Drillability prediction - geological influences in hard rock drill and

Geol Rundsch (1997) 86: 426 - 438, Offprint
with Contributions to "Prediction in geology", Vrije Universiteit Amsterdam, February, 22nd-24th 1996
426
Original Paper
K. Thuro
Drillability prediction - geological influences in hard rock drill and
blast tunnelling
Abstract* 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 to 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 improved 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 and especially geological factors with measured bit wear and drilling velocity are shown.
Drilling velocity is dependent on a lot of geological
parameters: Those principal parameters include jointing of rock mass, orientation of schistosity (rock anisotropy), degree of interlocking of microstructures,
porosity and quality of cementation in clastic rock, degree of hydrothermal decomposition and weathering of
a rock mass. Drilling bit wear increases with the equivalent quartz content. The equivalent quartz content
builds the main property for the content of wear-relevant minerals. For various groups of rock types different connections with the equivalent quartz content
could be detected. In sandstone bit wear is also dependent on porosity or the quality of the cementation. Finally an investigation program is submitted, which
helps to improve the estimation of rock drillability in
planning future tunnel projects.
Key words: Drillability ⋅ Drilling rate, Bit wear,
Destruction work, Anisotropy, Joint spacing,
Equivalent quartz content, Porosity
Drilling equipment - technical introduction
For drilling blastholes in hard rock, today the rotary
percussive drilling is standard in underground mining
and tunnelling, providing maximum performance under most circumstances (Cohrs 1988). The hydraulic
drill hammer is a combination of a rotary drilling machine and a percussive drill and uses a separate rotary
and percussive mechanism.
Whereas percussive drilling is controlled by jerkily
moving of the drilling rod with only a loose contact of
the drilling bit to the bottom of the borehole, rotary
percussive drilling is characterized by continuous rotation - comparable to rotary drilling. By means of
high feed pressure (12 - 20 kN), lying more than a decade above those in percussive drilling, the drilling bit
is always tight to the bottom of the borehole. Since the
torques are much stronger, crushing work is carried
out also by shearing between the impacts.
Fig. 1 Operation of rotary percussive drilling and the main machine parameters
K. Thuro
Lehrstuhl für Allgemeine, Angewandte und Ingenieur-Geologie,
Technische Universität München
D-85747 Garching, Germany
Fax: +49 89 289 14382
e-mail: thuro@ mineral.min.chemie.tu-muenchen.de
Regarding just the procedure, the rotary percussive
drilling is superior to the rotary drilling and the percussive drilling (Feistkorn 1987). The hydraulics facilitate an optimum energy transfer from the percussive
mechanism to the drilling rod. Parameters are the
427
technical specifications of the drill hammer, flushing
system and the design of the drilling bit (Fig. 1).
Fig. 2 Drilling rig: Atlas Copco Rocket Boomer H 175 with 3
booms and service platform
Typical tunnelling rigs consist of a diesel-hydraulic
rubber-wheeled tramming carrier, carrying up to three
booms with hydraulic drifter feeds and rock drills. The
range comprises units for hydraulic drilling with a selection of of different carriers, booms, feeds, and rock
drills (Fig. 2).
Fig. 4 Typical button drill bits with six, seven, eight and nine buttons and different flushing systems mainly used in hard rock
Fig. 4 shows typical button bits used in underground
excavation in rotary percussive drill rigs. 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 (wolfram carbide
with a cobalt binder, MOHS´ hardness 9½). Possible
sorts of button types and their main characteristics are
shown in Fig. 5.
Button Types
spherical
(semi-)
ballistic
conical
(ballistic)
Characteristics
! "non aggressive" shape
! minimum drilling rates
! low bit wear
! excavation mainly
by impact
! "aggressive" shape
! moderate drilling rates
! moderate bit wear
! excavation mainly
by shearing / cutting
! "very aggressive" shape
! maximum drilling rates
! high bit wear
! excavation mainly
by shearing / cutting
Fig. 5 Button types of drilling bits used for rotary percussive drilling and their main characteristics
Fig. 3 Hydraulic boom BUT 35 of the AC-Rocket Boomer H 175.
Centre-mounted feed with double rotation devices, which makes it
possible to position the feed vertically on both sides of the boom,
with accurate parallel holding, roof drilling and cross-cuts
For example the COP 1440 hammer (20 kW impact
power) mounted on the AC Rocket Boomer H 175 is
the most popular hydraulic rock drill presently in use.
