underground drilling and equipment selection 2011
TRANSCRIPT
The Southern African Institute of Mining and Metallurgy
Drilling and Blasting 2011
G Harper
Page 1
UNDERGROUND DRILLING AND EQUIPMENT SELECTION
G.S. Harper 1. Introduction
The current method used to extract the majority of ore from deep level South African
gold and platinum mines is one of drilling and blasting. At 2010 production levels, this
method of mining requires the drilling of approximately 700 000 holes each day at an
estimated annual cost of R6 Billion and is achieved using an estimated 28 000 hand held
pneumatic drills of which at any one time approximately 50 per cent are in use, the
remainder either being under repair or held as spares. Virtually all of these rock drills
are mounted on thrust legs and are controlled by hand using a crew of one or two men,
depending on the size of the drill and the mining conditions.
Hand held pneumatic drills have been successfully used by the South African mining
industry for the past 117 years1 and although considerable resources have been invested
to improving pneumatic drilling, the benefits achieved to date have become marginal
and the overall efficiencies of compressed air systems and the rock drills themselves
remain very low. Furthermore, for reasons to be discussed in this presentation, the
performance of pneumatic rock drills decreases in the increasingly fractured rock
conditions encountered in deep mines.
Hydraulic powered rock drills with their higher power output overcome the
disadvantages of the pneumatic drills and provide further advantages of lower noise
levels and the elimination of the fogging associated with the exhaust air of pneumatic
drills.
2. Background
While the South African de facto tool for underground drilling of blast- and support-
holes is the hand-held pneumatic rotary-percussive rock drill, this machine can trace its
origins to the middle of the 19th
century2. Human power and steam as the motive force,
over time, gave way to compressed air, and pneumatic machines reached commercial
proportions in 1861 during the excavation of the alpine Mont Cenis tunnel3. Patents for
hydro-powered rock drills are reported as being issued in 18564, with the first electric
drills appearing in 18875. All of these early machines operated on a rotary-percussive
basis, emulating the manual method of a hammer-strike on a chisel, which was then
twisted to access a new surface.
The succeeding 140 years has seen the power source for rock drills being predominated
by compressed air, owing to its ease of production and suitability for transmission over
long distances. It was only during the last two decades of the last century when hand-
held hydro-powered drills regained some prominence in selected South African mines,
owing, in part, to a need for a more powerful drill at deeper levels of mining. Recently,
electrically powered drills have re-emerged and are currently undergoing production
trials in a few selected South African gold and platinum mines.
The Southern African Institute of Mining and Metallurgy
Drilling and Blasting 2011
G Harper
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To say that the pneumatic machines have not progressed since the late 18th
century
would be irresponsible. Advances in the understanding and modelling of the stress-
wave propagation and transfer from the percussion element through the drill rod (or
string) to the rock6-10
have allowed more efficient designs of drilling machine.
Similarly, developments in metallurgy, rock-specific bit design, the use of knock-off
bits and thrust legs, amongst others, have allowed greater advance rates and more
efficient drilling.
Despite this, however, the fundamental mechanical design of the pneumatic rock drill
has changed little since its early development. Ergo, little significant work has been
done, and reached commercial application, which controls or limits sound emissions
from available pneumatic drills to below acceptable levels. While some manufacturers
do offer exhaust-muffled drills as a standard product, un-muffled drills are freely
available and tend to attract a higher market share.
3 Blasting Requirements
To obtain effective efficient rock breaking by the drill an blast method requires:-
1. The correct amount and type of explosive,
2. Correctly positions and,
3. Detonation with the right sequence and timing.
Effective methodologies have been established for the determination of the amount and
placement of explosives and shock tube initiation systems and electronic delay
detonators have overcome the deficiencies of fuse and igniter cord. The remaining
crucial factor to effective rock breaking is the drilling accuracy.
The most critical factor is the position of the bottom of the hole, since this controls the
burden at the toe, whereas the drilling angle to the face and hole spacing at the collar are
of secondary importance and consequently the suggested location tolerances are 50 mm
diameter at the hole bottom and 300 mm diameter at the collaring position.
Figures 1 and 2 demonstrate the variability of the face angle for manually drilled stope
holes and table one shows the increase in burden that results from non-parallel holes.
The Southern African Institute of Mining and Metallurgy
Drilling and Blasting 2011
G Harper
Page 3
Drilling Angles
0
10
20
30
40
50
60
50-59 60-69 70-79 80-89 90-99
Hole Angle (Degrees)
Pe
rce
nta
ge
of
Ho
les
Figure 1 Distribution of angles of drilled stope face holes
Figure 2 Example of top and bottom drilled stope face holes
The primary stope drilling requirements of the South African mining industry must,
therefore, be the cost effective drilling of accurately placed and directed blast holes with
minimal exposure of the operators to environmental and health risks.
