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1. INTRODUCTION

Site characterization in weathered rock mass is important

in rock mechanics and rock engineering. It is often basedon the factual data from drillhole exploration andgeological mapping. To obtain an accuratecharacterization of ground rock mass condition, moreand additional factual data about the ground condition

are always useful and needed.

One of the additional factual data which can be used to

characterize weathered rock mass is the data from

automatic and digitally monitoring of drilling in groundinvestigation and in construction. The digital data from

drilling monitoring can provide additional geologicalinformation for better site characterization. Particularly,

air-driven rotary-percussive drilling with down-the-holehammer can be used to drill hundreds and thousands

production holes in geotechnical construction.

Over the last ten years, Yue and his research team [1, 2,3, 4, 5, 6] have developed a new in-situ technique for

characterization of rock mass and the associated spatialdistribution in ground. It is the DPM system. It

comprises seven pressure transducers and digital data

logger. It can automatically monitor the drilling processof air-driven rotary-percussive drilling machines in

real-time and in a digital manner. Based on themonitored digital DPM data, a data analysis method has

been developed.

This paper reports a continued effort on the DPMmethodology for rock mass characterization. To examinethe influence and interaction of the most importantvariable factors concerned in rock drilling- namely, air

pressures, work condition of drilling machine anddrilling rate, two drilling machines are used to drill two

adjacent holes and the corresponding drilling processesare monitored by DPM system. A full drill analysis isapplied to divide the monitored drill data into manysub-processes such as pushing-in, pulling-back, anddrilling. Work conditions of the two drilling machinesare examined in these sub-processes by two aspects:

thrust motor effect and effect of drill rod, down-the-holehammer and drill bit.

The case study demonstrates that the different drill workpowers on the drilling data can be identified and theDPM methodology can be applied into different drilling

machines.2. DPM METHODOLOGY

2.1. Air-derived rotary-percussive drillingmachine

(A) Composition of drilling machine

As shown in Fig. 1, the drill equipment system

consists of the following main parts:

A manual control panel;

A swivel drill chuck (fixed with a pneumaticrotation motor and a shank adaptor);

A pneumatic thrust motor;

A steel gauge loop chain;

A straight steel frame with two sliding channelsand a centralizing clamp at the front end;

A number of steel cylindrical open tubes of 1, 2 or

3 m long and with couplers for extension, and

A down-the-hole hammer of 1 or 3m long andwith a drill bit at the bottom end.

(B) Air pressures of drilling machine and theirfunctions to drill

ARMA 08-219

Case Study of Drilling Process Monitoring from Two Adjacent Holes wit

Two Drills

J. Chen, W. Gao, Z.Q. Yue

Department of Civil Engineering, the University of Hong Kong, Hong Kong, China

ABSTRACT: The drilling process monitoring (DPM) has been used in site characterization. Information from the drilling

monitoring can be added to the existing geological information for more accurate identification of the rock discontinuities and

weathered degrees of soils and rocks, because a larger number of drilled holes from rotary-percussive drilling can be used. In this

paper, two sets of DPM data from two adjacent holes are examined. The two holes were drilled with two different

rotary-percussive drills in soil nailing construction. Based on the monitored data, the interaction of the most important drilling

parameters is assessed. The parameters are the drilling rate, the air pressures and the effect of work powers of different drills. The

effect of the different drill work powers on the drilling data is identified. So, only the rock dependant variation can be left for site

characterization. The results demonstrate that the drilling parameters from different drills can be normalized.

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Rotation pressure, thrust pressure and percussionpressure drive the two motors and the down-the-holehammer in a drilling machine to finish different actions.The functions of the rotation pressure are to remove

chippings from hole wall and the bottom of the hole toensure the drilling smoothly and to ensure the rotation

between succeeding hammer blows from the impactmechanism so that the bit for every blow works on anew part of the bottom of the hole and to connect or

disconnect the drill rods one by one.

The function of thrust pressure is to ensure that thepercussion energy is transmitted to the rock and to push

in and pull back the drill rod and drill bit. This meansthat the thrust keeps the bit in contact with the bottom of

the drill hole when the stress wave from the hammerreaches the drill bit. Furthermore, the thrust also keeps

the rods are in contact with each other.

The function of percussion pressure is to let the hammerto produce impact force on the hole bottom and breakthe geomaterial in the front of drill bit and to flush

chippings off with the air flow.

