drillability in hard rock drill and blast tunnelling

GEOMECHANICS
DRILLABILITY IN HARD ROCK DRILL AND BLAST
TUNNELLING
By Dipl.-Geol. Dr. Kurosch Thuro and o. Univ.-Professor. Dr. phil. Georg Spaun
Usually the main subject in preliminary site investigations
prior to tunnelling projects is the prediction of tunnel stability. During the last years in conventional drill and blast
tunnelling, problems have occured also connected with the
accurate prediction of drillability in hard rock. The drillability is not only decisive for the wear of tools and equipment but is - along with the drilling velocity - a standard
factor for the progress of excavation works. The estimation
of drillability in predicted rock conditions might bear an
extensive risk of costs. Therefore an improoved prediction
of drilling velocity and bit wear would be desireable.
The drillability of a rock mass is determined by various
geological and mechanical parameters. In this report some
major correlations of specific rock properties as well as
geological factors with measured bit wear and drilling rates
are shown. Apart from conventional mechanical rock
properties (unconfined compressive and tensile strength,
Young's modulus) a new property for toughness refering to
drillability has been introduced: the specific destruction
work WZ. This new property makes it possible to understand better the connection between drilling velocity and
the main mechanical rock character. As well as mechanical
rock properties the influences of geological parameters on
drillabillity have been the main topic of a recently published dissertation (THURO 1995, 1996).
Parameters of drillability
Drillability is a term used in construction to describe the influence of a number of parameters on the drilling rate
(drilling velocity) and the tool wear of the drilling rig. The
interaction of the main factors is illustrated in Fig. 1.
geological parameters
Rock & Rock Mass
machine parameters
influence on choice
ditions also depend on the geological history, containing
weathering, hydrothermal decomposition and the structure
of discontinuities. Together they build the basic parameters
for drillability.
mineral
mineral composition
micro fabric
equivalent quartz content
porosity
rock
elastic/plastic behaviour
mechanical rock properties
destruction work
compressive strength
Young's modulus
tensile strength
ratio of of σu/σt
rock density
rock mass
rock mass conditions
discontinuities
anisotropy
spacing of discontinuities
weathering
hydrothermal decomposition
Fig. 2: Geological parameters: General view of the characteristics
of mineral, rock and rock mass.
According to rock conditions the corresponding drilling rig
will be choosen. The machine parameters are depending on
the drilling method: In underground excavation the rotary
percussive drilling is standard, providing maximum performance under most circumstances. Parameters are the
technical specifications of the drill hammer, flushing system and the design of the drilling bit. Typical tunnelling
rigs consist of a diesel-hydraulic tramming carrier, carrying
up to three booms with hydraulic drifter feeds and rock
drills. For example the COP 1238 (15 kW impact power)
and the COP 1440 (20 kW impact power, both made by
Atlas Copco) are the most popular hydraulic rock drills in
use on the marked today. Fig. 3 shows typical button bits
used in underground excavation in rotary percussive drill
rigs.
Drilling Rig
percussive drill hammer
power transfer
drilling bit
rock mass conditions
mechanical rock properties
drilling velocity
tunnelling performance
Drillability
wear of drilling tools
drilling bit wear
excavation system & logistics
operation & maintenance
of the tunnelling rig
Working Process
Fig. 1: Illustration of the term „drillability“ and the main influencing
parameters.
First of all, the geological parameters generate the specific
characteristics of rock material and rock mass (Fig. 2). These characteristics may be at least partly put into figures with
the help of mechanical rock properties. But rock mass con-
Fig. 3: Typical button drill bits of various manufacturers with 6 - 9
buttons and different flushing systems mainly used in hard rock.
The drilling bit is the part of the rig which carries out the
crushing work. The bit consists of a carrier holding the
actual drilling tools: buttons of hard metal (wolframcarbide
with a cobalt binder, MOHS´ hardness 9½). Possible sorts of
Both authors are members of Department of General, Applied and
Engineering Geology, Technical University of Munich.
Felsbau 14 (1996) Nr. 2
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THURO & SPAUN: Drillability in hard rock drill and blast tunnelling
Button Types
spherical
(semi-)
ballistic
conical
(ballistic)
quartzphyllite
105
100
95
drilling rate [%]
button types and their main characteristics are shown in
Fig. 4.
The shape of the button and the design of the bit (geometry
and arrangement of buttons, flush holes and draining channels) have a severe influence on bit wear and drilling performance. For example, using ballistic 9-button bits, a maximum penetration performance has been obtained in
quartzphyllite of the Innsbruck area. In Fig. 5 drilling rates
relative to the average of the quickest bit type are plotted
comparing 6-, 7-, 8- and 9-button bits.