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Drilling and Blasting 2011
G Harper
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Table 1 Effect of drilling accuracy on burden
Drilling Angle to the Stope Face (Degrees)
Hole Length (mm)
Hole Spacing at Collar
(mm) Hole 1 Hole 2
Hole Spacing at Toe (mm)
Per Cent Increase in
Burden at Toe of Hole
1100 500 70 70 500 0
1100 500 70 80 670 34
1100 500 60 80 830 66
1100 550 70 70 550 0
1100 550 70 80 720 31
1100 550 60 80 880 60
The most direct and obvious solution to the provision of accurately positioned blast
holes is the use of drill guides or drill rigs. However, whilst numerous drill rigs have
been developed and implemented over the last five to seven decades, each reportedly
providing improved productivity and drilling accuracy, the majority of blast holes are
still drilled by hand and we must recognise why this should be since rigs provide the
following significant advantages:
• Holes can be accurately placed and spaced.
• Drills can be thrust at optimum levels to maximise the drilling rate.
• ‘In-line’ thrusting reduces bending loads on the drill steel and reduces drill steel
breakage
• ‘In-line’ thrusting reduces wear on drill shanks and hex-inserts
• ‘In-line’ thrusting permits the application of the noise reduction strategies
described later.
• By reducing the number of operators the overall exposure to falls of ground is
reduced.
• Drill rigs are amenable to remote control operation enabling further
improvement to the health and safety of operators both from falls of ground and
noise exposure.
Further to the engineering and practical difficulties encountered during the evaluation of
drill rigs, human behavioural factors were reported as a major negative influence on the
general acceptance of drill rigs11
. These factors include11
:-
• High mobility rate amongst black workers,
• Changes in senior management,
• Change and resistance to change,
• Reimbursement of workers,
• Administrative difficulties; having a team to set up the project that was different
from the line management who would run it, and
• Communication problems;
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Drilling and Blasting 2011
G Harper
Page 5
Examples of some drill rigs are shown in Table 2 whereas Figure 3 shows a hydropower
rig in operation.
Table 2 Drill Rig Details
Type Power Cost (1989 Rand)
Comments
TDS Twin Boom Compressed air R170 000
MME Twin Boom Hydropower R395 000 Requires a power pack or
hydropower
Sulzer Twin Boom Hydropower R135 000 Requires a power pack or
hydropower
Novatek Single Boom Hydropower R40 000 Requires a power pack or
hydropower
Figure 3 Hydropower rig in operation
4 Drill Design Requirements
Percussive or rotary-percussive drilling methods have long been accepted as one of the
most efficient ways of drilling blast holes in hard rock. Percussive drilling is a process
of energy transformation in which the kinetic energy of a relatively slow moving rock
drill piston is transformed, during impact with the drill rod, to strain energy in a fast
moving strain pulse within the drill steel. The drill steel acts primarily as a conduit for
this energy. Drilling is achieved following the removal of the rock fragments produced
by the interaction of the strain pulse with the rock face. The efficiency of drilling is
therefore determined to a large extent by the efficiency of energy transfer from piston to
drill steel to rock.
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The first item for consideration in percussive rock drilling is the generation of energy by
accelerating the piston and allowing the piston to impact the drill steel.
4.1 Engineering Requirements
With the following identities:
F = actuator force (N) provided by whatever mechanism is selected
L = stroke (m)
M = mass of piston (kg)
The impact velocity, frequency, impact energy and drill power can be estimated as
follows:-
Impact velocity v (m/s) is given by:
m
Flv
.
..2=
(4.1)
Assuming the full actuator force operates over the full length of the stroke. Hustralid
determined a generic equation for the piston velocity of pneumatic rock drills that
includes a factor B0 to compensate for these assumptions.
m
FlBv
.
..20=
(4.2)
The value of B0 for pneumatic machines is, according to Hustralid6, 0.68
Frequency f (Hz) is given by :
ml
Ff
..25.0=
(4.3)
(Assuming return stroke time is the same as the drive stroke and there is no dwell at
reversal)
Impact Energy Ei (J) is given by:
FlEi .=
(4.4)
Drill Power Pw is given by :
m
FlPw
.2
.5.0
3
= (4.5)
Assuming that the new percussive device should provide a drilling performance at least
equivalent to existing pneumatic machines then a device of at least 4kw percussive
power is required (to accommodate the assumptions and simplifications) and a
minimum blow energy of 25 joules for a hole diameter of 36 mm.
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Figure 4 shows the relationships between blow energy, frequency, impact velocity and
the required actuator force for a 4kW device with a piston mass of 1 kg.
