fractures

lon board,
No, 100, heavy ‘weight
1%" hick sheats)

7 32h pat concrete

Figure 6. Manufactured block with simulated inclined
fractures

Laboratory Drilling Experiments

The drilling operation was conducted by constructing a drilling
frame above the roof bolter and drilling in a vertical direction
(fig.1). The 6 manufactured blocks were loaded into the
Supporting structure above the roof bolter and the system was
Programmed for the specific rotational speed and penetration rate
desired. A new bit was added at the start of each hole and the
Parameters of thrust, torque, rotational speed, and penetration rate
were recorded every 0.1 second throughout the drilling. The data
was collected by connecting a computer to the feedback control
system on the roof bolter. The drilling parameters were recorded
directly on the computer and covered the complete drilling passes
from hole collaring to break through on the other side of the
manufactured blocks. The drilling experiments on blocks 1-4
were completed in two weeks. Drilling on blocks 5 and 6 was

conducted several months after the drilling of the first 4 blocks.

Drilling experiments on blocks 5 and 6 were conducted over a
two week interval.

The manufactured blocks with the rock layers were drilléd in
the direction of the long axis (78-in) and required two drilling
passes of the roof bolt feed arm due to the height restriction on the
test frame. The bit was initially collared into the cement block at
a distance of about %-in and the holes was drilled to a depth of
60-in. The drill rods were removed from the hole, an extension
was added, and the remaining 18-in was drilled. As mentioned
before, the bit was replaced after each hole was drilled. For the
holes drilled in the 78-in long blocks, two separate data files had
to be merged to cover the complete drill distance of each hole.
With the exception of the block with the foam inserts, drilling in
the manufactured rock layer blocks was fairly smooth.
Occasionally a problem occurred with plugged bits, worn bits,
machine shut downs or inadequate drilling parameters for the rock
being drilled. Since the temperature of the drill bit is often quite
high, the foam layers melted and plugged the bit during a large
number of holes drilled in block 1.

‘The manufactured blocks with the cardboard layers to simulate
fractures and bedding separations only required a single pass of
the roof bolt feed arm since the drilling distance on these blocks
was'49-1/2-in, This simplified the drilling and the data analysis
operation since each hole drilled produced one data file. For most
of the holes drilled in these two blocks, a new bit was used for
cach hole but there were several holes drilled with the same bit to
determine the impact of bit wear, The biggest problem that
occurred on drilling the blocks with the cardboard layers was bit
plugging when the lower rotational speeds were used.

The blocks were drilled using various combinations of
controlled drilling parameters. Table 2a shows the experimental
design for the manufactured blocks with the foam layers (block 1)
and the rock layers (blocks 2-4). Two sizes of bits, 1-3/8 and 1-
1/32-in in diameter, were used during the drilling of blocks 1-4 to
determine the impact of bit diameter. For the 1-3/8-in bits
(indicated with an X in table 2a), tests were conducted using 5
levels of rotational speeds (range of 150 to 500 rpm's) and 3
levels of penetration rates (range of 0.6 to 1.5 in/sec). A total of
50 tests was conducted with 14 tests in the manufactured block
with the foam layers and 36 tests in the blocks with the rock
layers. For the 1-1/32-inch bits (indicated with an O in table 2a),
11 tests were conducted. Seven of the 50 drilling tests used the
control system of the roof bolter ("Machine Control” in table 2).
"Machine Control” refers to a proprietary feedback control system
developed by J. H. Fletcher that automatically adjusts the drilling
parameters as the bit encounters various conditions during
drilling. The machine control system was modified to allow for
specific control of rotational speed and penetration rate as needed
for the experiments discussed in this paper. For each hole that
was drilled in the manufactured blocks, a new bit was used to
reduce the impact of bit wear on the drilling parameters and to
simplify the data analysis. Bit wear was significant in a number
of holes and, in some cases, the bit had to be replaced before the
drilling could be completed through the block (78-in).