(C) Drill actions of drilling machineDPM system records all drill actions of drilling machine.The drill actions are listed as:

Drilling: This operation refers to that the drill bitdrilling into geomaterials;

Pushing-in: This operation refers to the downwardmovement of the chuck due to the forward pullingof the loop chain. The hammer bit is not drillingthe material at the bottom of the hole. This processcan be considered as the drilling in air from the

definition. The moving speed of the chuck isdescribed as the pushing-in rate;

Pulling-back: This operation refers to the upward

movement of the chuck due to the backwardpulling of the loop chain. The moving speed of thechuck is described as the pushing-back rate;

Flushing: This operation refers to the downward orupward movements of the chuck. The hammer bit

is not drilling the material at the bottom of thehole. The compressed airflow for percussion is

being applied.

From above introduction, we can know that the mostimportant actions concerned in drilling are thrust action

and drilling action. The thrust action is finished by airdriven thrust motor. The drilling action is finished by air

driven drill rod, down-the-hole hammer and drill bit. Thework conditions of thrust motor, drill rod, down-the-holehammer and drill bit are very important to rock drilling.

2.2. DPM SYSTEMThe DPM methodology includes a hardware system forin-situ automatic monitoring and recording of drilling

parameters in real time sequence and a software package

for analyzing and presenting the monitored digital datain time series.

(A) Hardware systemAs shown in Fig. 1 to Fig. 4, the hardware system is a

portable, flexible, reliable and economic in-situtechnique. It can be easily and non-destructivelymounted onto any existing drilling machines forcharacterizing rock mass mechanical strength in real

time.

Fig. 1. A pneumatic rotary-percussive drilling machine withdown-the-hole hammer equipped with a DPM device in Hong

Kong

Fig. 2. Mounting of position transducer

Fig. 3. Mounting of rotation transducer

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Fig. 4. Mounting of pressure transducers

(B) Software system

The software system comprises two parts: data-sampling program and data analysis program. The

data-sampling program collects data from seven sensorsand sends them to computer. With the help of dataanalysis software, the basic geological information along

the drill hole can be obtained without delay.

3. SITE CONDITION AND TEST ROCEDURES3.1. Site conditionThe DPM tests were carried at a cut slope in Hong Kong

(Fig5). The slope is 147 m long and 71 m high and has

a slope angle of 40 degree to horizontal. Ground

investigation drillhole and geological mapping results atthe slope indicate that the geomaterials forming the slope

are completely decomposed tuff (CDT), highlydecomposed tuff (HDT), the weathered rock betweenCDT and HDT, and moderately or slightly decomposed

tuff.

Fig. 5. DPM test site in Hong Kong

3.2. Test procedures

Selected two locations for soil nail drilling. Thetwo locations have the same vertical heights andthe distance between the two holes is about 2 m.

Carried the soil nail drilling at the Hole P15 at onelocation with the first drilling machine.

After the drilling at the Hole P15 was done,replaced the second drilling machine.

The second drilling machine was the same as thefirst one except it had a new thrust motor, a set of

new drill rods, a new down-the-hole hammer witha fresh and sharp drill bit.

Carried out the soil nail drilling at the Hole TH1 atan adjacent location.

Monitored the full processes of the drilling work

for two holes with the DPM system. Analyzed the DPM data. The entire monitored

data were divided into sub-processes including thepushing-in, the pulling-back, and the pure drilling.

Examined the work conditions of the first andsecond drilling machines in the sub-processes withrespect to two aspects: the thrust motor effect andthe combined effect of drill rod, down-the-hole

hammer and drill bit.

4. DPM DATA AND ANALYSISThe soil nail drilling of the Hole P15 was carried outwith the first drilling machine associated with an old setof thrust motor, drill rods, down-the-hole hammer and

drill bit. The soil nail drilling of the Hole TH1 wascarried out with the second drilling machine associated

with a new set of thrust motor, drill rods, down-the-holehammer and drill bit. The two adjacent holes were

located at the same vertical height and their horizontaldistance was 2 m. The surface of the two adjacent holeswas covered with CDT soils that had been exposed due

to cutting into the hillside slope about 15 years ago. Thedrilling work was carried out by one team of two

operators on two different days.

4.1. Drill parameters in pure drillingIn this special case, the parameters affecting the puredrilling can be summarized as follows:

1) Work condition of thrust motor;

2) Work condition of drill rods, down-the-hole

hammer and drill bit;

3) Downward thrust pressure;

4) Percussion pressure;5) Drilling rate;

6) Rock materials.