Features such as rapid and exact boom positioning
with roof drilling and cross-cuts are performed with
the BUT 35 boom shown in Fig. 3.
The shape of the button and the design of the bit (geometry and arrangement of buttons, flush holes and
draining channels) have a strong influence on bit wear
and drilling performance. In Fig. 6 drilling rates relative to the average of the quickest bit type are plotted
comparing 6-, 7-, 8- and 9-button bits. For example,
using ballistic 9-button bits, a maximum penetration
performance has been obtained in (tough) quartz phyllite of the Innsbruck area. This impression is less
distinctive in brittle rock types as can be seen in
limestone from the German Muschelkalk. The highest
drilling rates in this limestone have been archieved using an 8-button bit, giving an optimum between button stress and button area in brittle rock.
428
quartz phyllite (Innsbrucker Quarzphyllit)
105
100
Machine Parameters
Rock & Rock Mass
Drilling Rig
mechanical
rock mass
90
percussive
tunnelling
rock properties,
95
drilling rate [%]
Geological Parameters
drill hammer,
performance
drilling velocity
power transfer,
conditions
drilling bit
85
Drillability
80
wear of drilling tools
drilling bit wear
75
Working Process
70
6 x 45 s
7 x 45 s
8 x 45 s
8 x 45 b
9 x 45 s
9 x 45 b
excavation system & logistics,
operation & maintenance of the tunnelling rig
button bit type
limestone (Muschelkalk)
105
Fig. 7 Illustration of the term "drillability" and the main influencing parameters.
100
drilling rate [%]
95
mineral
mineral composition
micro fabric
equivalent quartz content
porosity / cementation
rock
elastic/plastic behaviour
mechanical rock properties
destruction work
compressive strength
Young's modulus
tensile strength
ratio of compressive and
tensile strength
rock density
90
85
80
75
rock mass
70
6 x 45 s
7 x 45 s
8 x 45 s
8 x 45 b
9 x 45 s
9 x 45 b
rock mass conditions
discontinuities
anisotropy
spacing of discontinuities
status of weathering
hydrothermal decomposition
button bit type
Fig. 6 Drilling rates in quartz phyllite and limestone depending on
the button type and drilling bit. 9 x 45 b = 9 button type, ∅ 45 mm,
b ballistic; s spherical
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. As could be seen in the technical introduction,
drillability is - first of all - influenced by the machine
parameters of the chosen drilling rig. Therefore, only
tunnel projects with the same drilling equipment can
be used for drillability studies (Thuro 1996). The interaction of the main factors is illustrated in Fig. 7.
Apart from technical parameters, especially the geological parameters will basically influence the drilling performance and the wear of the drilling rig (Fig.
8). The specific characteristics of rock material and
rock mass may be at least partly put into figures with
the help of mechanical rock properties. But rock mass
conditions also highly depend on the geological history, weathering conditions, hydrothermal decomposition and the structure of discontinuities. Therefore, one has to go through three levels of investigation:
mineral - rock type - and rock mass - meaning also
three levels of dimension!
Fig. 8 Geological parameters: General view of the characteristics
of mineral, rock and rock mass
The last important factor influencing drillability is the
working process itself. Firstly, smooth operation and
permanent maintenance of the tunnelling rig contributes to a successful drilling performance. Secondly, a high penetration rate at the tunnel face does not
automatically lead to a high performance of the tunnel
heading (Thuro and Spaun 1996a). Therefore, it is a
matter of understanding the entire excavation system
before applying expertise to the investigation of drillability.
The necessity of drillability studies
But why is prediction of drillability necessary? The
following figures will show the effects of increased
drilling time on the performance of the tunnel heading.
As an example, the excavation works of the Altenberg
Tunnel in Idar-Oberstein are presented as circle diagrams in Fig. 9 - in calculation (left side) and during
final construction (rigth side; Thuro 1996).
As can be seen from the drilling segment, the entire
drilling time of one round has increased nearly three
times from calculation to final construction. The time
for charging of the explosives during construction has
increased five times as compared with calculation. Finally, the time for excavating one entire round has be-
429
been doubled, as can bee seen by the bigger diameter
of the construction circle, and heading performance
has been cut in half.