90
85
80
Characteristics
75
70
m "non aggressive" shape
m minimum drilling rates
m low bit wear
m excavation mainly
by impact
6 x 45 s
Drilling performance at the Inntaltunnel/Innsbruck
9 x 45 s
9 x 45 b
Often it is a matter of time dependent expenses rather
than the pure cost of materials that increase the construction costs of a tunnel. So first of all it is of high interest, how
drilling velocity may influence heading performance. Taking the Inntaltunnel as an example in Fig. 6 two excavation classes have been compared to show how net drilling
time at the tunnel face raises the whole time for drilling one
entire round. In the crown heading a 3-armed Atlas Copco
Rocket Boomer H 185 was used with COP 1440 hydraulic
rock drills mounted. From excavation class III to class IVb
the drilling time decreases nearly at the same ratio as the
net drilling time. Therefore the share of drilling is reduced
Excavation Class III
Excavation Class IVb
miscellaneous 3%
support
miscellaneous
support
31%
24%
10%
37%
drilling
27%
drilling
18%
15%
21%
14%
charging
transport
charging
transport
Comparison of Excavation Class III & IVb
III
IV
2,3 m/min
3,0 m/min
drilling depth
3,0 m
2,4 m
excavation depth
2,7 m
2,2 m
78 sec
48 sec
drilling velocity
net drilling time
105 min
68 min
charging
59 min
53 min
round length
6,5 h
6,3 h
drilling time (round)
One of the most striking tunnelling projects in Austria was the 12.7 km long
Inntaltunnel nearby Innsbruck. During
running excavation works of the Inntaltunnel, poor drilling and blasting conditions have been recorded over long distances. Drillability of the rock mass has been determined by foliation of the Innsbrucker Quarzphyllit and by its geotechnical character.
8 x 45 b
Fig. 5: Drilling rates in quartzphyllite depending on type of button
and drilling bit. 9 x 45 b = 9 button type, ∅ 45 mm, b - ballistic
(s - spherical).
m "very aggressive" shape
m maximum drilling rates
m high bit wear
m excavation mainly
by shearing / cutting
The third main factor influencing drillability is the working process itself. First
of all smooth operation and permanent
maintenance of the tunnelling rig is contributing to successful drilling performance. Secondly, a high penetration rate
at the tunnel face is not automatically
leading to a high performance of the heading as will be demonstrated in the following case study of the Inntal tunnel. So
it is a matter of understanding the entire
excavation system before applying expertise to the investigation of drillability.
8 x 45 s
button bits
m "aggressive" shape
m moderate drilling rates
m moderate bit wear
m excavation mainly
by shearing / cutting
Fig. 4: Button types of drilling bits used for rotary percussive drilling and their main characteristics.
7 x 45 s
rounds per day
3,7 rounds/day
difference in percent
based on
excavation class IVb
heading performance
-40
-20
10,0 m/day
0
20
3,8 rounds/day
8,4 m/day
40
difference in %
Fig. 6: Excavation class III and IVb in the Inntaltunnel. Effects of decreasing net drilling time
on the entire drilling of one round.
Felsbau 14 (1996) Nr. 2
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GEOMECHANICS
Drilling rates and mechanical rock
properties
Nevertheless net drilling times are a result of changed drilling velocities in different rock types. But what are the
changed drilling velocities based on? To get information on
this point, drilling rates have been measured periodicaly during running excavation works and cores have been taken
out of the rock mass to get mechanical rock properties of
representative sections (SPAUN & THURO 1994). In this way
drilling progress could be connected with some main rock
parameters. Two of the most frequently used rock properties are the unconfined compressive strength and the tensile
strength. But the correlation between drilling rate and compressive strength as well as tensile strength of the tested
rock types was rather poor (Fig. 7, Fig. 8). These rock
properties were not likely to describe the toughness of the
rock material.
To get further on this point it is necessary to get a better
understanding of the crushing mechanism at the bottom of a
borehole.
COP 1440 - 20 kW
very high
4,0
drilling rate [m/min]
Coming from studies by high-speed photography and analysis of thin sections of rock below the area of disc cutter
tools of TBM´s, three main destruction mechanisms could
be detected (OZDEMIR et al. 1977, WANG et al. 1978,
BLINDHEIM 1979). Those results can be generalised and
transfered on the crushing process below the buttons of a
drilling bit (Fig. 9).
rotation
drilling bit
button
3
rotation
button
1
3
2
1 crushed rock powder
2
2
2 radial cracks
10 mm
3 detached fragments
Fig. 9: Crushing process in rotary percussive drilling. Destruction
mechanism under the bit buttons.
drilling velocity
5,0
high
3,0
medium
2,0
low
1,0
y=a+b·ln x yσ(n-1)=0,76 m/min n=20 R2=20%
very low
0,0
0
20
40
60
80
100
120
unconfined compressive strength UCS [MPa]
Fig. 7: Drilling rate correlated with unconfined compressive
strength. The quality of the correlation is very poor.
drilling velocity
5
COP 1440 - 20 kW
very high
4
drilling rate [m/min]
Crushing process underneath a drilling bit
penetration
approximately by one third (27% to 18%, Fig. 6). The extension of drilling time had no real influence on the length
of one round in class III, because support works were less
extensive than in class IVb.
high
3
medium
2
low
1
y=a+b·ln x yσ(n-1)=0,58 m/min n=20 R2 =54%
very low
0
0
2
4
6
8
indirect tensile strength TS [MPa]
10
12
Fig. 8: Drilling rate correlated with indirect tensile strength. The
quality of the correlation is rather poor.