Data for 4kW Percussive Actuator
(Piston mass 1 kg)
0
100
200
300
400
500
600
0 10 20 30 40 50 60 70 80
Stroke (mm)
Blo
w E
ne
rgy
(J
) / F
req
uen
cy
(H
z)
0
2
4
6
8
10
12
14
16
Ac
tuato
r F
orc
e (
kN
)
Im
pa
ct
Velo
cit
y (
m/s
)
Frequency (Hz)
Blow Energy (J)
Actuator Force (kN)
Impact velocity (m/s)
Figure 4 Data for a 4kW Percussive Actuator
It is important to note that most of the engineering restrictions result directly from the
constraints of a hand-held rock drill and would be avoided by the use of a remote
controlled drill rig.
4.2 Drill operation requirements
Having generated the requisite energy within the piston it is important to consider how
this energy is to be transferred to the rock to effect breaking. In the most common
configurations the energy is transferred to a drill bit via impact of the piston on a drill
steel. The piston energy is thereby converted to a strain pulse in the drill steel. The
effective transfer of this energy requires that the striking face of the piston and the drill
steel should be as close in diameter as possible. The shape of the strain pulse is highly
dependant on the shape of the piston as shown in figure 5.
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Page 8
Piston(Mp,Vp) Drill steel Rock
Rockthresholdstress level
Length = l
e
2l/c
Piston(Mp,Vp) Drill steel Rock
Rockthresholdstress level
Length = l
e
2l/c
Strain pulse
Strain pulse
Hydraulic
Pneumatic
Figure 5 Strain pulse shape
The final stage in the process is the transfer of the energy within the strain pulse to the
rock. This transfer required that the drill bit be maintain in contact with the rock by
thrusting the drill. The thrust force requirement Ft has been determined by Hustralid6
as:-
( )Ff
t = + ∫301
0
β σ
τ
idt
When a rock drill is not thrust correctly the strain energy is not fully transferred to the
rock and a significant reflected wave is generated within the drill steel with significant
consequence with regard to the fatigue life of the drill steel (Figure 6).
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Drilling and Blasting 2011
G Harper
Page 9
Time
Strain
Correct thrust
Under thrust
Figure 6 Effect of thrust on strain within a drill steel
The overall effect of the thrust force on the performance of a rock drill is clearly shown
in Figure 7
Thrust
PR2
PR1
F1 F t
Figure 7 Effect of thrust force on drill penetration rate
4.3 Energy Considerations
The power usage of the pneumatic rock drills is calculated from the air mass flow rates
and air supply pressure as follows.
The Southern African Institute of Mining and Metallurgy
Drilling and Blasting 2011
G Harper
Page 10
ss QPE .=
Where:
E is the energy in watts,
sP is the supply pressure in Pascal and
sQ is the mass flow rate (at the supply pressure )in kg per second.
A combination of E and the time to drill a one metre hole of diameter 34 mm provides
the power usage of the pneumatic rock drills in kWh/m.
The energy usage for several rock drills to drill a 34mm diameter hole one metre deep is
presented in table 3 below.
Table 3 Power usage of rock drills to drill a 34 mm diameter hole one metre deep
Power usage kWh/m
Surface Air supply pressure (kPa)
Underground Air supply pressure (kPa)
Drill Type
350 450 550 350 450 550
Boart Longyear S215 std 0.201 0.224 NA 0.089 0.100 0.094
Boart Longyear S215 muffled 0.194 0.185 NA 0.117 0.130 0.108
Boart Longyear S215 muffled
AWS 0.194 0.185 NA 0.117 0.130 0.108
Sulzer ADDS 0.230 0.244 NA 0.126 0.148 0.159
Hilti TE MD20 (205) 0.212 0.107
Hilti TE MD20 (250) 0.215 0.108
The cost of drilling a blast hole is determined from a summation of the costs of
consumables, maintenance, power and manpower and therefore, while the energy
requirement for drilling a blast hole with a pneumatic rock drill supplied at a low air
supply pressure may be similar to that when using a high supply pressure, the total cost
of drilling will be substantially higher because of the increased manpower costs. This is
particularly true of the Hilti TE MD20 rock drill, which, while having one of the lowest
energy requirements, has the lowest penetration rate, and would therefore attract the
highest manpower cost for drilling. Finally, it should be noted that the total energy
efficiency for the pneumatic systems of an established mine is of the order of only one
to two percent.
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5. Alternative technologies
O’Brien et al.12
details numerous studies conducted on new and emerging works that
have the potential to either generate holes in rock at sound pressure levels below that of
the conventional pneumatic drill, or provide effective attenuation of the drill generated
noise. The techniques discussed are summarised in the following sections, highlighting
the most promising techniques.
5.1 Rotary-percussive drilling
Studies of rotary percussive drilling are primarily concerned with the understanding of
the transmission of the stress waves from the hammer (or piston) to the rock / bit
interface. In-depth understanding of these mechanics has the potential to increase the
efficiency of transmission of the percussive energy, diminishing inefficiencies or lost
energy, which has the potential to be converted to sound energy. However, little work
appears to have been done on noise itself12
.