Table 4. The maximum, minimum, and average values of SED and UCS for rock layers.

ameter pt Concrete_| Red Sandstone | Brown Sandstone | White Marble illite _| Light Brown Sandstone
Max SEI “84162 ~ 37,428 87,815 86,104 87, eee sone |
Ave SED 40,324 28,007 44,849 44,862 46,735 50,804
Min SED 15,943 15,265 20,197 20,983 13,489 15,934
Max SED, 56,244 37,069 54,262 71,892 62,129 60,311
Ave SED, 36,987 27,901 36,400 40,553 45,850 48,904
Min SED, 24,549 20,920 23,952 24,081 11,154 37,293
Max UCS 6,468 8,098 12,070 19,226 28,011 34,696
Ave UCS 4,020 6,986 9,995 17,418 20,445 27,359
Minucs [2,496 6,188 7,920 15,343 16,488 10,695

Teo
bit Sune
cry

[ROCK LAYERS AND UNCONFINED COMPRESSIVE STRENGTH, pal

Figure 9. Correlation of specific energy of drilling
and UCS for rock layers

This approach was used to reduce the variation in the SED
caused by non-homogeneity of the rocks and drilling
inefficiencies. Figure 10 shows the data points and the linear
regression of the SED,, and the unconfined compressive strength
of the rocks. The correlation coefficient for the linear regression
is 0.841 a significant improvement over the SED without
weighting.

3
2
Fd
3
3.
2
5
g
a
3
Fs
5
3
Es Coe Ro Bowe
“ne ot a w

ROCK LAYERS AND UNCONFINED COMPRESSIVE STRENGTH,

Figure 10. Correlation of weighted specific energy
of drilling and UCS of rock layers

SUMMARY AND DISCUSSION

The data collected from a series of laboratory experiments
using manufactured blocks for simulating rock layers indicates
that drilling parameters collected during routine roof bolt drilling
‘operations can be used to identify the relative strength of the

rocks. The data was collected from a large number of holes
drilled through 3 blocks that had rock samples embedded in them.
The rock samples included marble, argillite, concrete and three
different sandstones, All of the rocks drilled were sampled for
physical property testing and the characteristics of the rocks were
identified. The drilling experiments included a series of
controlled tests where the penetration rates and rotational speeds
were held constant and the torque and thrust were allowed to vary.
The control parameters were selected to match what is normally
encountered in underground roof bolting operations and the size
of the bit and design of the laboratory drilling experiments
simulated normal mining conditions. The specific energy of
drilling, as defined by Teale, was found to have a good correlation
with the unconfined compressive strength of the rocks being
drilled. An approach using Teale’s original equation with
additional weight given for the velocity parameters of rotational
speed and penetration rate was found to improve the fit between
the SED and the unconfined compressive strength of the rocks.
There is a large variation in the SED (table 4) even for a specific
rock type drilled under laboratory conditions. Given this large
variation it will be necessary to develop secondary and possibly
tertiary parameters for identifying rock types during drilling. A
series of pattems was identified during the drilling experiments
for transitional zones from strong to weak rocks, weak to strong
rocks, rock to fractures and fractures to rock. In each case a
dominant pattern emerged and will be useful for developing the
secondary and tertiary parameters.

Fractures and bedding separations can be positively identified
by changes in the drilling patterns (most notably thrust) but
additional study is needed to characterize the dimensions of the
fractures (size and orientation). A significant amount of data has
been collected from the drilling experiments and will be further
analyzed for a more complete characterization of the fractures.

Field studies. will also be necessary to investigate the
application of the methodology for real-time identification of roof
lithology and roof characterization. Preliminary studies have
been completed in underground coal mines in Utah, Illinois and
West Virginia but additional field studies are needed based on the
findings of the laboratory experiments.


Table 2b shows the experimental design for the two
manufactured blecks with cardboard layers for simulating
fractures and smaller bedding separations. The drilling
‘experiments on the blocks with the simulated fractures were
similar to the experiments on the blocks with the rock and foam
layers. The rotational speed was controlled but varied between
150-600 rpm’s and the penetration rate was controlled but varied
between 0.4-1.5 in/sec. The manufactured block with the
simulated horizontal fractures (block 5) had 69 holes drilled in it
with 39 of them ending prematurely (before the hole could be
drilled completely through the block). The majority of the holes
ending prematurely were the result of bit plugging when the
cardboard layers were intercepted. For block 6, which had the
layers inclined, 50 holes were drilled through it with 18 of them
ending prematurely. The rotational speeds were varied between
300-500 rpm’s with penetration rates varying between 0.4-
1.5 in/sec. The lower rotational speeds were not used while
drilling block 6 since the bit plugged often at these speeds while
drilling through the cardboard layers in block 5. The higher
rotations! speeds helped with cutting and removing the cardboard
layers. All the holes drilled in the two blocks with the cardboard
layers used the 1-3/8-in diameter bits.”