Around the contact of the button a new state of stress is induced in the rock, where four important destruction mechanisms can be distinguished:
1) Under the bit button a crushed zone of fine rock powder
is formed (impact).
2) Starting from the crushed powder zone, radial cracks are
developed (induced tensile stress).
3) When stress in the rock is high enough (respectively if
enough cracks exist ±parallel to the bottom of the borehole), larger fragments of the rock can be sheared off
between the button grooves (shear stress).
4) In addition to the mechanisms above stress is induced
periodical (dynamic process).
Looking at the drilling mechanism it is obvious, that besides compressive and tensile strength (percussive process)
and shear strength (bit rotation) the elastic characteristics of
rock material will be of crucial importance. To be precise,
the bit is always drilling through pre-cracked rock (see
MÜLLER-SALZBURG 1963: 104) and we have to devote a
great deal of our time with the post-failure behaviour of
rock to get closer to the crushing mechanism below the
drilling bit.
Monitoring destruction work
With other words, a new property is needed, describing
both brittleness/toughness of rock and the quantity of energy necessary to build new surfaces (cracks) in rock. To get
this newly defined rock property, the deformation process
of a rock sample under unconfined compression is studied
in Fig. 10.
Felsbau 14 (1996) Nr. 2
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THURO & SPAUN: Drillability in hard rock drill and blast tunnelling
"brittle"
marble
Something similar to the destruction work has been
established several times in literature: The Coefficient of
Rock Strength CRS (PAONE, MADSON & BRUCE 1969),
failure
post-failure-section
pre-failure-section
point
the Rock Impact Hardness Number RIHN (RABIA &
BROOK 1980, 1981), and the Swedish Brittleness Test
failure point
(SELMER-OLSEN & BLINDHEIM 1970), all based on the
Protodyakanow impact test (PROTODYAKANOW 1962),
perform a method, where a rock sample is crushed by a
weigth (represents a definite destruction work) and the
obtained grain size is a measure for brittleness. The only
destruction work
disadvantage is, that this sort of rock property is not a
Wz = σ d ε
real physical quantity but an index value lacking a physical unit.
Very similar to the destruction work is the specific
strain ε
energy es introduced by HUGES (1972). It may be
Fig. 10: Stess-strain curve of a brittle marble without and a typical phyllite with described as the specific work in the stress-strain curve
a distinctive post-failure behaviour under unconfined compression.
until the failure of the sample occures during unconfined compression. In this property, however, is missing
Especially in the rock types of the Innsbrucker Quarzphyllit
the post-failure section of the curve and thus the main point
(quartzphyllite,
sericite-chlorite-phyllite,
carbonateof the newly defined quantity.
(quartz)-phyllite) samples showed a distinctive post-failure
Returning to our example of the Inntaltunnel, the debehaviour during unconfined compression (class I behastruction
work WZ proved as a highly significent parameter
vior). In contrast to the phyllites the marbles indicated a tyfor
correlation
with the drilling performance. Fig. 12 shows
pical brittle behaviour with no post-failure section at all
the
mean
values
of drilling rates recorded in 20 different
(class II behavior). Whereas in phyllites drilling conditions
tunnel sections of the Inntal tunnel compared with the dewere poor, submitting low drilling rates, the marbles were
struction work of the rock material belonging to them. The
quite easy to drill, providing high drilling performance. It is
diagramm indicates the close correlation between drilling
obvious, that the area under the envelope of the stress-strain
velocity and destruction work.
diagram of the phyllite sample is much larger than the area
of the marble sample (Fig. 10) - so why not connect those
drilling velocity
5
results? From the physical sight, the envelope curve is notCOP 1440 - 20 kW
marbles
very high
hing else but the energy (or work), required for destruction
4
of the rock sample.
sericite-chlorite-phyllite
high
The newly defined rock property has been determined
3
as the specific destruction work (in short: destruction work)
quartz phyllites
medium
WZ [kJ/m³] and gives the possibility to compare rock mate2
rials refering to brittleness/toughness using drills or cutters.
sericite-chlorite-gneisses
low
Whereas the YOUNG´s modulus submits the gradient
carbonite phyllites
1
(derivation) of the linear section, the destruction work is
estimated out of the area under the stress-strain-envelope
y=a+b·ln x yσ =0,17 m/min n=20 R =96%
very low
0
(integral, Fig. 11). As a product of both - stress and strain 0
50
100
150
200
250
destruction work represents the work of shape altering indestruction work Wz [kJ/m3]
cludung the post failure section.
"tough" phyllite
drilling rate [m/min]
stress σ
σ
2
(n-1)
pre-failure section
A
B
Fig. 12: Drilling rate correlated with destruction work. The quality of
the correlation is very good.
post-failure section
C
σu
D
Applying expertise to other projects
stress σ
Youngs modulus
destruction work
E=δσ/δε
WZ =
σdε
δσ
δε
strain ε
Fig. 11: Stess-strain curve with a distinctive post-failure behaviour of
the sample. Determination of Young´s modulus E and specific destruction work WZ.