5.1.1 Blow frequency
An argument for the abatement of noise induced hearing loss is to shift the generated
noise spectrum to above that detected by the adult human ear. Studies conducted on
dental workers and operators of ultrasonic cleaning and welding equipment have
indicated that whilst damage does occur, it is of a lower severity than from equivalent
sound pressure levels within the audible spectrum23
. This, superficially, would increase
the equivalent exposure time and allow drill operators to work for longer periods in a
noisy, but high frequency, environment.
Applying this to the rotary-percussive drill, only anecdotal evidence exists for the
deliberations surrounding the use of a high-frequency short-pulse wave, versus a low-
frequency long-pulse wave. Pemberton24
stated that Boart Longyear had developed the
“Nova” machine, a 3,5 kW, independent rotation hand-held drill that operates at 53 Hz
and delivers 66 Joules of blow energy. During testing of this machine, the rock-removal
rate per kilo Watt of power was the same as for a rifle bar machine operating at 35 Hertz
and 97 Joules blow energy. Further, there is evidence of a trend for other drill
manufacturers to move towards a higher-frequency lower-blow-energy machine design,
with Tamrock and Atlas Copco either developing, or offering drills with (relatively)
high blow frequencies (115 Hz and 102 Hz, respectively)24
. Tamrock have additionally
developed their “KHZ” machine, which is stated to operate in the kilo Hertz range, with
a variable blow frequency that can be tailored to the rock being drilled25
, and is reported
to have achieved five metres per minute in hard granite.
The reasons for moving to these higher frequencies are not clear, and it is assumed, in
the absence of published literature, that the bulk of the research work on the influence of
percussion frequency is either incomplete or company confidential.
5.1.2 DTH and churn drilling
Down-the-hole (DTH) hammer drills are inherently quieter than the top-hammer drills,
due to the percussive element advancing into the hole as the penetration progresses, thus
using the rock to muffle the noise. Under optimal thrust considerations, (theoretically)
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DTH and churn drilling are more efficient than conventional top-hole drills for rocks
with low penetration resistance26
. At higher penetration resistances, the three (top-hole,
DTH and churn) drills approximate one another.
However, to maintain adequate blow energies, the piston size limits the minimum
diameter at which such drills can work effectively (in a churn drill the piston and the bit
are a single mechanism). However, recent advances in magnetostrictive and shape
memory steels, and piezoelectric elements may indicate the use of these for down-the-
hole purposes27
, allowing the potential to reduce the diameter of the in-hole percussive
components.
5.2 Drag-bit cutting
Drag bit cutting is an established technology, predominantly used in the oil- and gas-
well industry. Haase28
has demonstrated that penetration rates comparable with rotary-
percussive drilling can be achieved but, in order to do this, expensive bits and
complementary water jetting are required. Table 2 presents normalised penetration
rates, required thrust forces and bit costs for various types of rotary drag bits,
normalised against the equivalent data for a hydraulic percussive rock drill.
Table 2: A comparison of hydraulic percussive drilling and rotary drilling28
Bit Type Dimensions Rate of penetration
Maximum thrust
Bit Costs
(mm) (m/min) (kN) (R/m) Hydraulic percussive rock
drill 40 1,0 1,0 1,0
(*)
Impregnated 48 x 32
37 x 23 0,11 – 0,16 25 2,5 – 10,1
Natural diamond, surface
set
48 x 32
37 x 23 0,07 -0,11 25 5,0 – 5,9
Polycrystalline diamond,
surface set
37 x 23
60 x 42
0,14
0,55 – 1,27
8
8
10,9
18,5
(*) Cost for the complete drill, thrust leg and bit, based on 1989 Rands (R1,19/m).
Table 2 shows that the thrust forces required to achieve comparable penetration rates are
considerably higher than those for conventional rotary percussive drilling, implying the
use of hydraulic actuators to achieve the required forces.
5.3 Fluid-jet cutting
The use of water jets, with and without abrasive entrainment, has been used for some
years, and is reaching maturity in the engineering materials field of application for the
cutting and shaping of ceramics, reinforced composites and heat-sensitive materials.
Water-jet cutting has typically three guises, viz:
• Pure water at high pressure, supplied as either a continuous, pulsed or cavitating
jet;
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• Water at high pressure with entrained abrasive and / or air; and
• Mechanically assisted water-jet cutting, where any of the two above techniques
operate in conjunction with a mechanical drill or cutter.
The cutting action of the jet is generally a function of the stagnation pressure as it
impinges onto the rock, implying that the actual delivery pressure of the fluid to the
nozzle does not necessarily have to exceed the compressive strength of the rock. These
parameters define what is commonly termed the “threshold pressure” of cutting, or the
minimum pressure required to cause fracturing. With mechanically assisted water-jet
cutting, the jet causes a form of strain softening of the rock ahead of the cutter, implying
that it may not be necessary to provide as high a pressure as for pure water-jet cutting29
.