In order to verify the location of the embedded rocks and the
condition of the rocks themselves, each of the holes drilled in the
blocks was observed with a borehole camera. The interfaces
between the rocks and the embedded cement were measured and
the condition of the rock units was recorded. Selected areas of the
blocks were core drilled with a |-in-diameter core barrel to collect
rock and cement samples for physical property testing. Two-in-
diameter cores were available for testing on 3 of the rock units
(argillite, white marble and brown sandstone) and it was not
necessary to collect 1-in-diameter cores on these units.

Experimental Results

The data collected during the drilling experiments was
analyzed to determine the impact of the embedded units on the
drilling parameters and to determine if changes in the parameters
could be used to identify the material being drilled. Since all 6
blocks had multiple holes drilled through them, multiple data sets
were obtained for each block and for each unit within the blocks.
‘The procedure used for analyzing the multiple data sets included
a pattem identification approach for variations in drill parameters
and conversion of the drilling parameters to the specific energy of
drilling (SED). The areas of greatest interest are those zones
where the drilling transitions from one rock layer (or fracture) to
another. The identification of these transition zones and the
characterization of materials in each zone are of particular interest
for characterizing roof rocks.

Variations in Drilling Parameters with Rock Types and
Simulated Fractures

An example of the drilling parameters collected during the
drilling of block 2 in the laboratory experiments is shown in
figure 7. The penetration rate for the example was set at
0.6 in/sec and the rotational speed was 300 rpm’s. Since the

example shown is block 2, the total distance drilled was 78-in and
is indicated by the depth on the left hand side of the figure. The
variation of the penetration rate is similar to what was measured
during all the laboratory experiments as it fluctuates considerably
based on the mechanics of the drilling process. For this example,
the penetration rate varied from 0.1 to 1.8 in/sec. The fluctuation
was somewhat consistent when transitions zones between weak
and strong rocks were encountered and the system often required
several inches of drilling before the penetration rate retumed to
the programmed setting. The rotational speed remains fairly
constant during the entire drilling operation but requires 4 to 8-in
of initial drilling into the block before the programmed level is
reached. Past researchers have reported similar results and have
recommended not using the drilling data from the first 4 inches of
drilling since it produces inconsistent results. The thrust and
torque levels were not controlled but were allowed to change as
needed to achieve the controlled penetration rate and rotational
speed. The. thrust levels shown on figure 7 varied from about
2,000 to over 8,000-lbs. The torque followed a pattern consistent
with the thrust variations and has been shown in past studies to be
directly related to rock strength. On the example: shown, the
compressive strengths of the embedded rocks ranged from 4,020
to 27,359 psi and the variation in thrust and torque levels would
be expected for drilling through a sequence of rocks with a large
variation in strengths.

Bere, BB pe
80
70
60
‘
= so
8 sok
j
30
5
2
‘0
eC 20
PENETRATION =-RPM
RATE, in/sec

Figure 7. An example of the drilling parameters for
manufactured blocks with rock layers. Controlled
parameters of 0.6 in/sec penetration rate and 300 rpm.
rotational speed.

Patterns in the drilling parameters were also analyzed to
determine trends as the drill bit transitioned from one rock layer
to another. The pattems were identified on the basis of changes
in the drilling parameters (thrust, torque, rotational speed and
penetration rate) and the SED. Changes in the parameters and the
SED were studied for the cases where the hole transitioned from
strong to weak layers and weak to strong layers. The changes
were noted as each hole was drilled and the frequency of
occurrence for each pattern was counted. The pattems identified


Table 2a. Drilling parameters used during laboratory experiments on
manufactured blocks with rock layer.

Penetration rates, in/sec Holes

Hole terminated prematurely

06

41 15

drilled

No.

Reason

Block

<pP<| [p< p<

xX

xoo

Bit plugged from

melted foam

Block

op

Wom bits - 2
Broken bits - 2

6 |Machine shut down - 2

Block

fod dad tad
>< P<

Machine shut down

XX

500

Le be
be be fo be be | > pe ae J foe | ne fe oe 28

lO P< P< P<

Machine control

XXX

X= 1-3/8-inch diameter bits. = 1-1/32-inch diameter bits.