Because the distance between the two parallel holes is 2m, it was highly possible that the drilling for the twoholes would encounter the same geomaterial at each hole

depth. Under this assumption, the differences in the twoset of DPM data for the Hole Nos. P15 and TH1 can beconsidered mainly due to the differences in the replacedcomponents in the first drilling machine.

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4.2. DPM dataFig. 6 and Fig. 7 present the DPM data in real-timeseries of the Hole Nos. P15 and TH1. Fig. 6a shows the

complete time-history of the drill bit position in the HoleP15. Fig. 6b shows the complete time-history of the

corresponding percussion pressure punching the DTHhammer in the real time series. Fig. 6c shows thecomplete time-history of the corresponding downward

thrust pressure pushing the rod downward. Fig. 6d showsthe complete time-history of the corresponding upward

thrust pressure lifting the rod upward. Fig. 6e shows thecomplete time-history of the corresponding forwardrotation pressure rotating the rod clock-wisely. Fig. 6f

shows the complete time-history of the correspondingreverse rotation pressure rotating the rod

anti-clock-wisely.

Figs. 7a to 7f show the complete time-history of thecorresponding bit position and associated compressedair-flow pressures in the Hole TH1, respectively.

-2

0

2

4

6

8

10

12

14

16

18

20

22

24

10:43:00 10:57:24 11:11:48 11:26:12 11:40:36 11:55:00

Time of Full Drill Process (hour:minute:second)

DrillBitPosition(m)

Adding rods and

drilling

Retrieving rods

(a)

0

0.2

0.4

0.6

0.8

1

1.2

10:43:00 10:50:12 10:57:24 11:04:36 11:11:48 11:19:00 11:26:12 11:33:24 11:40:36 11:47:48 11:55:00


PercussionPressure(MPa)

(b)

0

0.2

0.4

0.6

0.8

1

1.2

10:43:00 10:50:12 10:57:24 11:04:36 11:11:48 11:19:00 11:26:12 11:33:24 11:40:36 11:47:48 11:55:00


DownwardThrustPressu

(MPa)

.

(c)

0

0.2

0.4

0.6

0.8

1

1.2

10:43:00 10:50:12 10:57:24 11:04:36 11:11:48 11:19:00 11:26:12 11:33:24 11:40:36 11:47:4811:55:00


UpwardThrustPressure(MPa)

(d)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

10:43:00 10:50:12 10:57:24 11:04:36 11:11:48 11:19:00 11:26:12 11:33:24 11:40:36 11:47:48 11:55:00


ForwardRotationPressure(MPa)

(e)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

10:43:00 10:50:12 10:57:24 11:04:36 11:11:48 11:19:00 11:26:12 11:33:24 11:40:36 11:47:48 11:55:00


ReverseRotationPressure

(Mpa

(f)

Fig. 6. The DPM data for the whole drilling process in

real-time series in forming Hole P15 in the slope

02

4

6

8

10

12

14

16

18

20

22

24

26

28

1 3:5 7: 47 1 4:0 4: 59 14 :12 :1 1 14 :1 9:2 3 1 4:2 6: 35 14 :33 :47 14 :40 :5 9 1 4:4 8: 11 14 :5 5:23


DrillBitPosition(m).

Adding rods and

drilling

Retrieving rods

(a)

0

0.2

0.4

0.6

0.8

1

1.2

1 3:5 7: 47 1 4:0 4: 59 14 :1 2:1 1 1 4:1 9: 23 14 :26 :35 14 :3 3:4 7 1 4:4 0: 59 14 :48 :11 1 4:5 5: 23


PercussionPressure(MP

a).

(b)

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

13 :57 :47 1 4:0 4:5 9 14 :1 2:1 1 1 4: 19 :23 14 :26 :3 5 1 4: 33: 47 1 4:4 0:5 9 14 :48 :11 1 4: 55: 23


DownwardThrustPressure(MPa)

.

(c)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

13:57:47 14:04:59 14:12:11 14:19:23 14:26:35 14:33:47 14:40:59 14:48:11 14:55:23


UpwardThrustPressure(MPa)

(d)

0

0.2

0.4

0.6

0.8

1

1.2

1 3:57:47 1 4:04:59 1 4:12:11 1 4:19:23 14 :26 :35 14 :33 :47 14 :40 :59 14 :48 :11 14 :55 :23


ForwardRotationPressure(MPa).