Fanglomerate composition
quartzite
support 67 min
weathered
volcanic
rock
40%
Calculation
18%
27%
drilling
62 min
22%
10%
8%
mucking
90 min
support 157 min
charging
20 min
10%
volcanic rock
vein quartz
schist
compressive strength [MPa]
31%
Construction
0
20%
mucking
79 min
drilling
167 min
vein quartz
charging
102 min
Comparison:
200
quartzite
33%
33%
16%
100
volcanic rock
Calculation
Construction
swellability
swellability of the weathered volcanic rock
40
net drilling time
round length
2.5 m/min
60 sec
4.2 h
1.3 m/min
121 sec
8.4 h
swelling [%]
drilling rate
30%
30
20%
20
10%
5%
10
heading performance
13.3 m/day
7.6 m/day
Fig. 9 Working round in the Altenberg Tunnel in calculation and
final construction. Effects of increased drilling time on the performance of the tunnel heading
The reason for this fatal fault in prediction is evident
in Fig. 10: The composition of the fanglomerate
(Waderner formation, Rotliegend) coming up along
the entire length of the tunnel. The fanglomerate is
composed of quarzite, vein quartz and schist of the
Hunsrück range and volcanic rock of the Idar-Oberstein volcanic area. But about one half of the volcanic
rock has already been deeply weathered and decomposed to a clay-siltstone with swelling minerals ranging from high to very high swellability. The range of
the compressive strength of the components ranges
from over 250 MPa (quartzite) to nearly zero
(completely weathered volcanic rock). This was the
reason for stucked drilling rods, blocked water flushing, collapsed boreholes and - above all - bad
drilling and blasting conditions during running
excavation
That is why
works.
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 improved prediction of drilling
velocity and bit wear would be desireable.
0%
equivalent Ca-montmorillonit content
25%
36%
very high
high
moderate
low
no swellability
0
0
5
10
15
20
25
30
swelling time [h]
Fig. 10 Composition of the fanglomerate (Waderner formation),
compressive strength of the components and swelling ability of the
weathered, decomposed volcanic material
Monitoring and classification of drilling rates and bit
wear
To get information on the correlation between drilling
rate, bit wear, mechanical rock properties and geological parameters, extensive field studies and laboratory
work was carried out. Until now, nine tunnel projects
in Germany, Austria and North India have been followed more or less extensively, measuring drilling rates
periodically during running excavation works.
Furthermore, rock samples have been analysed to get
mechanical rock properties of representative sections
(Thuro 1996). Based on engineering geological mapping of the tunnels, mean values of 25 different rock
types or homogeneous areas were taken for correlation
analysis. In this way, drilling progress and bit wear
could be connected with some of the main rock parameters.
Before going into a detailed analysis of drillability
parameters, a classification of drillability is given,
contributing up-to-date experience. Firstly, a drill-
430
borehole depth
meters
case studies in 9 tunnel projects in Germany, Austria
and overseas (North India) were plotted into the chart
in Fig. 11.
The investigations were carried out using a 20 kW
borehammer (Atlas Copco COP 1440). The matrix
was 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. The drilling
rates range from 1 meter per minute to about 5 meters
per minute. The bit life-span ranges from 50 meters to
over 2,000 meters per bit. Therefore drillability ranges
in our classification from extremely poor to easy.
net drilling time
minutes
Mechanical rock properties
ability classification should rely on values easily 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 lifespan.
drilling velocity
drilling rate =
total boremeters
meters
bit life-span =
number of drill bits
bits
Formula 1 Determination of drilling velocity and drilling bit wear
low
moderate
high
very high
extremely high
The drilling performance is taken as the drilling velocity or drilling of one simple borehole. The drilling
bit wear is taken as the bit life, which means the total
of boremeters drilled with one bit (Formula 1).
To get an impression of how wide values of bit
wear and drilling rates may vary, mean values of different rock types or homogeneous areas derived from 25
Bit Wear
The most frequently used rock properties are the unconfined compressive strength, the Young´s modulus
and the tensile strength. As a derived rock property,
the ratio of unconfined compressive strength and tensile strength often is designated as toughness (or brittleness) of a rock material. Many authors tend to take
one or more of those properties as main parameters of
drillability (Schimazek & Knatz 1970, Wanner 1975,
Habenicht & Gehring 1976, Blindheim 1979, Movinkel & Johannessen 1986). Thus extensive rock testing
has been carried out based on the ISRM suggested methods (Brown 1981, ISRM 1985) to gain representative mean values of the properties of the drilled rock types.
