underground drilling and equipment selection 2011

The Southern African Institute of Mining and Metallurgy

Drilling and Blasting 2011

G Harper

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.



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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.



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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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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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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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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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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.



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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.



G Harper

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.



G Harper

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.



G Harper

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.



G Harper

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



G Harper

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



G Harper

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.



G Harper

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blast hole drilling system, SIMRAC GEN 207.



G Harper

18. Scanlon, T. and Harper, G.S., 1998. Evaluation and further development of a

‘quiet’ non atmospheric polluting balst hole drilling system, SIMRAC report GEN

311.

19. Otterman R.W., Burger, N.D.L., von Wielligh, A.J. and de Wet, P.R., 2001. Design and development of a quiet, self-thrusting blast hole drilling system,

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by localized microwave radiation, 8th Ampere Proc., Bayreuth, Sept. 2001.

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36. Haase, H.H., 1997. Development and testing of a plasma device for creating roof

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electric-pulse discharge (EPD), Mechanical Technology, April, 1998.

38. Moeny, W., 2005. Personal electronic correspondence relating to the current

status and developments with the PHM technology.



G Harper

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while drilling using the USDC and integrated sensors, Eurosensors XVII

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underground drilling and equipment selection 2011

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