(e)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

13:57:47 14:04:59 14:12:11 14:19:23 14:26:35 14:33:47 14:40:59 14:48:11 14:55:23


ReverseRotationPressure(Mpa).

(f)

Fig. 7. The DPM data for the whole drilling process in

real-time series in forming Hole TH1 in the slope

The full drilling processes in the Hole Nos. P15 and TH1in Fig. 6a and Fig. 7a can be divided into two major

parts: 1) adding rods and drilling 2) Retrieving rods.They can be further divided into many actions such as

drilling, pulling-back and pushing-in. The drillinformation of the two holes is summarized in Table 1.

From Figures 6a and 7a and Table 1, it can be observedthat the total time used by the first machine for drillingthe Hole No. P15 is much higher than that used by thesecond machine for drilling the Hole No. TH1, whichindicate that the second machine had much higher

drilling power than the first machine. However, theminutes used for retrieving rods from the two holes by

the two machines are similar, which may indicate thedifferent thrust motors did not induce noticeabledifferences in retrieving rod from the holes. This is

because the time used for pulling-back during retrieving

is a small part of the total used time. Further analysis anddiscussions are given below.

Table 1. Drill information of Holes Nos. P15 and TH1

Time (min.) used for drill actions

Drill actions Hole P15 of 22.76

m deep formed

with the first

machine

Hole TH1 of 25.01

m deep formed

with the second

machine

Adding rods

and drilling62.41 34.03

Retrieving rods 14.39 16.85

Pure drill

process36.60 20.52

4.3. Net drill analysis(A) Net drill processes of two adjacent holesThe DPM results Figs. 6 and 7 can be used to identify

the zones of constant drilling rates and the associatedzones of uniform geomaterial resistance to drilling along

the drill hole in slope. The identified results are shown inFigs. 8 and 9, respectively. In each of the two figures,the drill bit depth is plotted with respect to the net

drilling time for each hole. Besides, the associatedpercussion pressure, downward thrust pressure and the

forward rotation pressure with respect to the bit depthare plotted. The average drilling rate for each of theidentified linear zones in the bit depth versus the net

drilling time curve is also plotted against the bit depth.

A total of 23 drill rods were used to drill the Hole P15 of

22.76 m in length. Each rod is 1.0 m long. Three majorzones can be obtained in the curve of bit depth versus itsnet drill time for the Hole P15. A total of 25 drill rodswere used to drill the Hole TH1 of 24.94 m in length.Three major zones can also be obtained in the curve of

drill bit depth versus its net drill time for the Hole TH1.

(B) Downward thrust pressures of the two holes innet drill process

The purpose of downward thrust pressure in pure drillingprocess is to ensure that the percussion energy istransmitted to the rock. This means that the thrust keeps

the bit in contact with the bottom of the drill hole as thepercussion stress wave reaches the drill bit. So, whetherthe downward thrust pressure is adequate in pure drilling

process is determined by work status of thedown-the-hole hammer and bit.

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Major zone 1: 1.090m/min

2: 0.813m/min

2-1 0518

2-2 0818

3 0.488m/min

3-1 0499

3-2 0397

3-3 0.504

3-4 1449

3-5 0.475

3-6 r 712

3-7 0391

3-8 057

P15

0.50 1.0 1.5 2.0

0

2

6

8

10

12

14

18

22

24

26

DrillBitDepth(m)

20

16

4

Net Drill Time (Minute)

4

16

20

DrillBitDepth(m)

26

24

22

18

14

12

10

8

6

2

0

38363432302826242220181614121086420 0 0.20.40.60.81.01.2

0

2

6

8

10

12

14

18

22

24

26

DrillBitDepth(m)

20

16

4

0

2

6

8

10

12

14

18

22

24

26

DrillBitDepth(m)

20

16

4

0.80.60.40.20

Downward Thrust

Pressure (MPa)

4

16

20

DrillBitDepth(m)

26

24

22

18

14

12

10

8

6

2

0

Forward Rotation

Pressure (MPa) Percussion Pressure (MPa)

1.21.00.80.60.40.20

Major

zone 2

Majorzone 3

Major

zone 1

Zone drilling rate

(m/min)

Fig. 8. DPM results in identifying the zones of constant drilling rates and the associated zones of uniform geomaterial resistance to drilling along Hole P15 in slope.