The threshold pressure for damaging typical South African quartzitic rock is 100 MPa28
.
The typical drawbacks to this form of cutting are the high quantities of water required,
as well as the size of the power packs needed to generate the high pressures and jet
momentum (flow rate).
Kollé30
states that for the abrasive slurry drilling, the mass of abrasive used per mass of
rock removed is always greater than unity, and may go as high as 20 times that of the
displaced rock. Extrapolating from Kollé’s example given for granite, a blast hole of 34
mm diameter by 1,2 m depth would require 74 kg of abrasive. Thus when abrasives are
being used, these can be expensive adding an additional logistical load to the mine’s
transport system. Their recovery post-cutting may also prove an additional process
burden prior to reaching the dewatering pumps.
5.4 Thermal methods
O’Brien et al.12
considers several methods of using heat sources to melt or spall holes of
varying diameters in rock. These methods included direct heat sources (thermal lances,
or jets, for example), lasers, and microwave concentration. All have been demonstrated
to create holes in rock and concrete. The direct heat source methods, however, were
generally discarded for detailed examination as typical mining tools, owing to their
inherent practical and safety implications.
5.4.1 Laser drilling
Lasers have received considerable attention recently in the US as tools for developing
oil wells, and it has been shown that they operate on rock by either spalling (thermal
stress induced) or direct melting and vaporisation. In general, however, the literature is
consistent in reporting that when blind holes are being drilled, the melt pool tends to
shield the laser beam, resulting in a deceleration of the penetration rate with depth22
.
5.4.2 Microwave drilling
Jerby and Dikhtyar32
describe the microwave drill (US patent number 6,114,676) as
being capable of drilling into many non-conductive (to electro-magnetic radiation)
materials, such as (amongst others) concrete, rocks, ceramics, wood and glass. The
principle of operation is to concentrate the microwave energy using a wave-guide into a
small spot underneath this “near-field concentrator”, causing a localised hot spot, which
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melts the material. The pin of the concentrator is then pushed into the melt pool,
forming a hole. On retraction of the concentrator pin, the molten material solidifies,
leaving a lined aperture. This principle is illustrated in Figure 8.
Jerby and Dikhtyar32
claim to have drilled into concrete using a 600 W microwave drill,
creating a two millimetre diameter hole, two centimetres deep, in less than one minute.
Correspondence with the patent holder has indicated that this device is capable of being
scaled to 40mm in diameter, and that larger diameter drilling of concrete has been
achieved33
. While the authors claim that the microwave drill is quiet and does not
produce dust, there are safety concerns arising from the potential for operator exposure
to the microwaves and radio frequency interference with other electronic devices.
Microwaves in
Coaxial waveguide
Concentrator
Concentrated energy Figure 8: Microwave drill principles
32
5.5 Plasma drilling
The CSIR assessed the potential for use of a new type of rock drill that used the shock
wave produced by the generation of a high-frequency, short-rise-time electric
discharges generated under water to break rock35
. Patents for this device, known as the
plasma hole maker (PHM), are held by the Tetra Corporation (US). Several studies of
the device have been undertaken by the CSIR, primarily for use as a roof bolt drill
owing to its relatively compact design, its capability for drilling long holes with a low
reaction force and, hence, suitability for use in narrow stoping widths35,36
. Indications
are that the typically existing in-stope electrical system would be sufficient to power the
device, thus obviating the need for any additional electrical infrastructure to the panel.
The concept of operation is shown in Figure 9.
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Tests conducted by the CSIR36
have indicated the ability to penetrate typical South
African formations. A-weighted peak-level sound measurements recorded during some
of these tests indicated 88,6 dB(A) at a distance of 2,5 m from the device. Noting that
the noise emission is the spark created at the rock interface under water, it can be
anticipated that this sound level will diminish with penetration into the rock.
To second machine
Support mechanism
Feed drive mechanism
PHM
Power in
Water in
Pulse generator
Figure 9: Conceptual plasma roof bolt drill
36
While penetration rates have been estimated to be of the order of 0,5 m/min37
, numerous
mechanical and electrical failures of the machine were encountered. Recent
communication with the intellectual property owner38
has indicated that the PHM has
been redesigned, and that a new patent has been taken out to cover the changes made.
This new design is stated to be five to ten times more efficient that that tested by
Haase36
, and should consume as little as two to five Kilowatts for the same penetration
rates.