Table 2b. Drilling parameters used in the test program on.

manufactured blocks with fractures.
. . Holes terminated
i Penetration rates, in/s ematurel
Fracture orientation eri — patel Fr y
spect ms Toa] 06 | 13 ll 15 No._|__Reason
150 X_[XXXX] XX xX
200 XX | XXXX| XXX x .
S00 hax KK X x Bit plugged
Horan incurs 400 XX | XXX [XXXXX x xX 69 39 Machine
300 XXX| XXX | XXX XXXX stopped
600 x | x x x x
Machine control XXXXXXX
150
200
Bit plugged
Inclined fractures (Block 300 px en XKKX 50 18 .
f) ao [xx POX XXXXK Machine
x stopped
XXXX XXXXXXX
500 LX x PRT oo | *

X = 1-3/8-in diameter holes.



Table 3b. Variation trends of drilling parameters while drilling in manufactured blocks with simulated fractures and

bedding separations (Blocks $ and 6)

Transition | Patem | Thrust | Torque {PHOT Rea] SED | Frequency | Percentage
1 i i + Ca rm 294

2 {nc | NC . Can 1 294

3 Pe a ne [1 1 294

Rock to fracture |“ 1 NC 1 NC] 41 4 7
5 * * ‘ Coe 5 1471

i 7 1 D ne} 2 471

1 1 1 * + | 1 2.94

2 1 1 i ne] 1 294

3 t ! 1 bd 1 1 2.94

4 1 I 1 * tT 1 2.94

Fracture to rock [~~ 1 NC . ne [tt 2 5.88
a oa . + 2 5.88

7 + . . TF 5 i471

8 t t 1 NC 1 21 61.77

Legend: | = increase, | = decrease, NC = no change, * = no dominant trend

Correlation of Rock Strength with Specific Energy of Drilling
(SED)

The specific energy of drilling (SED) as defined by Teale (6)
was used for conversion of the drilling parameters. His definition
of the specific energy is defined as the work done per unit volume
of rock excavated. The specific energy is dependant upon a
number of factors including rock properties, the mechanics of the
drilling operation and the chip removal process. According to
Teale (6), as the efficiency of the drilling operation increases
{appropriate thrust arid rotational speeds are reached) the specific
energy value decreases until it approaches a near constant value.
The specific energy of drilling is determined by the following:

2nNT Q)
Au

F
e=z—t
A

Where:

¢ = specific energy of drilling, psi

F = thrust, lbs

A =cross-sectional area of the hole, in?
N = rotational speed, rpm

T =torque, in-Ibs

= penetration rate, in/sec.

‘The specific energy of drilling (SED) for cach data point along
the hole length was computed for each hole (see Fig. 2). The SED
for each rock type that was embedded in the manufactured blocks
(Blocks 2 - 4) in all holes was then computed, summarized, and its

average SED determined. This average SED was compared to the
average strength of the rocks based on the unconfined compressive
strength tests (UCS). The maximum, minimum and average
values of the SED and the UCS are listed in table 4. Comparing
the average SED to the average UCS for the rock types, the SED
was 1.9 to 4.5 times greater. The variation in the minimum and
maximum values of the SED was much greater than the variation
in the UCS of the rocks. This variation is the result of the drilling
process and is related to discontinuities, drill cuttings removal and
drilling inefficiencies such as binding of the rods and dulling of
the bits.

Figure 9 shows a plot of the average UCS and the SED of the
rocks and concrete used for blocks 2-4. Using the Teale equation
for determining the SED, a linear regression was obtained with a
correlation coefficient of 0.734. Given the wide variation in the
rock strengths and in particular in the SED the regression equation
gives a reasonable approximation of the data. In order to
determine if the correlation could be improved, a suatistical
approach was used where the different drilling parameters were
weighted to determine which had the greatest effect on reducing
the variability of the SED. The two parameters which had the
greatest effect were found to be the rotational speed and the
penetration rate. The weighted SED using the rotational speed
and penetration rate was obtained using the following:

SSEDxy xn

SED, = - 2)
wxn

Where:
SED,, = Weighted specific energy of drilling. psi

= penetration rate, in/sec
N= rotational speed, rpm.


An Approach to Identifying Geological Properties
from Roof Bolter Drilling Parameters

Gerald L. Finfinger, Senior Scientist
NIOSH - Pittsburgh Research Laboratory
Pittsburgh, PA

Syd Peng, Chairman and Charles T. Holland Professor
Quanzhong Gu, Graduate Research Assistant
Department of Mining Engineering
West Virginia University
Morgantown, WV

ABSTRACT

Identifying the properties of overlying rocks in underground
mining operations is important to ensure the appropriate roof
support design is used to maintain stability of the mine entries.
Recently J. H. Fletcher & Co..developed a monitoring and
control system for roof bolters for the underground mining
industry. The system records the drilling parameters used during
roof bolt drilling and the information can provide insight into the
physical properties of the roof strata, The parameters include
thrust, rotational speed, torque and velocity and the
measurements are collected every 0.1 second during the
operation.