.

TH1

0.50 1.0 1.5 2.0

0

2

6

8

10

12

14

18

22

24

26

DrillBitDepth(m)

20

16

4

Net Drill Time (Minute)

4

16

20

DrillBitDepth(m)

26

24

22

18

14

12

10

8

6

2

0

38363432302826242220181614121086420

Major zone 1:1.786m/min

z 2 1.284

z 3 1.134

Major

zone 1

Major

zone 3

Major

zone 2

Zone drilling rate

(m/min)

Percussion Pressure

(MPa)

4

16

20

DrillBitDepth(m)

26

24

22

18

14

12

10

8

6

2

00 0.4 0.8 1.2

0

2

6

8

10

12

14

18

22

24

26

DrillBitDepth(m)

20

16

4

Downward Thrust

Pressure (MPa)

0.80.60.40.20

4

16

20

26

24

22

18

14

12

10

8

6

2

0

0 0.4 0.8 1.2

Forward Rotation

Pressure (MPa)

Fig. 9. DPM results in identifying the zones of constant drilling rates and the associated zones of uniform geomaterial resistance to drilling along Hole TH1 in slope.

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The accumulative histograms of the downward thrustpressures in the pushing-in and pure drilling processesfor the major zones 2 and 3 of the Hole Nos. P15 andTH1 are shown in Figs. 10 and 11, respectively.

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Fig. 10. Accumulative Histogram of downward thrust pressure

in pushing-in process and pure drilling process of major zone

2 of the Hole Nos. P15 and TH1

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Fig. 11. Accumulative Histogram of downward thrust pressure

in pushing-in process and pure drilling process of major zone

3 of the Hole Nos. P15 and TH1

The monitored data show that the pushing-in process for

drilling the major zone 2 in Hole TH1 was not used. So,Fig.10 has no the pushing-in curve for TH1. Figs. 10 and

11 show that a majority of the downward thrustpressures in pure drilling for P15 are in the rangebetween 0.1 to 0.6 MPa. The majority of the downward

thrust pressures in pure drilling for TH1 are in the rangebetween 0.4 and 0.7 MPa. The average downward thrust

pressure for TH1 was 0.52 MPa and the averagedownward thrust pressure for P15 was 0.32 MPa.

A majority of the downward thrust pressures for thepushing-in process are of high values. When a hammerand bit can break geomaterials easily or hammer and bit

are pushing in through an open zone, a high value ofdownward thrust pressure is used to drive the drill bit toadvance further.

(C) Drilling rate of the two holes in net drill processSince the distance between the two holes is short, it can

be assumed that the geomaterials encountered in the twoholes were the same along the hole depth. The difference

between the two drilling rates along a same depth in thetwo holes can be induced by the different workconditions of the two drilling machines (i.e., drill rods,down-the-hole hammers and drill bits).

Hence, the drilling rates in three major geomaterialzones and associated pressures of the two holes arefurther summarized in Tables 2 and 3.

Table 2. Major zone depths and associated drilling rates and

drilling pressures and their statistical data of Hole P15. (Note:

CLC = coefficient of linear correlation, SD = Standard

deviation)

Hole P15 Major

zone 1

Major

zone 2

Major

zone 3

Starting

depth

0.000 0.709 11.969

Ending

depth

0.709 11.969 22.757

Zone depth

(m)

thickness 0.709 11.260 10.788

Rate

(m/min)

1.090 0.814 0.488Average

drilling rate

CLC 0.925 1.000 0.999

Mean 0.106 0.269 0.356

SD 0.048 0.145 0.150

Downward

thrust pressure

(MPa) SD/Mean 45.28% 53.90% 42.13%

Mean 0.207 0.926 0.959

SD 0.089 0.095 0.045

Forward

rotation

pressure

(MPa)SD/Mean 42.99% 10.26% 4.69%

Mean 0.539 0.859 0.905

SD 0.228 0.069 0.052

Percussion

pressure(MPa) SD/Mean 42.30% 8.03% 5.74%

Table 3. Major zone depths and associated drilling rates and

drilling pressures and their statistical data of Hole TH1. (Note:

CLC = coefficient of linear correlation, SD = Standard

deviation)