5.6 Ultrasonic drilling
Distinct from the high frequency / ultrasonic rotary percussive drilling researchers at the
Los Alamos National Laboratories (LANL) have developed an
ultrasonic/sonic/drilling/coring (USDC) device that drills into rock without the need for
rotary motion of the bit39
. A piezoelectric stack is excited to between 20 kHz and 23
kHz, and a free-floating end-effector (drill or corer) converts this to a combination of
high frequency and sonic waves in the 60 Hz to 1 000 Hz frequency spectrum. Free
masses on the opposite end of the piezoelectric stack further enhance the drilling action
and springs maintain the contact between the drill and the stack. The USDC is stated as
being capable of clearing the hole of debris via its ultrasonic vibration, implying that no
flushing water or fluid is required.
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While no quantified specific energies of drilling are provided, Bar-Cohen et al.40
state
that the device was capable of drilling a six millimetre hole in basalt to a depth of 25
mm in some two hours, using and average of ten Watts of power (peak at 25 W). It is
unlikely that this device can be scaled to a size suitable for blast hole drilling.
6. Noise Issues
The noise emitted from the un-muffled machine typically exceeds 115 dB(A), and these
machines are recognised as leading contributors to noise induced hearing loss (NIHL).
Despite mines issuing HPDs, the compensation payments to affected individuals by the
mining industry is high. In recognition of the effects of excessive noise in the
workplace, the Mine Health and Safety Council (MHSC) of South Africa published two
milestones at their 2003 annual summit12
, requiring that the hearing conservation
programmes implemented by industry must:
By 2008, ensure that there is no deterioration greater than ten per cent in hearing amongst occupationally exposed individuals; and By 2013, the total noise emitted by all equipment installed in the workplace must not exceed 110 dB at any location in the workplace.
In is extremely important to recognise that: -
• Seldom, if ever, are rock drills used in isolation and,
• There is a significant change in sound power level between the free field
conditions under which the sound pressure level (SPL) of a machine is usually
measured and reported and the conditions generally prevailing within a stoping
environment.
The difference in SPL for a standard pneumatic rock drill under free field (surface)
conditions and an underground stoping environment is shown in figure 10 and table 3
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Standard Pneumatic Rock Drill Mean SPLs
50.0
60.0
70.0
80.0
90.0
100.0
110.0
120.0
Laeq
(dBA)
calc
16 Hz 32 Hz 63Hz 125 Hz 250Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz 16000 Hz
SP
L (d
B)
Surface 500
Surface 450
Surface 350
Underground 550
Underground 450
Underground 350
Figure 10 SPL spectra of a standard pneumatic rock drill under free field (surface)
and underground stoping conditions at air supply pressures of 350, 450 and 550 kPa.
Table 3 SPL results of several rock drills determined under free field conditions and a
stoping environment at different air supply pressures.
Rockdrill Type Air Supply Pressure
(kPa)
Free field SPL
(dBA)
Underground SPL
(dBA)
Change (dBA)
350 104 112 8
450 107 116 9 Standard pneumatic
550 109 119 10
350 96 108 12
450 100 108 8 Muffled pneumatic
(a) 550 101 110 9
350 97 102 5
480 101 106 5 Muffled pneumatic
(b) 550 102 108 6
Electric NA 95 102 7.
6.2. Rotary-percussive pneumatic rock drills
The current exposure limits of mine workers to physical pollutants, under the Mine
Health and Safety Act12
is 85 dB LAeq,8h, with a peak sound pressure level of 135 dB(A).
A previous study sponsored by the MHSC12
have indicated that the operator equivalent
noise exposure from an un-silenced pneumatic drill, Neq, for a five-hour shift, to be in
the range 113,8 dB to 116,8 dB at supply pressure of 400 kPa and 600 kPa respectively.
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A silenced drill exposes the operator to marginally less noise over the same time period,
namely 105,4 dB to 109,6 dB at the same respective supply pressures. These sound
pressure levels, therefore, place a legal onus on employers to monitor workers for
NIHL, effect noise control programmes, and issue affected workers with suitable
hearing protection devices12
.
The source of the noise emitted by an un-muffled or un-silenced pneumatic drill has
been studied14,15
, and can be summarised in Figure 11.
Figure 11: Discreet sources of noise from a pneumatic rock drill
14
Evident from Figure 11 is that the expansion of exhaust air from the exhaust ports, the
drill steel and the drill body, rank, in order, as the major contributors to the overall
sound emission from a pneumatic rock drill. Table 4 categorises the noise sources
further14
:
Sound pressure level measurements recorded underground for an un-silenced hand-held
pneumatic rock drill at two air supply pressures are depicted in figure 12, and show a
marked increase above 500 Hz13
.
Table 4: Noise sources in pneumatic rock drills by frequency15
Frequency Range (Hz) Source 40 to 100 Impact between the piston and drill steel and impact
between the drill steel and rock
100 to 2000 Exhausting air from the exhaust ports
2000 and above Resonance of the steel parts of the drill and resonance
of the drill steel
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70
80
90
100
110
120
63 Hz 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz
Centre Frequency
SP
L (
dB
)
400 kPa 600 kPa
Figure 12: Sound pressure level distribution – un-silenced pneumatic drill13
From Table 4 and Figure 12, the exhaust noise and the resonance of the mechanical
parts, including the drill steel ring, are therefore the dominant noise sources.