The drilling parameters were analyzed to determine the
application of identifying the strength of rocks being drilled from
the measurements. The data was converted into the specific
energy of drilling which is a measure of the amount of energy
required for removing a given unit of rock during a drilling
operation. The laboratory studies completed to date indicate a
fairly high correlation between the specific energy of drilling and
the unconfined compressive strength of ihe rocks that were
drilled. Additionally, the drilling parameters were shown to be
effective for identifying the presence of fractures or bed
separations between rock layers. The thrust, torque and specific
energy of drilling were all good indicators for identifying the
fractures or separations. Regardless of the drilling parameters
used during the drilling experiments, the location of the fractures
were identified.

In order to determine the application of the drilling parameters
for identifying roof rock properties, two series of experiments
were conducted. The first series of experiments used three
"manufactured" roof layers that had various rock samples
embedded in concrete blocks. The rock samples included three
types of sandstone, marble, and argillite. Another concrete block
was poured with foam inserts to simulate large bedding
separations (2 to 8-in). Two other manufactured blocks were
constructed using high-strength concrete with cardboard layers

Gene Wilson, New Product Development Manager
JH. Fletcher & Company
Huntington, WV

Bob Thomas, President
Structured Mining Systems
Canonsburg, PA

embedded to simulate smaller fractures or bedding separations.
The size of the cardboard layers varied from 1/8- to 1-in thick.
One of the blocks had the cardboard layers embedded at an
inclined angle to determine the effect of the orientation on the
drilling parameters. A series of experiments was conducted with
the rotational speed and the penetration rate held constant and the
thrust and torque allowed to vary. The information collected from
the experiments is used to determine the application of the drilling
parameter measurements for identifying rock properties, fractures
and bedding separations.

INTRODUCTION

There has been considerable interest in the past for using
drilling parameters to determine geotechnical properties of rocks.
Most researchers have focused on methodologies for determining,
the strength of rocks with unconfined compressive strength being
the mostcommon. Asearly as 1926, Protodyakonov(/) proposed
and studied methods for determining mechanical properties of
rocks by measuring drilling parameters. Following this early
research, there were a widespread number of studies conducted by
other researchers on drillability rock measurements (2, 3). The
interest for determining the rock properties primarily lies in the
area of improved drilling efficiency. Improved efficiency would
increase penetration rates, decrease drilling times, increase bit life
and ultimately reduce the cost of operation. Researchers followed
the early investigations with studies relating the drilling.
parameters to the competency of the rocks being drilled with a
focus on both drilling efficiency and structural integrity of the
rocks (4-5). The structural integrity of the rocks has a direct
bearing on the stability of the rocks for mining and construction
purposes and for rock removal determinations such as blasting or
cutting operations. In 1965, Teale (6).developed his concept of
the specific energy of rock drilling where the drilling parameters
of thrust, torque, penetration rate and rotational speed were used
to determine the compressive strength of rocks. His research
showed a close correlation between these parameters and
compressive strength of the rocks. The U.S. Bureau of Mines,


REFERENCES

1. Protodyakonov, M.M. Mechanical Properties and
Drillability of Rocks, Proceedings in the 5* Symposium on Rock
Mechanics, University of Minnesota, Ed. By C. Fairhurst, The
MacMillan Company, New York, 1962, pp. 103-118.

2. Wells, E.J. Penetration Speed of Percussion Drill Bits.
Chemical Engineering and Mining Review, July 1949, Australia,
p. 362.

3, Somerton, W.H. A Laboratory Study of Rock Breakages by
Rotary Drilling. AIME Transcript, 1959, vol. 216, p. 92.

4. Rollow, A.G. Estimating Drillability in the Laboratory.
Proceedings of the 5* Symposium on Rock Mechanics, C.
Fairhurst, editor, University of Minnesota, Pergamon Press, 1962,
Oxford, pp. 93-102.

5. Maurer, W.C. The “Perfect-Clesning” Theory of Rotary
Drilling, J. Pet. Tech, Nov 1962, Trans., AIME, p 225

6. Teale,R. The Concept of Specific Energy in Rock Drilling.
Int. J. Rock Mech. Min. Sci. & Geomech, 2, 1965, pp. 57-73.

7. Frizzell, E.M. Hazardous Roof Rock Revealed by Feature
Extraction From Roof Bolter Drilling Parameters. In Proceedings
of the Intemational Symposium on Mine Mechanization and
Automation, Colorado School of Mines, Golden, CO, June 10-13,
1991, 14 pp.

