Chapter37
THERMALLY
ASSISTED
CUTTING
OF
GRANITE
by Parviz F. Rad
1 andFrederickJ. McGarry
2
1Civil Engineer,TwinCities MiningResearch
Center,Bureau
of Mines,
Assistant,
Institute
Minneapolis,
Minnesota
(formerly
Department
of Civil
Engineering,
of Technology,
Cambridge,
Mass.)
Research
Massachusetts
2professorof Civil Engineering,Massachusetts
Institute
of
Technology,
Cambridge,
Mass.
ABSTRACT
Although
tunneling
machines have attained
very high
advance rates
in medium-hard
rocks,
the need for frequent
repairs
has slowed their
use in very hard rocks.
Laserassisted
tunneling
is expected
to improve tunneling
machine performance
by improving
either
specific
energy
or amount and quality
of muck.
The beneficial
effects
of lasing
are the initiation
and growth of microcracks
from differential
thermal
expansion
of grains,
and overall
spalling
in the vicinity
of the lased area.
Using a constant
thrust
rock cutting
device with disk
cutters,
the effects
of cutter
geometry,
cutter
diameter,
thrust,
and speed variations
on specific
energy and muck
were explored
and the improvement
in performance
of disk
cutters
caused by lasing
was evaluated.
It was found
that
the improvement
depends on beam power and density,
beam speed, pattern
of heating,
and cutting
pattern
and
procedure.
A focused
beam slightly
offset
from the cutter
path gives the most improvement
both when cutting
on a
smooth surface
and when cutting
in previously
cut grooves.
INTRODUCTION
Excavations
in the form of surface
mining are believed
to have first
been conducted
about 15,000 years ago (9)
with very primitive
equipment.
Later,
around 2000 B.C.,
721
722
D•A•c
fire-setting
and continued
explosives
Rock
MEC•-IANICS
was first
used as an aid in rock fragmentation
in general
use until
around 1700 A.D.
(8)when
were first
introduced
in mining and tunneling
operations.
The most recent
major change in excavation
operations
was introduced
around 1820 when the fLrst
tunneling
machine became operational.
The performance
of
tunneling
machines has been steadily
improved since then
and their
application
has been extended
from soft
rocks
to medium and medium-hard
rocks.
With the continuing
development
of more powerful
equipment
and new techniques,
it is expected
that
their
capability
will
be extended
to
operation
in very hard rocks during
the next few years
(3,
5,
13).
The demand for underground
excavations
is increasing
steadily
with the increase
in the number and size of the
projects
that involve
such operations.
Underground excavations
are generally
made for the purposes
of extracting
ores,
conveying
traffic,
water,
sewage, and utilities,
securing
the national
defense,
and creating
underground
shelters
and underground
storage
spaces.
There
excavation
have been noticeable
changes in the methods of
for economical
as well
as technological
reasons.
activities
were originally
surface
operations.
After
Mining
underground excavation
techniques
were developed,
mining
was mostly in the form of underground
excavations.
In the
past 70 years,
however,
the number of open-pit
mines has
increased
rapidly
because of the increasing
cost of underground excavation(12).
On the other hand, the amount of
land available
for mining is gradually
decreasing
with the
expansion
of residential
and recreational
areas.
Further,
the necessity
of restoring
the land has added a heavy cost
item to the mining operation.
The cost of restoration
is
fairly
high in most cases and sometimes it is not even
possible
to restore
the land to an acceptable
state.
It
is therefore
expected
(12) that before
long surface
mining
operations
will
largely
be replaced
by underground
operations.
The excavation
of an underground chamber consists
of
three major operations:
fragmentation,
materials
handling,
and ground control.
An important
element in the overall
rock excavation
operation,
fragmentation
directly
influences
the rate of progress
and cost of excavation.
With improved explosives
and other modifications
in
blasting
75 ft/day
and mucking processes,
advance rates
of about
(8) have been obtained,
and recent developments
in the effic--iency
of boring
machines have resulted
in
advance rates
of up to 500 ft/day
(8).
The data collected
has shown, however,
that the progress
of tunneling
machines
TItEBMALLY ASSISTEDCUTTING OF GRANITE
723
in very hard rocks is slow and accompanied
by frequent
cutter
repairs
and/or
cutter
changes due to excessive
wear and breakage
(3, 4).
The cost of excavation
as well
as the rate of progress
with the tunneling
machines is
substantially
improved
if the geological
conditions
of the
site
are favorable
for the use of tunneling
machines.