6.2.1 Acoustic isolation of the drill
Early MHSC sponsored work16
assessed the overall noise situation in both coal and hard
rock mines, and described work conducted predominantly by the United States Bureau
of Mines (USBM). In general, it was reported that the use of exhaust mufflers degraded
the penetration performance of drills, owing to an associated increase in exhaust back-
pressure. The USBM programme of work on hand-held drills encompassed four key
areas, viz:
• Redesign of the rotation mechanism, by the fitment of an independent rotation
motor (to counter noise from the rifle-bar arrangement; to allow redesign of the
inlet and exhaust valve arrangements; and to compensate for any degradation
caused by exhaust muffling);
• Design and fitment of an acoustic shroud (muffler) to cover the exhaust as well
as the drill body;
• Development of a drill-steel shroud; and
• Redesign of the drill controls to allow the operator to stand further away.
Two models of drill were produced, one for coal and the other for hard-rock mining,
differing mainly in the blow energy and the construction of the outer drill-body shroud.
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Figure 13: USBM Hard rock stoper drill
15
Both models were reported to be lighter than the drills they replaced, and sound-level
measurements showed an average decrease of 15 dB(A) for the coal drill, (down to
102 dB(A)), with drill-steel damping accounting for a five dB(A) decrease on its own.
The penetration rate of this coal drill was reported to be greater than the unmodified
drills. The hard-rock model, with a higher blow energy, was reported to emit some
104 dB(A), with the drill-steel shroud accounting for a three dB(A) decrease in sound
pressure level. Penetration rates were reported to be comparable to the unmodified
version.
Methods specifically for damping the drill-steel ring will be described later, albeit that it
was indicated that the shroud increased the time to change the drill steel, it obscured the
operator’s vision of the steel rotation, and interfered with the chip extraction.
Harper and Scanlon18,17
recognized the performance degradation implications of
muffling a standard pneumatic drill, as well as the problematic introduction of drill steel
shrouding. They opted to totally enclose the drill and steel in a sound damping shroud,
which would be offered directly to the rock as a single enclosed entity. The authors
reasoned that and drop of performance due to the exhaust muffling could be offset by
optimising the thrusting of the drill, and thus thrust was provided inline from within the
shroud. Figure 14 illustrates this machine, which utilises an unmodified SECO S215
drill.
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Figure 14: Totally enclosed drill of Harper and Scanlon
17,18
A constrained layer damping system was developed for the drill steel consisting of a
thin-walled tubular metal cover bonded to the drill steel by a visco-elastic material that
adhered well to both surfaces. Surface testing of the initial prototypes in Norite yielded
decreased sound pressure levels from 115 dB(A) to 93 dB(A), but showed that the
original thrusting performance was inadequate. Further modifications saw an increase
in sound pressure levels to 102 dB(A), and refinements of the method of thrusting18
resulted in a proof of concept that could be taken to full commercialisation.
Otterman et al.19
refined the concept of Harper and Scanlon further (Figure 15), making
use of custom moulded casings and a geared air motor – lead screw thrust arrangement.
Surface testing of the experimental rock drill (XRD), again in Norite, yielded a
reduction to 90 dB(A), with penetration rates equal to the unmodified drill.
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Figure 15: XRD (after Otterman et al.
19)
Underground testing revealed numerous shortfalls, the most significant of which were
the machine’s weight and associated handling difficulties; corrosion of metallic parts; a
build up of water inside the tube with an associated excessive air exhaust back pressure;
and an inability to reach the bottom footwall corner during drilling. Despite these
difficulties, Otterman et al.19
noted that the machine had the potential to be developed to
the point where one operator could manage several jig-mounted machines
simultaneously. This would allow the operator to be located further away from the
noise source.
Figure 16 graphically summarizes the progress of MHSC sponsored noise reduction
developments, depicting recent comparative work conducted by Heyns21
with earlier
work of Franz et al.13
.
Note that the work of Heyns21
was conducted in an artificial stope with differing
acoustic properties to the true underground environment. The figures reported by Franz
et al.13
were recorded underground. Thus figure 16 is included as an indicative
comparison only.
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65
70
75
80
85
90
95
100
105
110
115
120
Pneumatic Muffled
Pneumatic
Water
Hydraulic
Electric QRD, Normal
Steel
QRD, Cladded
Steel
So
un
d L
evel (d
BA
)
Heyns (2003) Franz et al. (1996)
Figure 16: Achieved noise reduction (MHSC sponsored work)
The influence of the expansion of the exhaust air from a pneumatic machine is evident
in Figure 16 by comparing the values recorded by Heyns21
for the water hydraulic and
electric drills. Franz et al.12
recorded sound pressure levels underground for a water
hydraulic drill that exceeded those of a silenced pneumatic drill (Figure 16), but ascribes
this to a higher drill-steel ring resulting from the greater blow energy.