But not only in foliated rock mass the specific destruction
work has turned out as a suitable rock property for monitoring drilling rates. As can be seen in Fig. 13 a large variety
of rocks were tested, showing a high significant correlation
graph. The rock material plotted in this diagram includes
clay-siltstones, sand- and limestones, conglomerates, marls,
marbles, schists and different cristalline rocks derived from
seven tunnel projects in Germany and Austria.
The destruction work has proved as a highly significant parameter for the evaluation of net drilling rates in drill and
blast tunnelling and therefore is the most important mechanical rock property for the investigation of drillability.
Felsbau 14 (1996) Nr. 2
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GEOMECHANICS
drilling velocity
5
COP 1440 - 20 kW
very high
dip angle of
90
high
foliation
75
3
100
60
medium
2
80
low
standard deviation
1
y=a+b·ln x yσ(n-1)=0,33 m/min n=64 R2=85%
very low
0
0
100
200
300
400
500
destruction work [kJ/m3]
destruction work [%]
drilling rate [m/min]
4
60
30
40
15
20
Fig. 13: Drilling rate plotted against destruction work. The quality of
the correlation is very good.
As can be seen in Fig. 14, drilling rates are especially
dependent on the impact power of the rock drill. There is a
distinct improvement of drilling performance from the COP
1238 (15 kW) to the COP 1440 (20 kW) resulting in up to
40% higher penetration rates.
45
0
0
Fig. 15: Correlation between destruction work and the orientation of
foliation for a quartzphyllite with flat and smooth (continuous line) respectively uneven, undulating discontinuities (broken line).
drilling velocity
5
COP 1440 - 20 kW
very high
dip angle of
90
3
medium
2
low
1
very low
COP 1238 ME - 15 kW
0
0
100
200
300
400
500
destruction work [kJ/m3]
Fig. 14: Drilling rate plotted against destruction work for both rock
drills. There is a distinct improvement of drilling performance from
the 15 kW to the 20 kW type.
Influence of anisotropy
Of course, rock properties and drilling rates are also highly
dependent on the orientation of weakness planes related to
the direction of testing or drilling. In the following figures
(Fig. 15 - Fig. 18) the correlations between rock properties
and the orientation of foliation for a quartzphyllite with flat
and smooth discontinuities (continuous line) and a quartzphyllite with uneven, undulating sericite-chlorite-partings
(broken line) are shown.
The striking point is, that whereas the highest values of
destruction work are gained parallel to foliation, the unconfined compressive strength is higher perpenticular to the
discontinuities - and vice versa. In both cases, the minimum
is located between 45° and 60°. This is set in advance by
the geometry of the sample (length to diameter ratio 2 : 1).
foliation
75
high
100
unconfined compressive strength [%]
drilling rate [m/min]
4
80
60
45
60
30
40
15
20
0
0
Fig. 16: Correlation between unconfined compressive strength and
the orientation of foliation for a quartzphyllite with flat and smooth
(continuous line) respectively uneven, undulating discontinuities
(broken line).
Regarding indirect tensile strength, the minimum values are
obtained parallel to foliation, presuming that in this case real stresses turn up perpendicular to both foliation and force
direction (Fig. 17). By an angle of 45°, stresses are pure
shear stresses and the test turns out to be a shear test along
a forced gap. It is certainly for this reason that the diagram
of drilling rates looks quite the same (Fig. 18) but velocity
is high, where tensile strength is low.
In Fig. 19 a mathematical model is given to describe the
behaviour of both drilling and shear (tensile) strength. As
supposed, strength and drilling rate are connected with the
geometry of induced stress.
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THURO & SPAUN: Drillability in hard rock drill and blast tunnelling
To understand the connection between stress field and drilling rates one has to study the destruction process down the
borehole once again (Fig. 20 and Fig. 21).
dip angle of
foliation
90
compressive/tensile stress
75
tensile strength [%]
100
60
shear stress
80
shear stress
45
60
30
40
15
20
0
0
Fig. 17: Correlation between tensile strength and the orientation of
foliation for a quartzphyllite with flat and smooth (continuous line) respectively uneven, undulating discontinuities (broken line).
testing
arrangements
UCS
dip angle of
75
100
60
drilling rate [%]
80
45
60
30
40
15
20
0
0
Fig. 18: Correlation between drilling rate and the orientation of foliation for a quartzphyllite with flat and smooth (continuous line) respectively uneven, undulating discontinuities (broken line).
high tensile stress
low tensile stress
100
100
75
75
50
50
graph equation
y = a + b·cos x
90
75
drilling rate [%]
indirect tensile strength [%]
drilling rate
25
tensile strength
25
60
45
UCS
TS
Fig. 20: Drilling process according to different orientations of discontinuities (foliation).
foliation
90
TS
30
15
0
dip angle of foliation
Fig. 19: Drilling rate and tensile strength plotted against the orientation of foliation.
