ORIGINAL CONTRIBUTION

Laboratory Investigations on Percussive Drilling

S. B. Kivade • Ch. S. N. Murthy • Harsha Vardhan

Received: 13 June 2012 / Accepted: 5 August 2013 / Published online: 22 October 2013

� The Institution of Engineers (India) 2013

Abstract The laboratory investigation was carried out on

ten rock samples using pneumatic drill with drill bits of

different diameters. In general, the process of drilling

always produces sound. Sound is generated from the bit-

rock interface regardless of the material of the bit used in

drilling. The predicted sound level and penetration rate are

a product of the drill power and the physical properties of

the rocks penetrated. Rock samples were collected from the

field and physical properties of the rocks were determined

in the laboratory. The sound level and penetration rates

were correlated with the rock properties. The compressive

strength and abrasivity exhibit strong correlations with the

sound level and penetration rate. It was concluded that,

among the rock properties included in this study, the

compressive strength and abrasivity values are the domi-

nant ones affecting the penetration rate and sound level of

percussive drills. Though ten rock samples have been

covered in this study, detailed analysis of only one of them

is presented.

Keywords Pneumatic drill � Rock properties �Rock samples � Exhibit � Dominant � Drill power

Introduction

Percussive drills have been extensively used in quarries,

open pit mines and construction sites. An accurate estimation

of drilling rate helps make the planning of the rock excava-

tion projects more efficient. The drillability of the rocks

depends mainly on operational variables and rock charac-

teristics. Operational variables known as the controllable

parameters are thrust, blow frequency, pressure, torque, like

bit type and diameter, penetration rate and rotational speed.

However, rock properties such as compressive strength,

porosity, density and geological conditions are uncontrol-

lable parameters [1, 2]. Penetration rate is the progression of

the drilling bit into the rock in a certain period of time, which

is generally expressed as ‘‘m/min’’. Depending on the factors

that affect penetration rate these can be classified as

changeable and unchangeable factors [3].

The major noise source in pneumatic drill is the driving

unit which emits high intensity low frequency noise due to

compressed air [4]. Of the total noise energy of pneumatic

drill, 87.5 % is contributed by the exhaust and the next

largest component is the impact between the piston and the

drill steel [5–7]. Some studies in the past attempted to reduce

the high frequency noise due to vibration of the drill steel

using rubber collars on the drill rod [8]. However, at that time

this method was not successful as the heat generated due to

internal friction deteriorated both the material in the collar

and the bending in the rod. Significant sound level reduction

in pneumatic drills could be achieved by eliminating two

large exhaust openings and substituting rows of holes around

the circumference of the cylinder. A 75 % reduction in the

total noise energy has been reported by incorporating the

above design modifications [6, 9]. A simple sleeve of rubber

hose clipped over the drill steel can also reduce the drill noise

[10]. Further, a closed case fitted with a muffler around the

S. B. Kivade (&) � Ch. S. N. Murthy � H. Vardhan

Department of Mining Engineering, National Institute of

Technology Karnataka, P.O. Srinivasnagar, Surathkal,

Mangalore 575025, Karnataka, India

e-mail: sbkivade@gmail.com

Ch. S. N. Murthy

e-mail: chsn58@gmail.com

H. Vardhan

e-mail: har7323@gmail.com; harshanitk@gmail.com

123

J. Inst. Eng. India Ser. D (October 2013–March 2014) 94(2):81–87

DOI 10.1007/s40033-013-0024-2

drill body can also be designed for the purpose of noise

reduction [11]. Replacement of normal steel collared rod a by

plastic collared rod in pneumatic drills has also been reported

to reduce the sound level of drills [12].

Previous Investigations

For rock engineering purpose, very limited publications are

available on the subject.

Vardhan and Murthy [13] investigated the influence on

sound level produced due to drilling in rocks of varying

physical properties like compressive strength and abrasiv-

ity. Five different rock samples were used to find out the

sound level at different positions like near the drill bit, drill

rod, exhaust, and operator’s position. Vardhan et al. [14]

carried out a research work, estimating the rock properties

using sound level produced during drilling. In this inves-

tigation, same data was used by Vardhan and Murthy [13].

