Feeding
smooth rotation is reached. Productivity and Methods
The temperature of the adapter
sleeve can be checked to ensure that During the past century there has
the drilling parameters are correctly been a rapid and impressive increase
set. Immediately after drilling, the tem- in efficiency and productivity related
perature should be 60-70 degrees for to top hammer drilling. Starting
dry drilling, and approximately 40 from hitting a steel manually by a
degrees for wet drilling. sledge hammer 100 years ago,
Low percussion High percussion Drilling problems, mainly related to today’s hydraulically powered rock
pressure pressure loose couplings, may arise whatever drills utilise the latest state-of-the-art
parameters are used. In order to tight- technology.
en the couplings during drilling, the Every drilling method has its pros
Fig 8 Feed force must be matched to friction of the bit against the hole and cons, making an objective com-
percussion pressure. bottom has to be increased. This can parison quite cumbersome. In view
be done by increasing the feed, of this, the table in figure 9 can serve
increasing the rotation rate, or chang- as a guideline when comparing the
ing the bit. various percussion drilling alternatives
Rotation which Atlas Copco can offer. The
Flushing choice of best drilling method to apply
The purpose of rotation is to turn the depends on hole size and type of
drill bit to a suitable new position for Drill cuttings are removed from the application.
the next blow. Using button bits, the hole bottom to the surface by air
periphery is turned about 10 mm blowing or water flushing. As the by Hans Fernberg
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