Hole TH1 Major

zone 1

Major

zone 2

Major

zone 3

Starting 0.000 1.053 12.130

Ending 1.503 12.130 24.942

Zone depth

(m)

thickness 1.503 10.627 12.812

Rate (m/min) 1.786 1.284 1.134Averagedrilling rate CLC 0.981 1.000 1.000

Mean 0.427 0.504 0.547

SD 0.208 0.124 0.130

Downward

thrust pressure

(MPa) SD/Mean 48.71% 24.60% 23.77%

Mean 0.568 0.825 0.811

SD 0.374 0.042 0.059

Forward

rotation

pressure

(MPa)SD/Mean 65.85% 5.09% 7.28%

Mean 0.695 0.965 0.956

SD 0.367 0.055 0.076

Percussion

pressure

(MPa) SD/Mean 52.81% 5.70% 7.95%

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The results in Tables 2 and 3 show the clear differencesbetween the drilling rates of the two holes in the threemajor geomaterial zones. They were caused by the workconditions of drill rods, down-the-hole hammers and

drill bits of the two machines.

5. COMPARISON BETWEEN TWO DRILLING

MACHINES

5.1. Comparison of thrust motors of twomachines by pushing-in process analysis

Pushing-in action refers to the downward movement ofthe chuck due to the forward pulling of the loop chain bythrust motor. The relation between pushing-in rate anddownward thrust pressure in this action is determined by

work condition of thrust motor. All pushing-in actions ofthe two holes are obtained and the relations between

pushing-in rates and downward thrust pressures are

compared. Based on the comparison, work conditions ofthree thrust motors are determined.

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Fig. 12. Relations between pushing-in rates and downward

thrust pressures of two machines for hole Nos. P15 and TH1

As shown in Fig. 12, the downward thrust pressure andits corresponding pushing-in rate for Hole P15 aredistinctly higher than those for Hole TH1. However, the

trends for the relationship between the downward thrustpressure and its corresponding pushing-in rate for HoleNos. P15 and TH1 can be expressed by the two

correlation equations:

51326ln58018 .)(p.v tp += for Hole P15 and

94226ln60809 .)(p.v tp += for Hole TH1.

Where ratein-pushing=pv and pressurethrust=tp .

The two equations may show that the new thrust motor

for Hole TH1 is slightly powerful than the old thrustmotor for Hole P15, which could contribute to the higherdrilling rate in Hole TH1.

5.2. Comparison of percussion pressures of twomachines in pushing-in process and drill

processThe purpose of percussion pressure is to break the rockin front of drill bit and flush chippings off. The values ofthe percussion pressure in the actions of pushing-in and

pure drilling can be different. All pushing-in actions andpure drilling actions of the two holes are obtained. Thepercussion pressures in the two actions are compared.The average values of percussion pressure in pure

drilling process and pushing-in process of the two holesare shown in Table 4.

Table 4. Mean values of percussion pressure in pushing-in

process and pure drilling process

Percussion pressure inpure drilling process

(MPa)

Percussion pressure inpushing-in process

(MPa)

Hole

numberMean value SD

Mean

valueSD

P15 0.93 0.07 0.71 0.03

TH1 0.96 0.11 0.78 0.04

Table 4 shows that the average percussion pressure inpushing-in process is lower than that in pure drillingprocess. The difference may be due to the difference in

the contacts of the drill bit with the geomaterial at holebase, as shown in Figs. 13 and 14.

Air flowFig. 13. Airflow of percussion in pure drilling process

Fig. 13 shows the airflow for percussion in thedown-the-hole hammer during pure drilling process. The

airflow holes in the bit are blocked by the geomaterial atthe hole bottom. Without leaking of the airflow throughthe holes in the bit, the percussion pressure can be built

up. With the airflow pressure building up, the hammerpiston will release a percussive force to the bit and to the

geomaterial at the hole base. The impacting will cause ashort release of the full contact between the bit and thegeomaterial. At this moment, the airflow pressure is

released through the bit holes and the pressure isdropped. The released airflow flushes the debris out of

the hole. This process for the building up of pressure, theaction of percussion force, and the release of the blockedairflow in the hammer is repetitive during the puredrilling process.

Fig. 14 shows the airflow of percussion in the

down-the-hole hammer during the pushing-in process.Since the holes in the bit are not blocked, the airflow can

pass through the bit and flush the debris of the drilledgeomaterials in the hole out of the hole.