When the direction of drilling is right-angled to the orientation of foliation, rock material is compressed right-angled
but sheared parallel to it (Fig. 21/1). Although cracks will
develop radial to compression, the cracks parallel to the
bottom of the borehole will be used for chipping. Usually
in this case the highest drilling velocities are obtained because of the favourable schist orientation. Drilling is controlled by the shear strength of the foliated rock material.
The minimum destruction work causes large sized chips
and a maximum drilling performance.
If the drilling axis is oriented parallel to foliation, compression also is parallel but shear stress is right-angled (Fig.
21/3). It should be clear, that less cracks will develop for
reasons of higher strength right-angled to the weakness planes. Drilling is controlled by the tensile strength parallel to
the foliation producing small sized fragments and minimum
drilling performance.
Generally, drilling is controlled by the dip angle of foliation (Fig. 21/2), submitting medium sized fragments during the crushing process. Drilling performance is - by
geometrical reason - mainly a cosine function of the dip
angle.
Anyway, it is for sure, that in the parallel case, rock
properties are the highest and drilling rates are low. In addition blasting conditions are often related with drilling. So
if the tunnel axis is parallel to the main foliation, drilling
and blasting conditions suppose to be very poor. In the case
of the Inntaltunnel the consequences were a lower heading
performance and higher drilling expenses than expected.
In comparison with the crushing process under a TBM
disk cutter (WANNER 1975), the process underneath a button drill bit is completely different in foliated rock. In contrast to rotary percussive drilling, TBM penetration rates
are a maximum when foliation is ± diagonal to the tunnel
axis.
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penetration
GEOMECHANICS
rotation
drilling bit
rotation
1 crushed rock powder
2 cracks, rigth-angled to foliation
button
button
1 Drilling is controlled
by shear strength
1
1
penetration
2
¦ minimum destruction work
¦ large sized fragments
¦ maximum drilling performance
Low values of destruction work have been
obtained in rock where quartz-feldsparmouldings often were cut through or dislocated by internal folding. The result is a
spongy fabric with the possibility to absorb
high distortions but not as tough as the
structure described above.
Of course there are a lot of other geological parameters strongly influencing drilling performance - not possible to be
discussed in this paper - such as spacing of
discontinuities, hydrothermal decomposition, status of weathering and porosity of the
micro fabric.
Drilling bit wear
In the second place, the wear of drilling
equipment may be a severe factor of costs in
tunnelling. As a leading parameter, the wear
button
button
of drilling bits has been examined in different rock types. Other tools such as drifter
2 Drilling is controlled
by the dip angle of foliation
rods, couplings and shank adapters have a
¦ medium sized fragments
1
life-span in average ten times the one of
1
¦ drilling performance is a
button bits.
cos-function
of
the
dip
angle
2
An important hint for surveying abrasivity of rock is the analysis of worn-out drilling bits. Bit wear occures in six basic
forms, generally combined according to
rock mass conditions:
1) Button wear. Wear of the hard metal
rotation
rotation
drilling bit
buttons according to high abrasivity of
the rock, such as granite, gneiss or ambutton
button
phibolite.
3 Drilling is controlled
2) Steel wear: Wear of the steel calibre in
by tensile strength
diameter as a result of chip grinding in
¦
1
maximum
destruction
work
1
¦ small sized fragments
weak and moderate strong rock with
¦ minimum drilling performance
2
high abrasivity, such as sandstones,
schists, weathered and decomposed
0
5
10 mm
rock.
Fig. 21: Physical destruction process in foliated rock. Crushing mechanism below the bit
buttons depending on the dip angle.
3) Button damage: Breaking of drill buttons because of high shear stress. If the
As a further result of anisotropy, problems may occure
drifter
rod
becomes
stuck or fixed according to jointing,
when drilling direction is ± diagonal to the tunnel axis:
hard components or steel support, severe damage of the
When the angle between drilling and tunnel axis is acutebuttons may result.
angled, drifter rods are deviated into the dip direction of
foliation, if obtuse-angled, into the normal direction of fo4) Total button removal: By the same reasons of button
liation. In any case, drill tracks may be seen as curvatures
damage buttons may break out as a whole.
and produce distinct borehole deviation.
5) Total wear out: When parts of hard metal or entire
Structure and texture of rock material and their influbuttons are chipped off, frequently those rotating pieces
ence on rock properties are discussed in detail by
- too big to be removed by flushing - rip out more butHOWARTH & ROWLANDS (1987). Though the effect on detons leading to a total wear-out of the button bit.
struction work is much greater than supposed. Mechanical
6) Steel shaft damage: Damage of the steel shaft below
rock properties of the Innsbrucker Quarzphyllit are severely
the buttons by reasons of steel quality or severe force.
dominated by the elastic-plastic behaviour of rock material.