To
extend the capabilities
of tunneling
machines to operation
in hard rocks,
greater
understanding
of the rock-cutter
interaction
procedures
with
cutting
is
for
desirable,
weakening
or the
as well
the rock
use of
as better
prior
to
cutters
of higher
methods
and
or concurrent
cutting
capabilities.
The purpose of this study is to investigate
the effect
of lasing
on the specific
energy and rate
of rock removal
by mechanical
disk cutters.
Improvements
in either
of
these quantities
may justify
laser-assisted
tunneling
by reducing
the energy requirements
for rock fragmentation
or by increasing
advance rates.
This investigation
therefore
has studied
the effects
of varying
such parameters as heat input and input rate,
cutter
size,
thrust
and speed,
and geometry
of lasing
and cutting.
ACKNOWLEDGEMENTS
This study was performed
at tke Massachusetts
Institute
of Technology
under a contract
from the United
States
Department
of Transportation,
and under the supervision
of Professor
F.J.
McGarry,
Head of the Materials
Research
Laboratory.
ROCK
CUTTERS
The four basic types of cutters
used in tunneling
machines are drag bits,
toothed
cutters,
disk cutters,
and carbide
studded cutters.
A drag bit simply gouges
out a groove in the rock; thus it is best suited
to working soft
rocks.
The toothed
cutter
and studded
cutters
cause failure
by creating
high stresses
at the tips
of
t• teeth
or studs and breaking
chips of rock between
them.
The disk cutter
looks and operates
much like
an
ordinary
glass
cutter:
pushing
it on the face of rock
causes chips
to form on either
or both sides.
The stresses
caused by a stationary
disk cutter
on the
surface
of a smooth rock can be approximated
by a line
load on a semi-infinite
medium; this
approximation
is
valid
only before
any crack propagates.
Prior
to the
propagation
of the crack,
the rock behaves more or less
724
DYNAMIC ROCK MECHANICS
like
a linear
elastic
material.
After
cracks propagate,
either
spalling
or fracture
occurs.
Analytical
characterization
of the rock then becomes complex because of changes
in cutter
geometry and boundary conditions.
If,
after
the
cutter
penetrates
into
the rock,
it is moved forward
by a
horizontal
force,
the cutting
action
can best be approximated by a frictionless
curved wedge.
Because of the
geometry of the cutter,
the horizontal,
as well
as the
vertical
force,
causes
the
cutter
to
exert
lateral
forces
on the rock.
The lateral
forces
become an important
factor
in more efficient
muck removal
when previous
channels
are sufficiently
close to the groove being
The lateral
forces
are also responsible
for tensile
fractures
observed
beneath
cutter
cut.
cutters.
The sequence of events occuring
in the fracture
of rock
(fig.
1) under a wedge-shaped cutter
or a stationary
disk
cutter
is:
a)
crushing
b)
elastic
c)
formation
cutter,
of
a zone
d)
formation
of
chips,
is
specific
This
that
of
deformation
sequence
has
not
surface
been
indented
repeated
in
the
modified
or
eliminated.
irregularities,
of
the
of
crushed
for
rock
underneath
penetration
before.
same location,
rock,
If
some of
into
the
indentation
these
effects
the
a surface
is
are
The amount of chipped
material
depends on the geometry
of the wedge as well as on the forces
acting
on it.
Chip
formation
will
be greater
with a combination
of sharp
wedges
With
and
large
small
thrusts.
angle
wedges,
curved
cracks
develop
between
the crushed zone and the free surface
forming chips.
As
the angle is increased,
more cracking
and less chipping
is observed,
although
at sufficiently
high loads the rock
ultimately
fails.
A larger
wedge angle causes the resultant
force
to be directed
more nearly
downward and
results
in a larger
fracture
path and higher
fracture
forces.
Although the sharp wedges penetrate
deeper for a given
input energy,
the amount of rock removed per unit energy
is similar
for the different
wedge angles
(5).
As a
result,
bit dullness
(wedge angle)
has much more effect
THERMALLY
ASSISTED
CUTTING
OFGRANITE
725
o
I
I
726
DYNAI•ic
on drilling
thrust,
rate
such
for
Rock
drills
as roller
which operate
bits.
change in the wedge angle
small angle wedges.