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Air flow

Fig. 14. Airflow of percussion in pushing-in process

The results shown in Table 4 may indicate that the newhammer for Hole TH1 has slightly higher average

percussive pressure than the old hammer for Hole P15and has a slightly higher flushing power than the oldhammer for Hole P15, which could contribute to a

higher drilling rate in Hole TH1. This is caused by thedifferent pistons of the two hammers. At the same airsupply level, the running speed of powerful piston ishigher than that of normal piston. The high-speed pistoncan build up more percussion pressure between drill bit

and drilled geomaterials or debris.

Figs. 15 and 16 further show the percussion pressures

with respect to the pure drilling time for Hole Nos. TH1and P15, respectively. Percussion pressure for Hole TH1was repetitively variable between 0.80 and 1.03 MPa.

The low value is close to the level of percussion pressurein pushing-in process, as shown in Table 4. The high

value is the percussion pressure indicating the contactbetween drill bit and drilled geomaterials. In Fig. 15,

1.00MPa can be used to determine whether the drill bitkeeps contact with drilled geomaterials. This indicates

that the new down-the-hole hammer can generate highimpact force to break the geomaterials in front of the bit.As a result, a thicker geomaterial could be taken away,

which repetitively caused the bit could not have fullcontact with the geomaterial at the drillhole bottom.Consequently, the measured percussion pressure

repetitively varied.

On the other hand, the percussion pressure for Hole P15was variable between 0.89 and 0.97 MPa. The highvalue is the percussion pressure indicating the contact

between drill bit and drilled geomaterials. In Fig. 16,

0.94MPa can be used to determine whether the drill bit

keeps contact with drilled geomaterials. The amplitudeof the percussion pressure variation is about 0.08 MPaand is much less than that for Hole TH1 (i.e., 0.23 MPa).The percussion pressure is also much high than the level

of percussion pressure in the pushing-in process asshown in Table 4. This indicates that the old drill bit in

front of the old down-the-hole hammer could contactwith the geomaterial at the hole bottom well. As a result,the compressed airflow in the hammer could not be

released easily from the holes in the bit so that thepercussion pressure could be build up. The detailed

distributions of percussion pressure in pushing-inprocess and pure drilling process of the Hole Nos. P15and TH1 are shown in Figs. 19 and 20.

TH1

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14 16 18 20 22Net drill time (Min)

Percussion

(M

Pa)

A

Fig. 15. Percussion pressure in pure drilling process of Hole

TH1

P15

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38

Net drill time (Min)

Percussion

(M

Pa)

Fig. 16. Percussion pressure in pure drilling process of Hole

P15

Fig. 17a shows the enlarged part A in Fig. 15 for details

of the percussion pressure. Fig. 17b to 17d respectivelyshow the corresponding drill bit position, the downwardthrust pressure, and the forward rotation pressure withrespect to the pure drill time for the part A in Fig. 15.

The part A may be divided into the two stages 1 and 2

by the level of percussion pressure. The percussionpressure was high and the drill bit was fully contacted to

the geomaterial at the hole bottom in Stage 1. However,the part of geomaterial was broken and collapsed inStage 2 when the percussion pressure was dropped, the

drill bit was not fully contacted with the geomaterial,and higher downward thrust pressure and increased

drilling rate could be observed. . Some of the relevantdata for the stages 1 and 2 are listed in Table6.

TH1-A

0.8

0.9

1

1.1

4 4.1 4.2 4.3 4.4 4.5Net drill time (Min)

Percussion

Pre

ssure

(MPa) Stage 1 Stage 2

(a)

5.4

5.6

5.8

6

6.2


Drillbitbitposition(m)

Stage 1 Stage 2

(b)

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0

0.2

0.4

0.6

0.8

4 4.1 4.2 4.3 4.4 4.5

Net drill time (Min)

DownwardThrus

pressure(MPa)

Stage 1 Stage 2

(c)

0

0.2

0.4

0.6

0.8

1


ForwardRotatio

Press

ure(MPa)

Stage 1 Stage 2

(d)

Fig. 17. Detail study about part A of Hole TH1

Table 6. Detailed information of Stages 1 and 2

Stage 1 Stage 2

Duration (Second) 5.0 12.5

Thickness (cm) 6.9 32.3

Mean downward thrust

pressure (MPa)0.35 0.43

Lowest percussion

pressure (MPa)0.98 0.83

Drilling rate (m/min) 0.828 1.550

Air flow

Stage 1

Air flow

Stage 2

Fig. 18. Work state of down-the-hole hammer and bit and air

flows in pure drilling process

Fig. 18 illustrates the two stages in Fig. 17 and Table 6.