In consequence it is possible to make a statistical analysis
For example high values of destruction work have been
of worn out tools to gain an impresson of the grinding efmeasured in rock material showing a tight and laminated
fect of rock fragmentation.
micro fabric and high grade of interlocking between quartzOf course, the range of tool wear can be measured by
feldspar-mouldings and mica layers. In addition, mica conrecording
the quantity of worn-out button bits refering to
sisted mainly of biotite, giving evidence of a higher grade
their
total
drilling length - the so called "life-span" of the
of metamorphism within the low grade (greenschist) zone.
bit. Life-span is reported in boremeters per bit [m/bit].
drilling bit
rotation
penetration
rotation
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THURO & SPAUN: Drillability in hard rock drill and blast tunnelling
Fig. 23: Wear characteristic of drill bits used in rock belonging to the
"Innsbrucker Quarzphyllit".
Drilling bit wear in the "Innsbrucker
Quarzphyllit"
Fig. 22 gives an impression of the wide variety of the rock
types contained in the "Innsbrucker Quarzphyllit". Commonly quartzphyllites, sericite-chlorite-phyllites and gneisses are put together under the term "quartzphyllite".
"Innsbrucker Quarzphyllit"
sericite-chloritephyllite
quartzphyllite
27%
25%
10%
15%
10%
4%
sericitechlorite-gneiss
4%
6%
graph equation y=a+b·ln x
greenschist
9
carbonate-phyllite
quartz marble
8
marble
Fig. 22: Rock types contained in the "Innsbrucker Quarzphyllit" and
the composition of samples taken from the Inntal tunnel.
In Fig. 23 an example for a statistical analysis is given
showing the wear characteristic of drill bits used in rock
belonging to the "Innsbrucker Quarzphyllit". In fact, this
wear statistic is something like a fingerprint of the examined rock. In quartzphyllites and associated rock types
steel wear is dominating compared to the marbles, where
total button removal comes first.
6
5
4
3
2
standard deviation = ½
1
n=24
R2=95%
0
1
quartzphyllites, serizitechlorite-phyllites & gneisses
10
100
1000
Rosiwal abrasiveness
(3) button damage
(2) steel
wear
Fig. 24: Correlation between MOHS hardness and ROSIWAL abrasiveness.
quartz
"quartzphyllite"
46%
24%
quartz
7
Mohs hardness
carbonatequartzphyllite
It is clear, that tool wear is a result of the mineral content harder than steel, especially quartz (MOHS´ hardness =
7). To include all minerals, the equivalent quartz content is
determined - meaning the entire mineral content refering to
the abrasiveness/hardness (after ROSIWAL 1896, 1916) of
quartz (ROSIWAL abrasiveness = 100). Therefore each mineral share is multiplied with its relative abrasiveness/hardness to quartz (quartz = 100%). An appropriate
correlation between MOHS hardness and ROSIWAL abrasiveness is given in Fig. 24. Taking the average mineral content of quartzphyllites (Fig. 25), the equivalent quartz content is approximately 46% and thus is slightly higher than
the pure quartz content of 42%.
42%
feldspar
10%
(4) total button
removal
7%
12%
6%
22%
(5) total wear-out
(1) button wear
sericite
24%
(6) steel shaft
damage (2%)
chlorite
biotite 2%
carbonate pyllites, marbles
& quartz marbles
(4) total
equivalent quartz content equ=45,5%
(3) button
button
removal
Fig. 25: Average mineral content of quartzphyllites, sericite-chloritephyllites and -gneisses ("quartzphyllite") and derived equivalent
quartz content.
damage
36%
16%
12%
20%
15%
(5) total
wear-out
(1) button wear
(6) steel shaft
damage (1%)
carbonate 3%
(2) steel
wear
In Fig. 26 the bit life-spans of rock types contained in the
"Innsbrucker Quarzphyllit" are correlated with their equivalent quartz contents. It is obvious that bit wear raises
with increasing equivalent quartz content. The expected
relation is also detected when plotting the properties of
other rock material into the diagramm (Fig. 27). For sandstones and decomposed rock other correlations than
discussed here have been found (THURO 1995, 1996).
Felsbau 14 (1996) Nr. 2
8
GEOMECHANICS
Bit Wear
limestone, marl, conglomerates, phyllites, marbles
2500
very low
y=a+b·ln x yσ(n-1)=144 m/bit n=22 R2=95%
bit life-span [m/bit]
2000
low
1500
standard deviation
moderate
1000
high
500
very high
extreme high
0
0
20
40
60
80
100
equivalent quartz content [%]
Fig. 26: Bit life-span of "Innsbrucker Quarzphyllit" rock types and
corresponding equivalent quartz content.
Fig. 27: Bit life-span of limestone, marl, conglomerates, together with
phyllites and marbles and corresponding equivalent quartz content.
Not suitable for sandstones and decomposed or weathered rock.
Classification of drillability
Finally a classification of drillability is given,
contributing up-to-now experience. First of all, a
drillability classification should rely on values
easyly obtained on the site. Secondly, the parameters should be expressive and provide a good
resolution of drilling rate and wear characteristic.
The system proposed here is based on net drilling
velocity, measured at the tunnel face and drilling
bit wear recorded as the bit life-span.