8. Frizzell, EM, Howie WL, Smelser TW. Automated
Geophysical Sensing and Data Processing Roof Drill. In
Proceedings of the 23" Intl. Symp. on Appl. of Computers and
Oper. Research in the Mining Industry, Tucson, AZ, 1992.
9. Utt, W.K. Neural Network Technology for Strata Strength
ization. International Joint Conference on Neural
Networks, Washington, DC, July 10-16, 1999, 7 pp.

10. Ttakura, K., Sato, K., Deguchi, G., Ichihara, Y.,
Matsumoto, H., Eguchi, H. Development of a Roof Logging
System by Rock Bolt Drilling. Trans of the Institution of Mining
and Metallurgy, Vol. 106, A95-184, Section A, 1997, pp. Al18-
A123.

11, fakura, K. Current Drill Monitoring System Using
Mechanical Data of Drilling Machine and Estimation of Roof
Rock Structure. Proceedings of the Aust.-Japan Tech. Exchange
Workshop in Coal Mining 98, Brisbane, Aust., Nov. 1998, pp. 8-
10.

12. Thomas, B. and Wilson, G. Control Technology for Roof
Drill Operations. Proceedings of the 18* Intemational
Conference on Ground Control in Mining, Morgantown, WV,
1999, pp. 216-221.


are listed in table 3a. Eight different patterns were noted when
drilling transitioned from strong to weak rocks with pattern
number 8 representing about 55% of the cases. This pattern had
a decrease in thrust, torque and SED, an increase then decrease in
penetration rate and no change in the rotational speed. Of the
remaining 7 patterns, none had a frequency of more than 10%.
For those transitions from weak to strong rocks, 9 patterns were
identified. The dominant patter, accounting for about 55% of the
occurrences, is characterized by increases in thrust, torque and
SED, a decrease in the penetration rate and no change in the
rotational speed. Of the remaining 8 patterns, only pattem 8
occurred more than 10% of the time (12.9%).

Figure 8 shows the drilling parameters obtained during the
laboratory experiments on block 5 to determine if fractures or
smaller bedding separations could be identified. The distance
drilled on this block was 49-1/2-in as indicated on the left side of
the figure. For this example, the penetration rate was set at 0.6
in/sec and the rotational speed was set at 500 rpm’s. While the
penetration rate fluctuated during the drilling process it was more
consistent than what was observed while drilling in the blocks
with the rock layers. Block 5 was mostly composed of concrete
with very small transition zones since the largest cardboard layer
was only 1-in thick. The fluctuations in the penetration rate were
possibly related to the concrete mixture itself since it contained
aggregates (sand and gravel). The block had 3 cardboard layers

to fractures and from fractures back into rock. The patterns
recorded during the drilling are listed in table 3b. For those cases
where drilling transitioned from rock into a fracture, 6 patterns
were identified. Pattern 6 was the dominant pattern accounting
for about 65% of the cases. Pattern 6 is characterized by
decreases in thrust, torque and SED, an increase in the penetration
rate and no change in the rotational speed, For drilling transitions
from fractures back into rock, 8 separate patterns were identified.
Pattern 8 accounted for more than 60% of the occurrences and is
characterized by increases in thrust, torque and SED, a decrease
in the penetration rate and no change in the rotational speed.

wy
FEIZED conte .20 pet carved

DRILLED DEPTH, in

R
R
Ey

Tp00 0 “4000 0 1800 0200000
embedded in it and the parameter that was most significant in PENETRATION RPM © THRUST, TORQUE,  SEDw
ie RATE, in/sac eld tosint

identifying the layers was thrust. Referring to the figure it is easy
to identify the "fractures" with the changes in thrust.

Variations in the drilling parameters were also studied to
identify patterns that occurred as drilling transitioned from rock

Figure 8, An example of the drilling parameters for
manufactured blocks with simulated fractures.
Controlled parameters of 0.6 in/sec penetration

rate and 500 rpm rotational speed.

Table 3a. Variation trends of drilling parameters while drilling in manufactured blocks with

embedded rock layers (Blocks 2, 3 and 4).