They suggested further work in this direction.

Kumar et al. [15] carried out investigation in a coal mine

to estimate some of the rock properties during blast hole

drilling. They reported that detailed study could not be taken

up in the field as it was difficult to get wide range of rocks

with varying compressive strength and therefore, it was also

difficult to determine the sound level produced. It was sug-

gested to extend this work by conducting detailed laboratory

investigation, to measure the equivalent sound level pro-

duced during rotary drilling on a wide range of rock samples

with varying compressive strength in controlled conditions

and thereby established relationship between rock properties

and sound level produced during drilling.

Objectives of the Study

The objective of this study is to determine physical prop-

erties of rocks which are determinant in their drillability.

The experimental studies were carried out using integral

steel chisel of different bit diameters. The relations

between mechanical characteristics of rocks and their

drillability and sound levels were examined, to establish

the influence of various physico-mechanical properties of

rocks on the drill performance, i.e. penetration rate and

sound level.

Laboratory Drilling Experiments

Brief Description of the Percussive Drill Machine

The experimental set up used in the present work was the

same as given by Vardhan and Murthy [13] and as shown

in Fig. 1. The important specifications of the jackhammer

are: (a) Weight of the machine-28 kg (b) Number of blows

per minute-2,200 (c) Type of drill rod-Integral steel chisel

(30, 34 and 40 mm) (d) Recommended optimum air pres-

sure-589.96 kPa (e) Thrust value as high as 1,000 N.

Experimental Procedure

The block, after trimming to size, was placed on the base

plate and thoroughly clamped by placing two mild steel

plates (1 cm thick 9 7.5 cm width 9 61 cm length) on its

top surface which was rigidly held with the help of four

numbers of bolts (Fig. 1). Collaring was done at optimum

air pressure (588 kPa) and lowest thrust (100 N). For

keeping the starting conditions common for all the tests, all

the rock samples were collared up to a fixed depth of

2.54 cm using an old drill steel. A new or resharpened drill

bit was used for each test. Drill runs were either 1 min or

30 s, depending on the rate of penetration. Runs were

timed with a stopwatch, and penetration distance was

determined by measuring the relative position of the drill.

The depth of drill hole was measured in mm/s carefully.

The corresponding reading of the pointer on the vertical

graduated scale was recorded as the initial reading.

The range of operating air pressures selected for the

study was 392, 441, 490, 539, and 588 kPa. Similarly, the

range of applied thrust selected was 100–1,000 N. The

rocks used in the present study are Shale, Dolomite, Sand

stone, Lime stone, Hematite, Dolerite, Soda granite, Black

granite, Basalt, and Gabbros.

In this investigation, the sound level and penetration

rates of percussive drills were measured in the laboratory

Fig. 1 Jackhammer drill setup for drilling vertical holes in rock

samples

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123

and correlated with the rock properties. Noise measure-

ments were carried out in open space (outdoor location) to

reduce the effect of reflecting noise. In this laboratory

investigation, total three drill bits were used. Three integral

steel chisel bits 30, 34, and 40 mm diameter and 42, 43,

and 62 cm length of chisel geometry. These bits were

selected from among the available sizes.

Types and Specification of Rock Samples

Sound level measurement on pneumatic drill set up was carried

out for ten different rock samples obtained from the field.

During sample collection, each block was inspected for mac-

roscopic defects so that it provides test specimens free from

fractures and joints. Sound level measurement on pneumatic

drill set up was carried out for ten different rock samples. These

rock samples were shale, dolomite, sand stone, lime stone,

hematite, dolerite, soda granite, black granite, basalt, and

gabbros. The size of the rock blocks was *30 cm 9 20 cm

9 20 cm. The different types of rock used in the investigation

and their properties are given in Table 1.

Methodology

Noise Measurement

Sound pressure levels were measured with a CENTER

make model 320, IEC 651 Type II sound level meter. The

instrument was equipped with a CENTER make wind-

screen for minimizing the sound effect produced from

wind, � inch elect ret condenser microphone, digital dis-

play, time weighting and level ranges. The microphone and

the preamplifier assembly were mounted directly on the

sound level meter. The sound level meter was calibrated

before taking up any measurement using an acoustic cali-

brator available in the institute. For all measurements, the

sound level meter was hand-held. The instrument was set to

measure A—weighted sound pressure levels in the range of

30–130 dB.