To get an impression how wide values of bit
wear and drilling rates may vary, mean values of
different rock types or homogeneous areas derived from seven tunnel projects have been taken
for the diagrams of Fig. 28. The investigations
have been carried out using 15 kW and 20 kW
borehammers (Atlas Copco COP 1238 ME and
COP 1440). The matrix is based on the experience, that high drilling rates (3 - 4 m/min) and
low bit wear (1500 - 2000 m/bit) should be
described as "fair" drillability.
Conclusion
An investigation program is submitted, which
should help to improve the estimation of rock
drillability in planning future tunnel projects.
First of all, with the discovered correlation
charts for mechanical and petrographic rock
properties, it should be possible to predict drilling rates and bit wear for the examined rock types in a satisfactory manor.
Besides rock properties - the main thing in
preliminary site investigation is - first of all simple and basic geological mapping. This sounds rather simple. But it is extremely necessary
to keep in mind all the parameters possibly influencing drilling performance. Therefore it is very
important to prepare all rock and soil descriptions in a suitable way, engineers are able to understand.
Fig. 28: Classification diagram for two rock drills (COP 1238 - 15 kW and COP 1440 20kW) enclosing drilling rate and bit wear.
Felsbau 14 (1996) Nr. 2
9
THURO & SPAUN: Drillability in hard rock drill and blast tunnelling
Investigation Program
anisotropy
spacing of discontinuities
weathering
hydrothermal decomposition
1.
preliminary site investigation engineering geological mapping
rock & soil description and classification
quantitative description of discontinuities
on basis of IAEG and ISRM standardization
2.
mechanical rock properties
sampling out of drilling cores
if possible, out of an investigation tunnel
destruction work
compressive strength
Young's modulus
tensile strength
ratio of of σu/σt
rock density / porosity
ð influence of anisotropy
or other factors
3.
petrographic description
mineral composition
micro fabric
equivalent quartz content
degree of interlocking
Fig. 29: Proposal of an investigation program, which should help to improve the estimation of rock drillability in planning future tunnel projects.
For that reason we would like to finish this paper with the
words of Priscilla P. NELSON (1993: 261):
„Whatever the reasons, it is clear, that neither geology
alone, laboratory and field testing alone, experience alone
nor equipment design and operation expertise alone can
get an engineer to the point where underground excavation
is a clearly defined engineering process. Integration of all
these knowledge bases is required to raise the level of engineering contribution to underground construction, and
the entire excavation system must be understood before
applying engineering expertise to the solution of expected
or developing problems.“
RABIA, H. & BROOK, N. (1980): An empirical equation for drill performance prediction. - 21st Symp. on Rock Mech. Univ. Missouri-Rolla,
May, 103-111.
RABIA, H. & BROOK, N. (1981): The effects of apparatus size and
surface area of charge on the impact strength of rock. - Int. J. Rock
Mech. Min. Sci. & Geomech. Abstr., 18., 211-219.
ROSIWAL , A. (1896): Neue Untersuchungsergebnisse über die Härte
von Mineralien und Gesteinen. - Verhandlg. d. k.k. geol. R.-A. Wien,
475-491.
ROSIWAL , A. (1916): Neuere Ergebnisse der Härtebestimmung von
Mineralien und Gesteinen. Ein absolutes Maß für die Härte spröder
Körper. - Verhandlg. d. k.k. geol. R.-A. Wien, 117-147.
References
SELMER-OLSEN, R. & BLINDHEIM , O.T. (1970): On the drillability of
rock by percussive drilling. - Proc. 2nd Cong. of the Int. Soc. for
Rock Mech., Belgrade, 65-70.
BLINDHEIM , O.T. (1979): Drillability predictions in hard rock tunnelling. - Tunnelling 1979, London, Inst. Min. Metall., 284-289.
SPAUN, G. & THURO, K. (1994): Untersuchungen zur Bohrbarkeit und
Zähigkeit des Innsbrucker Quarzphyllits. - Felsbau, 12., 2, 111-122.
HOWARTH , D.F. & ROWLANDS, J.C. (1987): Quantitative assessement of rock texture and correlation with drillability and strength
properties. - Rock Mech. & Rock Eng., 20., 57-85.
THURO, K. (1995): Geologisch-felsmechanische Untersuchungen zur
Bohrbarkeit von Festgesteinen beim konventionellen Bohr- und
Sprengvortrieb anhand ausgewählter Tunnelprojekte. - 156 S. Dissertation TU München.
HUGHES, H.M.(1972): Some aspects of rock machining. - Int. Journal
of Rock Mech. Min. Sci., 9., 205-211.
MÜLLER-SALZBURG, L. (1963): Der Felsbau. Bd.I, Theoretischer Teil,
Felsbau über Tage, 1. Teil. - 624 S., Nachdruck 1980, Stuttgart
(Enke).