Transition Pattern | _ Thrust Torque _| Penetration rate | RPM SED Frequency | Percentage
1 VU then t | 1%) then t 1 NC_| 1" 1 then! 1 2.38
2 t 1 1*f then | NC NC 2 4.76
3 4 1 i NC L 2 4,76
Strong to weak 4 1*t then! | It then! NC NC_| 1*1 then! 3 7.14
rock 3 1 U 11 then | NC 4 3 714
6 1 1 1 NC 1 4 9.52
7 t 1 1 NC i 4 9.52
8 J t 11 then | NC 1 23 54.76
1 1 NC 1" 1 then NC | NC i 1 3.23
2 U 1 NC i 1 1 3.23
3 1 * * NC * 1 3.23
Weak to strong 4 t + ; x : : 33
rock 1 3.23
6 1 t i t NC 2 6.45
7 1 | then NC i 1*1 then | NC_| 1*1 then t 3 9.68
8 1 t NC NC 1 4 12.90
9 t t J NC 1 17 54.84

Legend: | = increase, | = decrease, NC = no change, * = no dominant trend


The physical properties of the rock units and the concrete of
the manufactured blocks are shown in table 1. Depending on the
number of samples available the number of tests per rock unit
ranged from 2to 9. The test procedure followed ATSM standards
sand was conducted on rock cores of 2- and 1-in in diameter. Two
inch diameter cores were used when available. The average
values of unconfined compressive strength (UCS), Brazilian
tensile strength, Young’s modulus and density are listed. The
rock units were selected to include a variation of rock types and
a wide range of compressive strengths. The average compressive
strengths of the rocks varied from about 6,986 to 27,359 psi. The
three sandstones (designated red, brown and light brown) were
very consistent with no laminations or apparent bedding planes.
A\ll three sandstones are fine to medium grained. The white
marble tends to have zones of voids and is discontinuous in many
areas, The argillite, , a weakly metamorphosed shale, has zones
of healed and open fractures. The high-strength concrete that was
embedded as separate layers in blocks 3 and 4 had a compressive
strength of 2,830 psi. While initially intended to be a high-
strength concrete, these layers are the lowest strength units in the
manufactured blocks. The concrete used for embedding the foam
layers and the rock layers in blocks 1-4 had a compressive
strength of 4,020 psi, Overall, the rock units and concrete layers
in blocks 1-4 provided a series of drilling encounters simulating
transitions from weak to strong and strong to weak rocks. The
transitions were specifically designed to provide conditions
similar to roof rocks found in underground mining operations.

Blocks 5 and 6 were designed to simulate fractures or smaller
bedding separations and were constructed using a different high-
strength concrete mix than what was used for the separate layers
inblocks 3 and 4, This concrete mix had an average compressive
strength of 12,329 psi. Blocks 5 and 6 were constructed in wood
frames with a finished size of 36 by 49.5 by 60-in. The simulated
fractures and bedding separations were constructed by embedding
heavy weight illustration board (a dense cardboard) with a
thickness of 1/8-in in the blocks. The simulated fractures covered
the range of 1/8, 1/4, 3/8, 1/2, 3/4 and 1-in thick. For simulated
layers thicker than 1/8-in, multiple sheets of the illustration board
were used. One manufactured block had horizontal layers spaced
every 12-inches in the block as shown in figure 5. An additional
block was made with the simulated fractures oriented at an angle
from the direction of drilling to determine if the orientation has an
impact on the parameters or if the drilling parameters can be used
to determine fracture direction (fig. 6). The inclined layers were
intercepted by the drill bit at angtes of 15, 30, 60 and 75°. To
accurately determine the location of the simulated fractures, each
hole was visually observed witha borehole scope after the drilling
experiments were completed.

Table 1. Average rock mechanics properties of rock units in the test blocks.

Unconfined compressive Brazilian tensile .
; Location, block ity,| Youne’s
Unit and unit Strength strength nen *| Modulus, Comments
psi | Samples tested | 10° psi |Samples tested! 10° psi
Block 2-unit 4
Red sandstone ‘| Block 3-unit4 | 6,896 3 105 3 149 | 2.77 {Consistent
Block 4-unit 1
Light brown Block 2-unit 2
endstone Block unit | | 22359 4 193 3 158 ] 2.34 [Consistent
Block 2-unit 1 ;
Brown sandstone | piney writ | 9995 2 093 2 160 }| 1.94 [Consistent
j Block 3-unit 3
White marble Block 4-unit3 | 17418 5 137 4 tm. | 248 |Vugey
Argillite Block 4-unit 2 | 20,445 5 1.04 5 182 | 4.24 [Discontinuous
. Block 3-unit 2
Hirstength concrete] pies 4 unin4 | 2830 3 NA| NA 159 | 20.7 IGrainy
lEmbedding concrete} Block 1-4 | 4,020 9 047 9 132 | 0.65. [Standard Concrete
Embedding concrete
in blocks with Block 5,6 | 12,340 6 043 6 NA | 2.47 [High strength concrete
fractures

N/A = Not available.