Maurer
MECHANICS
(5) describes
Further,
bit
under
have a more critical
the transition
constant
dullness
and
effect
from crushing
on
to
chipping
in terms of a threshold
stress:
if the stress
is
below the threshold,
only crushing
and indentation
occurs,
otherwise
formation
of chips creates
a crater
around the
wedge. This fact is important in operating
roller
bits
where the total
contact area can increase
by 500 to 1,000
percent as the bit dulls
(5).
This threshold
pressure
ranges from 300,000 to 600•000 psi for hard rocks such
as basalt
and granite.
After
the formation
of a chip,
the load may either
drop to zero and then gradually
increase with a constant rate,
or remain constant during
the
subsequent
(11,
14).
be--•ie•d
penetration
The actual
to
be between
until
loading
the
two cases.
EXPERIMENTAL
Advantages
is
achieved
however,
is
TECHNIQUE
of Lasing
Previous
gested
cutters
equilibrium
condition,
studies
of laser
rock
damage (6,7)
have sug-
that laser
heating
in combination
with mechanical
may improve the performance
of contemporary
tunneling
machines.
The beneficial
effects
of lasing
are the initiation
and growth of microcracks
by differential
thermal expansion of grains and overall
spalling
in
the
vicinity
analytically
of
(10)
the
that
lased
be comparable •magnitude
stresses
from
the
area.
stresses
It
has been
producing
shown
spalling
to the mechanically
may
induced
cutter.
The objective
of this study was to measure the improvements in the efficiency
of rock cutting
due to prior
softening
by laser
irradiation.
The variations
of such
improvements
were studied
as a function
of total
heat
input,
input rate,
cutter
size,
thrust,
speed, and
geometry of cutting
and lasing.
Material
and Equipment
The specimen material
chosen as a standard
was Barre
Granite
(table
1), quarried
by the Rock of Ages Corporation
in Vermont.
Sample size was 4 in. by 14 in. by 13 in.;
the surfaces were not polished
after
sawing, but were
smooth to the touch.
All
samples were cut at one time
TI-IERMALLYASSISTEDCUTTI-X'GOF GRANITE
727
from a single
quarry
block with the grain
orientation
held
constant
to insure
uniformity.
A limited
number of larger
size blocks
(8 in. by 13 in. by 20 in.)
was obtained
for
testing
with the 11-1/2-in.-diam.
cutter
and with thrust
forces
higher
than 3,000
psi.
TABLE 1. - Properties
Geologic
of Granite
used in this
name ..............................
Locality
study
Barre
...................................
Granite
Graniteville,
Vermont
Compressive
strength
Shore hardness ...........
Rockwell
Mohs
C hardness
hardness
...................
scleroscope
psi..
units..
32,000
102
.........................
72
...............................
Quartz
7
Feldspar
6
Mica
Apparentdensity...................
Specific
gravity
Static
lb/ft3...
165.5
............................
2.64
Young's modulus.............
106 psi..
3.5 to 7.0
Color .......................................
Grain texture ...............................
Blue-gray
Fine grained,
3
Coefficient
of thermal expansion..in./in./øF
Water absorption
............
Petrographic
analysis
.......
6
pct.
pct.
mm
4 x 10-6
by weight..
by volume..
0.23
Feldspar
65
Quartz
Mica
27
8
The rock cutter
testing
device
consisted
of a fixed
vertical
frame holding
various
cutters
(3-,
4-,
5-,
6-,
and 11-1/2-in.
diam.,
60-deg.
included
angle)
which
pressed upon block samples.
The cutter
frame operated
on a constant
thrust
mode with
loads
adjustable
up to
12,600
psi.
The block
samples were placed
on a hydraulically
driven
table
traveling
with a constant
speed
adjustable
between
0.1 ips and 5 ips.
The
vertical
measured
block
strip
over
the
and
horizontal
by a dynamometer
and
work
table
chart
recorder.
time with a disk
and
forces
located
were
on
the
between
recorded
The horizontal
integrated
to
on
force
indicate
cutter
the
a
were
sample
two-channel
was integrated
work done by
cutter.
The laser
used for heating
nitrogen-helium
continuous-gas
output
of 750 watts.
was a carbon dioxidelaser with a rated maximum
The laser
beam was reflected
by two
728
DYnAstic Rock MECH.i•ICS
mirrors
onto the surface of the specimen.
Immediately
in
front
of the laser was an optically
flat
mirror.
The
second mirror was also flat
for investigating
the effects
of
the
unfocused
beam.