When the set of drill bit, down-the-hole hammer anddrill rods in good condition drills the rock, the energycan be transferred to rock efficiently and part of rock

breaks and collapses (Fig. 18). This needs moredownward thrust pressure to keep the drill bit contactwith drilled rock. When a large portion of thegeomaterial was broken, the debris can cause the drill bitnot fully contact with the new geomaterial. As a result,

the percussion pressure in the pure drilling process coulddrop to form a waveform variation with time.

On the other hand, if the hammer and bit are not strong,they can only damage and break a small portion of the

geomaterials as the bit advances into the ground.Consequently, the percussive pressure will keepconstants and do not have the clear waveform variationwith time.

Figs. 19 and 20 show that the accumulative histogramsof the percussion pressure in the pure drilling and

pushing-in processes for the major zones 2 and 3 in HoleNos. P15 and TH1, respectively. They clearly show that

the percussion pressure values for the pure drilling aremuch higher than those for the pushing in process. Again,the data demonstrate that the blockage of the airflow bygeomaterial in pure drilling process caused a build up in

the percussion pressure during pure drilling. Thepercussion pressure values for P15 were lower thanthose for TH1 due to the fact that a new down-the-homehammer was used for TH1. Table 7 gives a summary of

the percussion pressure during the pure drilling processfor the major zones 2 and 3.

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Fig. 19. Accumulative histogram curves for distributions of

percussion pressure values in pushing-in and pure drilling

processes of the major zone 2 in P15 and TH1

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Fig. 20. Accumulative histogram curves for distributions of

percussion pressure values in pushing-in and pure drilling

processes of the major zone 3 in P15 and TH1

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Table 7. The detailed distribution information of percussion

pressures in pure drilling process of major zones 2 and 3 of the

Hole Nos. P15 and TH1

Percentage of percussion pressure (%) Major

zone 2

Major

zone 3

Hole P15 (0.94

MPa)

44.15% 73.05%Percussionpressure

indicating thecontact between

drill bit anddrilled

geomaterial

Hole TH1 (1.00

MPa)

33.97% 31.02%

The above DPM data and analysis indicate that the olddown-the-hole hammer and bit for Hole P15 could keepa full contact with the geomaterials being drilled betterthan the new down-the-hole hammer and bit for Hole

TH1. The new down-the-hole hammer could generatehigher impact force to break the geomaterials in front ofthe bit. As a result, a thicker geomaterial could be takenaway in TH1, which repetitively caused the bit couldnthave a full contact with the geomaterial at the hole

bottom.

5.3. Summary of comparison between the two

machinesFrom above analyses in drill actions of the two holes, thetwo machines can be compared. The results are given in

Table 8.

Table. 8 Comparisons between the two drilling machines

Compared ItemThe firstmachine forHole P15

The secondmachine for HoleTH1

Thrust motor Less powerful More powerful

Down-the-hole hammer Less powerful More powerfulDrill rods similar similar

Drill bitIn normal workcondition.

In good workcondition and can

break rock easily.

Less mean value(0.93 MPa)

Higher mean value(0.96) MPa

Percussion pressure inpure drilling

Less percussiveaction

Higher percussiveaction

Less mean value

(0.32 MPa)

Higher mean value

( 0.52 MPa)Downward thrust

pressure in pure drillingKeep the bit

contact withgeomaterial

constantly

Keep the bit

contact withgeomaterial

repetitively

6. SUMMARY AND CONCLUSIONS

This paper has examined two sets of DPM data of twoadjacent holes drilled with two machines. The paper

presents the method for examining the work conditions

of the thrust motors and hammer percussion in the

pushing-in process. The functions of the percussive,thrust and rotation air pressures and work conditions ofdrill rods, down-the-hole hammer and drill bit can beexamined in the net drill process analysis. The results

show that the drilling parameters from differentmachines can be compared and normalized. The workconditions of drilling machines can be identified andevaluated with the DPM methodology.

7. ACKNOWLEGEMENTSThe work presented in this paper was financially

supported by the Research Grants Council of HongKong (Research Grant No. HKU 7137/03E). Mr. J. Chenand W. Gao thank The University of Hong Kong for

postgraduate scholarships.

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