Determining the Compressive Strength and Abrasivity

of Rock Specimens

Though the compressive strength of rock samples can be

determined accurately using direct methods, in this study it

was determined indirectly using Protodyakonov’s Apparatus.

The reason being difficulty in making core samples out of the

particular rock block after drilling has been carried out into it.

The method for indirectly determining the compressive

strength of rock samples using Protodyakonov’s Index

Apparatus was carried out as per International Society of

Rock Mechanics (ISRM) [16] suggested methods.

The compressive strength of rock samples was deter-

mined indirectly using Protodyakonov’s Strength Index. In

this method Protodyakonov’s apparatus was used. Five

samples weighing 50 g each of a particular rock was sepa-

rately taken in a Protodyakonov’s apparatus. Five blows

(n) were given using a weight of 1.8 kg from a height of

0.6 m. This material (5 9 50 = 250 g) was then transferred

to a 500-lm sieve. The fines which pass through the sieve are

taken in a volume meter (measuring cylinder) and the height

of the column (h) is noted down. Protodyakonov’s Strength

Index (PSI) = (20 n)/h, where, n = number of blows = 5

and h = height in the volume meter (cm). Using this index,

the compressive strength of a rock index, the compressive

strength of a rock sample was determined using the relation:

Compressive strength = 100 9 PSI (kg/cm2).

Abrasion test measures the resistance of rocks to wear.

This test includes its wear when subjected to an abrasive

material, wear in contact with metal, and wear produced by

contact between the rocks. The abrasivity of rock samples

was also determined in accordance with the ISRM sug-

gested methods.

For this purpose, Los Angele’s abrasion apparatus was

used. The abrasion test requires two different sizes of rock

samples i.e., 19.0–13.2 and 13.2–9.5 mm. One set of test

samples of 1,250 ± 10 g was prepared so that they pass

through a sieve of 19.0 mm and are retained on a sieve of

13.2 mm. Another set of test samples of 1,250 ± 10 g was

prepared so that they pass through a sieve of 13.2 mm and

retained on a 9.5 mm sieve. Both the test samples are

placed in the Los Angeles abrasion testing machine. The

abrasive charge consists of cast iron spheres *48 mm in

diameter and each weighing between 390 and 445 g. The

machine is rotated at a speed of 20–30 revolution/min for a

period of 15 min. The material is then discharged from the

machine and sieved on a 1.7 mm sieve. The material

retained on the sieve is weighed. The abrasion resistance is

calculated using the relation,Abrasion resistance or

Table 1 Compressive strength and abrasivity of different rock

samples

Sample no Rock type Compressive

strength (Mpa)

Abrasivity (%)

1 Shale 102.05 15.2

2 Dolomite 110.7 16.7

3 Sand stone 125.6 17.3

4 Lime stone 149.92 18.8

5 Hematite 155.72 19.5

6 Dolerite 162.6 20.1

7 Soda granite 179.2 20.5

8 Black granite 187.72 21.4

9 Basalt 199.8 22.5

10 Gabbros 223.5 23.8

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abrasivity = (loss) in weight of the samples/original

weight of the samples i.e. 5,000 ± 20 g) 9 100 %.

Results and Discussion

Compressive Strength and Abrasivity of Rock Samples

The results of the experimental study of the compressive

strength and the abrasivity of the rock samples are given in

Table 1. It has been observed that, for all the rock samples

compressive strength and abrasivity increases from shale to

gabbros.

Rock Properties vis-a-vis Sound Level Produced

by Pneumatic Drill

Influence of Air Pressure on Sound Level and Penetration

Rate

The effect of air pressure on sound levels at constant thrust

of 700 N and varying drill bit diameters of 30, 34 and

40 mm for the sample of gabbros near drill rod is shown in

Fig. 2. An increase in sound level was observed with

increase in air pressure values.