In
some
tests
this
second
mirror
was replaced
by a concave focusing
mirror
with a focal
length
such that it focused
the beam on the specimen
surface.
Testing
In
Details
most
of
the
tests,
the
cutter
passes
were
made in
parallel
grooves each about 10 in. long.
In a limited
number of tests,
however,
after
a series
of parallel
grooves were cut,
the block was rotated
90 deg. and a
second set of parallel
grooves was cut perpendicular
to
the original
ones.
The muck produced
by each pass of the cutter
was
gathered
by a handheld
vacuum cleaner
and then weighed
to the nearest
0.1 gram on a laboratory
scale.
The total
energy expended in each pass was calculated
from the
integration
of the horizontal
force.
The variable,
specific
energy,
is equal to the total
energy per pass
divided
by the volume of muck per pass.
In tests
involving
the use of laser,
the cutting
occurred
immediately
after
lasing.
With the beam aimed
ahead of the cutter
(fig.
2), the time lag between lasing
and cutting,
as well as the heat input per inch of beam
travel,
was determined
by the table
speed.
Some tests
in this
series
were run with the focusing
mirror.
In
a limited
number
repeatedly
under
produce parallel
these tests,
the
cooled
of
tests,
the
blocks
were
the laser beam at a specified
lased paths at equal intervals.
specimens were cut after
they
passed
speed to
In
had
completely.
The results
of experiments
are reported
and standard
deviation
of each set or data
by mean value
points.
When possible
and appropriate,
a linear
regression
curve
was calculated
by the least
squares technique.
In general,
the regression
correlation
coefficient
for the specific
energy data was 0.95 or better
and for muck removal was
0.70
or
better.
RESULTS
Untreated
Rock
AND
DISCUSSION
OF
RESULTS
TYIERMALLYASSISTEDCUTTINGOF GRANITE
729
o
730
of
I)¾NAM•C ROCK MECHA•ICS
To evaluate
the effect
of the laser
a disk cutter,
it was necessary
to
havior
of
the
cutter
alone.
In
effects
of cutter
geometry,
are reviewed
as background
the
this
on the
explore
section
force,
for the
performance
the bevarious
and speed variation
results
of tests
with
laser.
Groove
For
Spacing
a fixed
cutter
force
and
diameter,
there
existed
a
critical
spacing for parallel
grooves;
at larger
spacings
the grooves tended to be independent
and at smaller
spacings
chipping
occurred
between adjacent
grooves.
The
chips formed in this manner were considerably
larger
than
the powdery debris
produced in the independent
grooves.
The amount of muck per cut was higher
and the specific
energy generally
lower than for the wider spacings.
The
specific
energy was related
to the amount of fracture
surface
energy required
to produce the debris,
since the
amount of surface
large
chips.
As the
groove
area
per
spacing
unit
volume
was decreased
was
lower
below
the
for
critical
spacing,
the average
chip size increased
and then began
decrease
again;
the maximum chip width was seldom larger
than the groove spacing.
Thus, there
also existed
an
optimum spacing for specific
energy and muck.
Figures
3 and 4 show the variation
of these quantities
as the
groove
spacing
was increased
through
to
the optimum spacing,
about 1/4 in.,
toward the critical
spacing
of about 1 in.
For wider spacings the results
are independent
of spacing
since the grooves did not interact.
The blocks with
spacings
of 1/2 and 3/4 in. were turned 90 deg after
cutting
and another
series
of cuts was made perpendicular
to the first.
The greatly
reduced specific
energy again
is in keeping
with the concept of fracture
surface
energy;
perpendicular
cutting
permitted
the formation
of very
large
chips.
The critical
spacing
and optimum spacing were found to
decrease
as the cutter
diameter
increased
in the range of
diameters
tested
(3, 4, and 5 in.).
A small
cutter
has
a smaller
contact
area with the specimen
and hence a
higher
distances
Cutter
i.e.,
local
stress
than
which permits
do the
larger
chipping
to greater
cutters.
Speed
Like most other
the strength
materials,
and other
granite
related
is rate sensitive,
properties
change
TI-IERMALLYASSISTEDCUTTING OF GRANITE
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THERMALLY ASSISTEDCUTTI-X'GOF GRANITE
733
as the rate
of load application
changes
(2).
The load
application
rate in cutting
tests
is directly
related
to
the linear-cutter
speed.
Thus the cutter
speed affects
all
results
- muck and specific
energy,
optimum and
critical
spacings.