Figure 3. Drill bits used for drilling experiments

EXPERIMENTAL APPROACH
Rock Sample Blocks

The research project was designed to conduct a series of
experiments in a laboratory setting as well as some field studies
in operating underground mines. The laboratory experiments
used the Fletcher roof bolter and a series of "manufactured" roof

tock blocks. The manufactured roof rock blocks were designed
and constructed by researchers at the Spokane Research
Laboratory (SRL) and used a variety of quarried rocks embedded
in poured concrete structures. Four blocks were provided by SRL
for the drilling experiments and schematics of the blocks are
shown in figure 4. Each of the blocks was 2 ft by 2 ft by 6-'4 ft.
Each block had four different layers embedded. Counting the
concrete that was poured to embed the rocks, each block had 9
different layers. Block 1 had foam inserts to simulate large
bedding separations. The foam inserts were 2, 4, 6 and 8-in thick.

Blocks 2-4 had rock units embedded in them as shown in figure 4.
The drilling experiments were conducted along the long axis of
the blocks. The location of each embedded unit in the blocks was
verified using a borehole video scope inserted into the drilled
holes. The rock units were also visually observed with the video
scope to determine the relative consistency of the rocks within the
poured concrete. Additional confirmation of the embedded rock
units was done by core drilling.

Lagend
I 4000 pa core

Figure 4. Schematics of the manufactured blocks. Block 1 has foam layers, Blocks 2-4 have rock layers


Spokane Research Laboratory conducted research on using the
drilling parameters and Teale’s methodology for determining the
competency of roof rocks in underground mines as a design
methodology for roof support (7-9). The Bureau’s research was
focused on improving safety of underground mining operations
using drilling measurements collected from roof bolt machines.
In the early 90's, Itakura, et al, (/0, 1) studied the use of drilling
parameters as a means of determining bed separations and
fractures and developed a methodology for identifying the size
and orientation of these discontinuities. All of the above
researchers relied primarily on laboratory instrumentation
experiments although some conducted a limited number of field
investigations to evaluate the practicality of their approaches. In
1999, J. H. Fletcher & Company, in concert with Structured
Mining Systems, developed a computer-control/monitor system
for underground roof bolting machines that enhances the bolting
operation (/2). The roof bolting operation is enhanced by a
feedback system that controls the drilling operation, automatically
installs resin bolts to manufacturer specifications for hold and
spin times and reduces operator fatigue. The drilling operation is
controlled to match the conditions of the roof rocks being drilled
with the intent of improved speed of drilling and reduced bit wear
The successful development of this system permitted the
implementation of a research program for determining the
application of drilling parameter measurements to identify rock
properties by detailed laboratory and field experiments. The
system is approved for use in underground coal mines and studies
can be conducted as part of the normal drilling operation with
minimal impacts to the mining cycle. The remainder of this paper
will report on the results of the laboratory experiments using the
computer control/monitor system on the Fletcher roof bolter
machine.

DESCRIPTION OF EQUIPMENT

The drilling experiments were conducted using the J. H.
Fletcher & Company arm feed style twin head roof bolter fitted
with a dedicated intrinsically safe processor (figs. 1 and 2) (/2).
The processor monitors the drilling parameters of thrust, torque,
penetration rate, and rotational speed. Additional conditions are
monitored by the system but were not part of the research
experiments, The system has the capability of being programmed
for automated drilling operation (the feedback system) or for
specifically designed control drilling. The controlled drilling
approach was developed for the experiments conducted as part of
this research. The drill bits used during the experiments were
Kennametal’s Dust Hog design including 1-3/8 and 1-1/32-in in
diameter (fig. 3). The drilling operation was conducted using a
vacuum recovery system for bit cooling and cuttings removal.
The laboratory experiments were conducted at J. H. Fletcher &
Company's Huntington, WV, facility. The same roof bolter and
control system were used throughout the experiments.

Figure 1. Set-up for drilling parameters experiments

using a twin-boom roof bolter with Feedback Control
System