Similarly, it was observed for the entire rock blocks

tested from shale to gabbros at operator’s position, at

exhaust, and near drill bit. The penetration rate increases

when the air pressure is increased from 392 to 588 kPa at

varying drill bit diameters of 30, 34, and 40 mm and

constant thrust of 700 N as shown in Fig. 3. Stall condi-

tions were observed in many cases especially at low

operating air pressures and higher thrust. The drill steel was

bouncing back indicating the improper contact of the bit

with the rock. For a given drill machine, energy available

at the bit rock interface has a direct bearing on the air

pressure. Further, the blow frequency is also proportional

to the air pressure. Therefore, the increase in air pressure

produces higher penetration rates and sound level and vice

versa for a given bit rock combination. It was observed in

the investigation that for the air pressure of 392–588 kPa

and constant thrust of 700 N, the difference in penetration

rate for different rocks from shale to gabbros is 1.9 mm/s.

Penetration rate increases with an increase in air pres-

sure. With the increase in bit diameter, penetration rate

decreases along with increase in air pressure. This shows

that both the thrust and air pressure have a significant effect

on the penetration rate and the level of sound produced by

pneumatic drill. The increase in penetration rate, decreas-

ing the sound level along with increase in thrust reaches the

optimum and then starts decreasing. The maximum sound

level and penetration rates were observed at the drill rod for

all bit rock combinations. The operating air pressure of 392

to 588 kPa, at optimum thrust of 700 N for the rock blocks

tested was observed. It was also observed that penetration

rate is directly related to operating pressure, and penetra-

tion rate increase with increase in air pressure.

Influence of Thrust on Sound Level and Penetration Rate

Figure 4 shows the effect of varying thrust on sound level

near drill rod at varying drill-bit diameters of 30, 34, and

40 mm and constant air pressure of 588 kPa for gabbros. It

was observed that, at constant air pressure of 588 kPa and

varying thrust of 100–1,000 N caused an increase of

1.0 dB in the sound level in gabbros near drill rod. Further,

the same was observed for other rock samples too at dif-

ferent positions.

Figure 5 shows the effect of thrust on penetration rate at

constant air pressure of 588 kPa and varying drill bit

diameter of 30, 34, and 40 mm for gabbros. It is seen

clearly that the penetration rate increases with increase in

thrust level. When it reaches the optimum thrust level ofFig. 2 Effect of air pressure on sound pressure level at varying drill

bit diameter and constant thrust of 700 N

Fig. 3 Effect of air pressure on penetration rate at varying drill bit

diameter and constant thrust of 700 N

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123

700 N the penetration rate starts decreasing, even if we

keep on increasing the thrust, ultimately reaching the stall

condition. If the thrust is further increased beyond the

optimum thrust level (800, 900 and 1,000 N), the pene-

tration rate start decreasing, ultimately exceeding this limit,

‘‘stall’’ of the drill will occur. Very high thrusts do not

result in high penetration rates even at very high operating

air pressures.

The variation in the penetration rate from shale to

gabbros was 0.6–3.07 mm/s at constant air pressure of

588 kPa and a varying thrust of 100–1,000 N. The maxi-

mum penetration rate was observed at a thrust of 700 N for

all the rock samples from shale to gabbros at the operating

air pressure varying from 392 to 588 kPa for all the bit rock

combinations and all the measurement locations.

This is due to better contact time between the bit and the

rock and sufficient transfer of energy from the drill to the

rock. Very high thrusts do not result in high penetration

rates even at higher operating air pressures as exhibited in

Fig. 5. There should be proper match between the applied

thrust and operating air pressure. If the drill is operated at

this condition, it works with optimum efficiency, causing

less wear of the drill steel and is least susceptible to

mechanical breakdown.

Influence of Bit Diameter on Sound Level and Penetration

Rate

A small diameter bit has to remove a smaller amount of

rock in drilling a hole of given length than a bit of larger

diameter. Thus it is not unexpected that an increase in

penetration rate is obtained when the diameter of the bit is

reduced, all other operating conditions remaining constant.