Figure
5 shows the variation
of specific
energy with
cutter
speed for cuts on a smooth surface.
The higher
ratio
of crushed material
to chips observed
at high
speeds demonstrates
the increase
of specific
energy with
speed.
Figure
6 shows the corresponding
muck values
and
their
relation
to cutter
speed.
The
relation
between
the
amount
of
muck
and
cutter
speed was observed
to be independent
of the cutting
conditions
(cutter
diameter,
thrust).
Increasing
cutter
thrust
has the same effect
as reducing
cutter
diameter;
the critical
and optimum spacings
are increased
because
of the higher
stress.
The experiments
performed
at
higher
thrusts
showed that
the amount of muck was increased
with
decreasing
cutter
diameter
and speed and
with increasing
thrust.
The value of specific
energy,
however,
followed
an opposite
pattern
with variations
in thrust,
cutter
diameter,
and speed.
Laser-Treated
Rock
Local heating
of the rock by lasing
weakens the rock
by creating
intergranular
and transgranular
cracks as
well
as by causing
other
types of permanent
damage.
Furthermore,
the thermal
stresses
help the mechanical
stresses
of the cutter
to exceed
the strength
of the
rock.
The extent
of damage depends on beam power,
beam
speed, pattern
of heating,
pattern
of cutting
and whether
or not the rock is allowed
to cool before
cutting.
Unfocused
Laser
The extent
of residual
damage was determined
by lasing
the rock and then testing
it when the rock had cooled to
room temperature.
It is believed
that the residual
damage continues
to develop during the entire
period
of
cooling
to room temperature.
Figure
7 shows the
cutter
speed
several
times
at a specified
paths at equal
inch of travel
and heat
under
variation
dosage.
the
unfocused
of specific
Each
block
650-watt
energy
with
was passed
l'aser
beam
table
speed to produce parallel
lased
intervals.
The energy input per linear
was then inversely
proportional
to the
734
DYNA•c
Rock MECHANICS
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DYNAMICI•OCK MECHANICS
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TI{ERMALLY ASSISTEDCUTTING OF GRANITE
table
speed.
The cutting
same table
speed at which
737
was done 24 hours later
the rock was lased.
at
Figure
specimens;
specimens
8 shows the amount of muck produced in these
the improvement
achieved
by lasing
the
may not be very significant
considering
the
amount
scatter
of
observed
in
these
the
tests.
A slight
change of color
observed in the lased areas
indicated
a loss of moisture
or other minor physical
changes.
Such changes might be responsible
for the
apparent
increase
in strength
associated
with lower
heat input
values.
The results
of these tests
showed
that
although
the permanent
damage induced
by the laser
did not significantly
alter
the muck removal pattern,
the value of specific
energy was decreased.
In
addition
to
the
irreversible
residual
dama9e,
transient
thermal
stresses
are produced
in the rock at
the time of lasing.
These transient
stresses
prior
to
the action
of the cutter
aid the formation
of larger
chips,
and lower the specific
energy.
In another
series
of
tests,
ahead
the
of
the
650-watt
cutter
unfocused
such
that
beam
the
was
cutter
aimed
would
5 in.
follow
in the center
of the lased path.
The heat input was
varied
by changing
the table
speed.
Since there was
a fixed
separation
between
the cutter
and the beam,
this procedure
also changed the lag time between heating and cutting
for each input
level.
The muck values
at all heat inputs were higher
for
the uncooled
specimens
than those from the cooled
specimens.
Significant
improvements
were also obtained
in the value of specific
energy.
A series
of blocks
was
treated
in
the
same manner,
but
with
beam.
For the range of speeds and heat
the improvements
were not significant.
a 325-watt
inputs
studied,
The results
of these treatments
appear to indicate
that
a critical
heat input
value must be exceeded
if
any improvement
is expected.
If the heat energy applied
is lower
than this
value,
the muck removal
may even be
more difficult
and more inefficient
than what might be
expected
with an untreated
rock sample.
Focused
Laser
The magnitude of the thermal stresses
depends on the
temperature
of the heated surface;
this temperature
was
increased
by focusing
the beam with a concave mirror.
Figure
9 shows the variation
of specific
energy for
738
DYNAMIC ROCK M•CaA•ICS
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DYNAMIC ROCK MECHANICS
specimens which were heated with a 650-watt
focused beam
prior
to cutting.