As the drill steel is common to all drills, irrespective of their motive power, damping the
noise from the steel is discussed in more detail in the following section.
6.2.2 Reducing drill steel ring
Maneylaws et al.16
report that misalignment between the piston-chuck-drill steel can
account for as much as 20 per cent of the impact energy being redirected towards
establishing transverse waves in the drill steel. The presence of transverse waves not
only reduces the effective energy available for penetration, but also gives rise to
unwanted sound vibrations. These authors recommended closer tolerances in the
manufacture of the chuck and drill steel shank, including a longer drill steel shank to
assist with alignment in the chuck. Interalia, these comments also apply to the
maintenance of the drills and periodic, accurate dressing of the drill-steel shanks.
Recognising this latter aspect, the Chamber of Mines Research Organisation (CoMRO)
developed nylon chuck bush inserts that fitted between the drill steel shank and the
hexagonal bore of the chuck20
. By taking up the play between these two components,
not only was the wear on the drill-steel shank reduced, but the noise emissions from the
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drill dropped by approximately two to three dB(A), by eliminating chatter. Since the
cost of the drill chuck modifications required to accept these inserts was relatively high,
the inexpensive and disposable chuck bush inserts never gained commercial acceptance.
Addressing the damping of the drill steel itself, many methods have been employed.
The most common is the use of a thin steel outer tube with an outer diameter below that
of the bit outer diameter, and an inner diameter sufficient to fit over the hexagonal steel.
Maneylaws et al.16
reports that these loose shrouds were difficult to fit, and operators
frequently drilled without them, despite the shroud yielding a respectable lowering of
the noise level. Bonding these shroud tubes to the drill steel with a viscoelastic
compound required turning the hexagonal steel down to a circular section, and while
providing the sound damping properties, weakened the steel leading to bending and
failure under thrust20
. Ultimately, the shrouds added an additional cost to the drilling
operation, and were abraded by the rock.
The USBM developed “concentric” drill steels15
, consisting of an inner steel rod
designed to transmit the percussion force, and an outer torque tube to provide the
rotation. This outer tube additionally provided the sound damping. Figure 17 illustrates
this concept.
Field trials of these concentric steels in a hand-held drill yielded a three dB(A) reduction
in the incident sound pressure level, and did not influence the penetration rate19
. They
also stated that commercial production of these steels was being considered, but that
patent rights were under dispute. The present literature is devoid of direct reference to
commercial production of these drill steels.
Figure 17: Concentric drill steel
19
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6.3. Noise Conclusions
It is unlikely that the conventional pneumatic rotary-percussive drill will be completely
phased out within the time spans envisaged by the 2008 MHSC guideline. The un-
silenced pneumatic drill is the established workhorse of the South African mining
industry, and the drill most commonly encountered underground. The advent of
silenced pneumatic and electrically powered drills, offering approximately a seven
dB(A) to 15 dB(A) decrease in sound pressure levels respectively, morally obliges
employers to give these serious consideration for phasing into a mine’s drilling
programme. In the absence of regulations, suppliers will continue to ply un-silenced
machines, justified on a direct cost basis.
The totally enclosed rock drills of Harper and Scanlon17
and Otterman et al.19
appear to
offer an immediate method of reducing noise in the stopes of producing mines.
However, apart from exhibiting relatively minor technological shortcomings, these
machines are stated to be unwieldy in the underground situation, and have received
negative comment from miners and drillers. Nonetheless, their current development
status is the most advanced, with prototypes available for field acceptance testing on a
larger scale. In terms of the MHSC guidelines, therefore, these drills, as a new
technological offering, have the shortest critical path to implementation.
7. Conclusions
While there continue to be significant advances in the field of mechanized rock-
breaking there can be no doubt that drill and blast will remain the predominant method
of ore winning in South African deep level mines for decades to come. Given the
current importance of cost and safety and health issues further exacerbated by the
availability of skilled drill operators the mining industry finds itself the difficult position
of having no immediately apparent technical solution. The pneumatic rock drill is a
mature technology will very little prospect of any new major development and while
there are potential new technologies available for drilling application they will not
become commercially available for decades.
The primary stope drilling requirements of the South African mining industry must,
therefore, be the cost effective drilling of accurately placed and directed blast holes with
minimal exposure of the operators to environmental and health risks.
It is possible however to meet these requirements with currently available technology by
a concerted change from hand operated drilling to drilling using remote controlled drill
rigs. The use of rigs immediately addressed the accuracy of drilling whilst allowing the
application of all the current noise attenuation methods and other environmental
improvements without being constrained by the method of powering (pneumatic,
hydraulic or electric) and therefore allows the retrofitting of any new drilling
technology.
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