It is generally accepted that penetration rate varies with the

diameter of the bit and these trials confirm these relation-

ship within practical drilling limits. They also show how

the bit characteristics can affect these relationships.

Figure 6 shows the effect of bit diameter on sound level

near drill rod at varying bit diameters (30–40 mm) and

constant air pressure of 588 kPa and varying thrust for

different rock samples. The results of the study show that

with increase in bit diameter the sound level increases with

increase in air pressure.

This establishes the influence of bit diameter (integral

steel chisel) on the penetration rate in pneumatic drilling.

The effect of bit diameter on penetration rate at varying

thrust and constant air pressure of 588 kPa is shown in

Fig. 7. It was observed from the study that with an increase

in bit diameter, penetration rate decreases for all the rock

blocks tested at all the measurement locations. Very low

thrust results in low penetration rate, however, even very

high thrust does not produce high penetration rate at higher

operating air pressure.

Influence of Compressive Strength on Penetration Rate

and Sound Level

The effect of compressive strength of rocks on sound level

near drill rod, at constant air pressure of 588 kPa and

varying drill bit diameters of 30, 34 and 40 mm for a

Fig. 4 Effect of thrust on A-weighted sound pressure level near drill

rod and varying drill bit diameter and constant air pressure of 588 kPa

Fig. 5 Effect of thrust on penetration rate near drill rod and varying

drill bit diameter and constant air pressure of 588 kPa

Fig. 6 Effect of bit diameter on A-weighted sound pressure level

near drill rod at constant air pressure of 588 kPa and varying thrust

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constant thrust of 700 N for all bit rock combinations is

shown in Fig. 8. The effect of compressive strength on

penetration rate at constant air pressures of 588 kPa and

varying drill bit diameters of 30, 34 and 40 mm for con-

stant thrust of 700 N for different rock samples is shown in

Fig. 9. It shows that, increase in the compressive strength

results in the decrease in the penetration rate from shale to

gabbros as well as decrease in sound level.

There is a considerable difference in the compressive

strength of rock samples from one to ten as shown in Table 1.

It is observed from the studies that sound level increases to

maximum near drill rod, with the air pressure of 588 kPa.

The change in sound level near drill rod for constant thrust

values varies from 2.8 to 3.6 dB. This increase in sound level

near the drill rod is due to higher vibration of the rod while

drilling in rocks of higher compressive strength. The above

result shows that the increase in compressive strength of

rocks increases the sound level.

Conclusion

Predicting the penetration rate and sound level is very impor-

tant in percussive rock drilling. Sound level and penetration rate

are necessary for the cost estimation and the planning of the

project. Drillability is one of the important parameters affecting

the rock properties. The rock properties involved in this study,

i.e. the compressive strength and abrasivity are found to be the

dominant rock properties affecting the sound level and pene-

tration rate of percussive drills.

The investigation shows that the penetration rate and sound

level increases with increase in thrust, reaches a maximum, and

then starts decreasing even if we keep on increasing the thrust,

ultimately reaching the stall condition. If the thrust is further

increased beyond the optimum thrust level (800, 900 and

1,000 N), the penetration rate and sound level start decreasing,

ultimately exceeding this limit, ‘‘stall’’ of the drill will occur.

Very high thrusts do not result in high penetration rates and

sound level even at very high operating air pressures. The study

reveals that the sound level near the drill rod is comparatively

higher than that near the drill bit, exhaust, and the operator’s

position for all the rock blocks tested for a particular bit rock

combination.

The higher vibration near the drill rod accounts for the

increase in sound level at this point while drilling in the

rock blocks having higher compressive strength. It was

observed that both the thrust and air pressure have a sig-

nificant effect on the sound produced by the pneumatic drill

at all the measurement locations.

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Fig. 7 Effect of bit diameter on penetration rate near drill rod at

constant air pressure of 588 kPa and varying thrust

Fig. 8 Effect of compressive strength on penetration rate at constant

air pressure of 588 kPa and thrust of 700 N, and varying drill bit

diameter

Fig. 9 Effect of compressive strength on penetration rate at constant

air pressure of 588 kPa and thrust of 700 N, and varying drill bit

diameter

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