The focused beam, roughly
3/8 in. in
diameter,
was
aimed
5 in.
ahead
of
the
cutter
on the
same
path as the cutter
followed.
At very high energy input
values
partial
melting
and beading
of the heated
area
occurred.
Figure
10 shows the amounts of muck removed
for these specimens.
Compared with the specimens which
were treated
identically
with an unfocused
laser
beam,
the focused laser
gave a four-fold
increase
in the amount
of muck removed.
The comparison of energy for these two
cases
showed
the
unfocused
that
a
beam.
focused
beam
is
twice
as
effective
as
Offset
The results
estimate
suggested
the
of
the
the thermal
that
heated
the
zone
in
numerical
stresses
cutter
analysis
should
order
that
developed
in the rock
(•,
be located
thermal
stresses
can be more efficiently
chips and propagating
fractures.
and
utilized
to
6, 10)
adjacent
to
mechanical
in
forming
The variation
of specific
energy with offset
distances
shown in figure
11.
The energy went through a minimum
about 1/2 in. offset
and then increased
again.
Figure
12 shows the variation
in muck removal
for these
specimens;
with increasing
offset,
the amount of muck went through
a
maximum at about 1/4 in.
offset.
The optimum offset,
however,
varies
slightly
with cutter
speed and total
heat
input.
is
at
Figure 13 shows the variation
of specific
energy with
cutter
speed when the focused laser beam was aimed 5 in.
ahead of the cutter
with 1/2 in.
offset
from its travel
path.
Figure 14 shows the muck removal for these specimens
This combination
gave the best results
obtained in this
study.
The effectiveness
relative
size
of
of
the
lasing
zone
is
dependent
affected
by heat
upon the
to
that
of
the zone affected
by the mechanical
cutter.
Since it is
expected that the zone stressed
by the mechanical
cutter
increases
with thrust,
the improvement due to heating
will
not be as pronounced at high thrust
as that for the
lower
thrust.
Multiplicity
of
Cuts
and Passes
Tunneling machine cutters travel
cular paths and keep breaking chips
face
of
the
tunnel.
Two
factors
continuously
in ciraway from the roughened
contribute
to
the
muck
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I)¾NAMIC ROCK MECttANICS
removal
in multiple
to penetrate
further
passes:
first,
the cutters
continue
into
the groove on each pass by
removing additional
muck from the groove;
and second,
neighboring
grooves interacting
with each other
form
chips as large as the groove spacing.
The efficiency
of
this
process
depends on choosing
the correct
groove
spacing
for the particular
operating
conditions
of thrust,
diameter,
and speed.
This section
investigates
the consequences
of cutting
repeatedly
over the same path,
both
independently
and with interactions
between neighboring
grooves.
Independent
Cuts
When the cutter
was passed over the surface
of the rock,
the resulting
groove was roughly
0.1 in. deep.
Subsequent
passes increased
the depth,
although
by decreasing
increments.
Figure
15 shows the results
obtained
from three
repeated
cuts in an independent
groove.
The results
were
compared with
those obtained
with
the same cutter
but at
1/2 in. groove spacing,
where interaction
between grooves
was high.
For successive
passes,
the muck decreased
as
did the observed
size of chips produced.
After
the third
pass the groove was about 0.2 in. deep, and the wedging
action
of the cutter
was always sufficient
to split
the
specimen completely
in two if a fourth
pass were attempted.
Block splitting
was avoided
if conditions
allowed
removal
of a significant
amount of muck with each pass,
such as
when there was groove interaction.
Multiplicity
of
Interacting
Grooves
In actual
field
conditions
the cutters
repeatedly
traverse
the same path and most of the muck is removed
through
the interaction
of the neighboring
grooves.
After
a number of cutter
passes have been completed
on
a fresh surface,
the values of specific
energy and muck
will
reach a steady
state.
In the steady
state,
the
amounts of muck and the values
of specific
energy vary
about mean values
for that cutting
condition.
The optimum,spacing
for the steady state
results
might be different
from that
for a single
pass on a smooth surface.
Figure 16 shows the specific
energy values obtained
from multiple
passes on grooves with a spacing of 3/8
in.
A series
of parallel
grooves was made consecutively,
spaced across the surface of the block;
this series
comprised
one pass.
Subsequent
passes
were
made by re-
T]•ERMALLY
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CUTTINGOFGRANITE
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inches
I
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PASS NUMBER
I
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Cutter
speed
Thrust
0.5
•
.--• I00•.