NELSON, P.P. (1993): TBM performance analysis with reference to
rock properties. - in: HUDSON, J. (ed.-in-chief): Comprehensive rock
engineering. Principles, practice & projects. Vol. 4. Excavation, Support and Monitoring. - 849 S., Oxford, New York, etc. (Pergamon),
261-291.
THURO, K. (1996): Bohrbarkeit beim konventionellen Sprengvortrieb.
Geologisch-felsmechanische Untersuchungen an sieben ausgewählten Tunnelprojekten. - Münchner Geologische Hefte, Reihe B:
Angewandte Geologie, 1., 1 - 152.
W ANG, F.-D., OZDEMIR, L. & SNYDER, L. (1978): Prediction and experimental verification of disk cutter forces in hard rock. - in: Eurotunnel '78 conference, Basle, Switzerland (Basle: Congress Centre,
1978), 1st day, March 1st, 1978, pap. 4, 44 S.
OZDEMIR, L., MILLER, R. & WANG, F.-D. (1977): Mechanical tunnel
boring, prediction and machine design. - Annual report, CSM
(Colorado School of Mines) APR 73-07776-A03.
PAONE, J., MADSON, D. & BRUCE, W.E. (1969): Drillability Studies laboratory percussive drilling. - 22 S., USBM U.S. Bureau of Mines,
RI (Report of Investigation) 7300, Washington.
PROTODYAKANOV , M.M. (1962): Mechanical properties and drillability
of rocks. - Proc. 5th Symp. on Rock Mech. Univ. Minnesota, May,
103-118.
Felsbau 14 (1996) Nr. 2
10
GEOMECHANICS
Abstract
Thuro & Spaun: Bohrbarkeit von Festgesteinen beim
Bohr- und Sprengvortrieb.
THURO & SPAUN: DRILLABILITY IN HARD ROCK DRILL
AND BLAST TUNNELLING
Die Bohrbarkeit des Gebirges wird durch unterschiedliche
geologische und felsmechanische Parameter bestimmt. In
diesem Beitrag werden die wesentlichen Abhängigkeiten
zwischen den spezifischen Materialeigenschaften von Gestein und Gebirge und den meßbaren Parametern Bohrkronenverschleiß und Bohrgeschwindigkeit aufgezeigt. Neben
den konventionellen felsmechanischen Kennwerten (Druck, Zugfestigkeit und Elastizitätsmodul) wurde ein neues Maß
für die Zähigkeit bezüglich der Bohrbarkeit von Gesteinen
eingeführt: die spezifische Zerstörungsarbeit Wz. Die neue
Auswertemethode ermöglicht es, den ursächlichen Zusammenhang zwischen der Netto-Bohrgeschwindigkeit und den
felsmechanischen Eigenschaften eines Gesteins besser als
bisher nachzuvollziehen. Im Verlauf des bergmännischen
Tunnelvortriebs erwies sich das Gebirge des Inntaltunnels
über weite Strecken als schwer bohrbar und ebenso als
schwer sprengbar. Die Bohrbarkeit des Gebirges wurde
zum einen durch die Schieferung des Innsbrucker Quarzphyllits bestimmt, zum anderen durch seine geotechnischen
Eigenschaften. In diesem Beitrag werden Bohrkronenverschleiß und Bohrgeschwindigkeit in Abhängigkeit der wesentlichen geologischen und felsmechanischen Parameter
diskutiert. Es werden felsmechanische Kennwerte des Innsbrucker Quarzphyllits und bohrtechnische Daten in Abhängigkeit von der Schieferungsrichtung vorgestellt und die
Bedeutung des Mikrogefüges nachgewiesen. Abschließend
wird ein Vorschlag für ein Untersuchungsprogramm unterbreitet, welches bei künftigen Vorerkundungen für Tunnelund Stollenprojekte helfen soll, Gestein und Gebirge im
Hinblick auf die Bohrbarkeit besser zu erfassen.
The drillability of a rock mass is determined by various
geological and mechanical parameters. In this report some
major correlations of specific rock properties as well as
geological factors with measured bit wear and drilling velocity are shown. Apart from conventional mechanical rock
properties (compressive and tensile strength, Young's modulus) a new property for toughness/brittleness referring to
drillability has been introduced: the specific destruction
work WZ. This new method makes it possible to understand
better the connection between drilling velocity and the main
mechanical rock character.
During running excavation works of the Inntaltunnel,
poor drilling and blasting conditions have been recorded
over long distances. Drillability of the rock mass has been
determined by foliation of the Innsbrucker Quarzphyllit and
by its geotechnical character. In this paper bit wear and
drilling velocity in correlation with main geological and
mechanical properties are discussed. Rock properties and
and drilling datas of the Innsbrucker Quarzphyllit are
shown in correlation of shist foliation. Besides, the importance of the criystalline microstructure could be proved.
Authors
Kurosch Thuro, Dipl.-Geol. Dr.rer.nat
Georg Spaun, o.Univ.-Prof. Dr.phil.
Lehrstuhl für Allgemeine, Angewandte und IngenieurGeologie, Technische Universität München
Lichtenbergstraße 4, D-85747 Garching
e-mail: [email protected]
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