Diameter
zo
2 ips
3,000 lb
3 inches
IO
0.5
inches
groove IDocing
Fig.
o
15--Effect
specific
PASS
of multiplicity
energy
of independent
and muck.
cuts on
748
DYNAMIC
Rock
MECHANICS
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749
peating
the process,
cutting
along the same grooves.
The
value of specific
energy decreased
in the second and third
passes and appeared
to reach a steady state
on the fifth
pass.
The muck values
are shown in figure
17; the
specific
energy results
and the scatter
in the muck data
suggest
that
these values
have also reached
a steady
state.
When the fifth
pass was made, lateral
cutter
forces were occasionally
large enough to chip
ments between two neighboring
grooves.
Since
out fragnot all
the rock between grooves was. chipped in the fifth
pass,
more was removed in the later
passes.
In places where
chips had been removed, the grooves were deepened such
that more rock could be removed in the following
passes.
The optimum spacing of the single-pass
grooves is such
that the stresses
developed
are high enough to form
chips between neighboring
grooves.
For repeated
passes,
the optimum spacing is increased
because the deeper
grooves allow more cutter
penetration
and stresses
sufficient
to remove larger
chips.
The results
obtained
with multiple
passes at different
spacings indicate
that the optimum groove spacing for untreated
specimens
was about 3/4 in.
The laser
improved the results
at all
spacings by facilitating
chip formation
between grooves.
The optimum spacing,
however,
remained
3/4 in.
Effect
of
Laser
The rate sensitivity
of granite
caused an increase
in
the values
of specific
energy and a decrease
in the
amount of muck as the cutter
speed was increased.
Although
significant
for steady state values,
these values were not
as pronounced for multiple
passes as for single
passes on
a smooth surface.
Figures
18 and 19 show the effect
of
cutter
speed on the steady
state
results
for untreated
specimens.
Again,
as for single
passes,
the ratio
of
chips to fine material
was high at lower speeds, and
helped to account for the specific
energy curves.
It was found that
the best improvements
in the amount
of muck and the value of specific
energy were obtained
if the laser beam was focused on a path laterally
offset
from the cutter
path.
Using these results
and using the
3/4 in. spacing,
the effect
of laser
heat input on the
specific
energy of muck removed from the rock was investigated.
Figures
20 and 21 show the improvements
obtained
in values of specific
energy and muck, respectively.
Desirable
effects
were observed
in both of these
sets of values,
although
the increase
in the amount of
muck due to laser
treatment
for steady
state
was not as
pronounced
as
for
independent
cuts
on a smooth
surface.
750
I)¾N.tMIC
ROCK MECl•.tNICS
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Tt•ERMALLY
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Although
the amount of muck removed for any groove
length
was smaller
if the cutting
was done at higher
speeds,
the pass took less time so that tke amount of
material
removed per unit
time might be higher
if tke
cutting
were done at higher
speeds.
Thus, slow speeds
favor
economy on a strict
specific
energy whereas higher
speeds favor
increased
advance rate,
which is of equal
importance
in a practical
situation.
Lasing the rock,
however,
increased
the
amount
of
muck
removed.
This
im-
provement
diminished
as the cutter
speed was increased
because at higher
cutter
speeds a lower amount of heat
delivered
to
the
was
rock.
CONCLUSIONS
The data collected
and observations
made during
the
course of this
study justify
the following
statements:
(1)
For a cutting
condition
with
fixed
cutter
ttkrust,
speed, and diameter,
there exists
a critical
spacing for
parallel
grooves:
at larger
spacings
the grooves
tend to
be independent
and at smaller
spacings
there
is chipping
between
adjacent
grooves.
There also exists
an optimum
spacing
at which the most efficient
cutting
is performed.
The critical
and optimum spacings,
as well
as the amount
of muck, increase
with decreasing
cutter
diameter
and speed
and with increasing
thrust.
The value of specific
energy
is inversely
related
to the amount of muck.
(2)
Repeated cutting
in an independent
groove causes
a gradual
decrease
in the amount of muck and value
of
specific
energy.
Cutting
repeatedly
in
interacting
grooves
initially
causes a decrease
in the value
of
specific
energy and an increase
in the amount of muck.
After
several
passes,
a steady
state
is reached
and the
values
of specific
energy
and muck vary around mean
values
for the steady
state.
Cutting
at steady
state
is several
times more efficient
than cutting
on a
smooth surface.
The critical
and optimum spacings
for
steady
state
are greater
than those for single
passes.
(3)
The laser
beam decreases
the value
of specific
energy and increases
the amount of muck in both singlepass independent
cuts and steady state
cuts.
Lasing
weakens the rock by causing permanent
damage and transient
thermal
stresses.
The extent
of damage depends on beam
power,
beam speed,
and lasing
procedure.
For each
particular
procedure,
a critical
heat input
value must
be exceeded
if any improvement
is expected.
The greatest
improvements
energy were
path slightly
in
tke
obtained
offset
amount of muck and value
by focusing
the
from the cutter
of
specific
laser
beam on a
groove.
756
])¾NAMIC
ROCK MECHANICS
REFERENCES
1.
2.
Farra,
G.,
mental
Observations
C.R.
Nelson,
and F.
of
Radiation.
Rept.
Engineering,
April
1969,
Massachusetts
128 pages.
Green,
Tests
S.J.,
No.
Rock
R69-16,
and R.D.
at Strain
Moavenzadeh.
Failure
Laser
of
Civil
of Technology,
Uniaxial
Compression
from 10-4/Seconds
Geologic
Materials.
Manufacturing
General
Motors Corporation,
Rept.
April
1968,
46 pages.
Experi-
to
Department
Institute
Perkins.
Rates
Due
on Three
Development,
No. MSL-68-6,
3.
Hill,
G.A.
What's Ahead for Tunnelling
Machines?
Proc. of the American Society
of Civil
Engineers,
v. 94, No. C02, October
1968,
pp. 211-231.
4.
Hirschfield,
Report
R.C.
prepared
for
Hard Rock Tunneling
the
U.S.
at Massachusetts
Institute
15, 1965,
46 pages.
Investigation.
Department
of
of Technology,
Commerce
October
5.
Maurer,
W.C.
The State
of Rock Mechanics
Knowledge
in Drilling
-- Fracture
and Breakage
of Rock.
Society
of Min. Eng.,
AIME, 1966, p. 355.
6.
Moavenzadeh, F., R.B. Williamson,
and F.J. McGarry.
Thin Disk Technique
for Analyzing
Rock Fractures
Induced by Laser Irradiation.
Rept. No. R68-21,
Dept.
of Civil
of Technology,
Engineering,
Massachusetts
May 1968,
82 pages.
Institute
7.
Moavenzadeh,
F.,
R.B. Williamson,
and F.J.
McGarry.
Laser
Assisted
Rock Fracture.
Rept.
No. R67-3,
Dept.
of Civil
Engineering,
Massachusetts
Institute
of Technology,
January
30, 1967,
57 pages.
8.
Muirhead,
I.R.,
and L.G. Glossop.
Hard Rock Tunneling
Machines.
Trans.
Inst.
of Min. and Metallurgy,
v. 77,
p. A1, 1968,
18 pages.
9.
Nasiatka,
Thomas M.
an• Present.
10.
Nelson,
C.R.
Fracture
of
Massachusetts
152 pages.
Tunneling
BuMines Inf.
Investigation
Some Brittle
Inst.
of
Technology
Circ.
of
8375,
Modes of
Materials.
Technology,
--
1968,
Its
Past
12 pages.
Thermal
Ph.D.
Thesis,
September,
1969,
TI-IERMALLYASSm?•DC•?•
o• G••
757
11.
Paul, B.,
and D.L. Sikarskie.
A Preliminary
Theory
of Static
Penetration
by a Rigid Wedge into a Brittle
Material.
Seventh Symposium on Rock Mechanics,
Society
of Min. Eng., AIME, June 1965, p. 119.
12.
Pfleider,
E.P. Overview,
NAE/NRC Study on Rapid
Excavation.
Proc. of the Symposium on Research
and Development
in Rapid Excavation,
Sacramento,
California,
October 28-29,
1968, 10 pages.
13.
Robbins,
R.J.
Robbins Tunnel Boring Machines.
Conference
on Tunnel
and Shaft
Excavation,
University
of Minnesota,
Minneapolis,
Minnesota,
May 15, 1967,
8 pages.
14.
Sikarskie,
Penetration
Brittle
Science,
D.L.,
and Rafael
Benjumea.
A Note
of a Rigid Wedge into a Nonisotropic
Material.
v.
6,
International
1969,
p.
343.
J.
of
Rock
on the
Mechanics