Regarding the drilling rig, the drilling process is
fundamental for the choice of the investigation pa-
y
ilit
b
illa
Dr
very low
drilling bit wear
5
no
no
ea
sy
rm
al
very high
drilling rate [m/min]
high
3
po
o
2
ve
ry
ex
tr
po eme
or
ly
1
po
r
medium
et
dy
e
in
bta
to
o
n
or
low
percussive drill COP 1440 - 20 kW
Drilling Velocity
4
et
dy
e
n
i
bta
to
very low
0
0
500
sandstones
conglomerate &
fanglomerate
1000
1500
limestone & marl
quartzite
marble
2000
2500
[m/bit]
phyllites & gneiss
quartz-mica-schist
amphibolite
Fig. 11 Classification diagram enclosing 25 case studies of different rock types or homogeneous areas derived from 9 tunnel projects
431
failure point
UCS
stress σ
unconfined
compression
test
destruction work
Wz= σ d ε
drilling bit
button
3
strain ε
rotation
Fig. 13 Estimation of the specific destruction work Wz from the
stress-strain curve of a rock sample under unconfined compression.
UCS unconfined compressive strength
button
1
3
2
2
2
10 mm
1 crushed rock powder
2 radial cracks
3 detached fragments
Fig. 12 Crushing process in rotary percussive drilling. Destruction
mechanism under the bit buttons
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 (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 periodically (dynamic process).
Examining 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 is of crucial importance. To be precise, the bit always drills through
pre-cracked rock (see Müller-Salzburg 1963: 104).
Therefore the so-called specific destruction work Wz
has been introduced (Thuro 1996, Thuro & Spaun
1996a,b), as a measurement for the quantity of energy,
required for destruction of a rock sample or - in other
words - the work, necessary to build new surfaces (or
cracks) in rock. As a product of both - stress and strain
- destruction work represents the work of shape altering including the post failure section (Fig. 13).
In Fig. 14 drilling rates of the previously mentioned
rock material is plotted against destruction work including clay-siltstone, sand- and limestone, conglomerate, marl, marble, schist and different cristalline rock.
The destruction work proved to be a highly significant
parameter for correlation with the drilling performance. The chart indicates the close correlation between
drilling velocity and destruction work.
5
standard deviation
4
drilling rate [m/min]
rotation
post-failure-section
pre-failure-section
penetration
rameters. In Fig. 12 the crushing mechanism is illustrated. Coming from studies by high-speed photography
and analysis of thin sections of rock below the area of
disc cutter tools of tunnel boring machines, three main
destruction mechanisms could be detected (Ozdemir et
al. 1977, Wang et al. 1978, Blindheim 1979). Those
results can be generalised and transferred on the crushing process below the buttons of a drilling bit.
3
2
1
y=5.49-0.60·ln x yσ(n-1)=0.28m/min n=23 R2=89%
0
0
100
200
300
400
500
destruction work [kJ/m3]
sand- & claystone
conglomerate &
fanglomerate
limestone & marl
silicic dolomite
phyllite & gneiss
marble
Fig. 14 Drilling rates, correlated with destruction work of 23 rock
types. The correlation is very good
In contrast to the described connection, correlations
between the conventional mechanical rock properties
(unconfined compressive and tensile strength, Young's
modulus and the ratio of unconfined compressive
strength and tensile strength "toughness") and drilling
rates show less significance (see Thuro and Spaun
1996 b).
432
compressive/tensile stress
Geological parameters
testing
arrangements
UCS
TS
TS
UCS
Fig. 15 Drilling process according to different orientations of foliation (after Spaun and Thuro 1994).
high tensile stress
low tensile stress
100
100
75
75
50
50
25
graph equation
y = a + b·cos x
90
75
drilling rate [%]
drilling rate
indirect tensile strength [%]
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. This
has been discussed in detail by Thuro & Spaun (1996,
also see Spaun and Thuro 1994).
When the direction of drilling is at right angles to
the orientation of foliation (Fig. 15, left side), rock
material is compressed at right angles but sheared parallel to it. 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 (Fig. 16).
If the drilling axis is oriented parallel to foliation
(Fig. 15, right side), compression also is parallel but
shear stress is at right angles. It should be clear, that
fewer cracks will develop for reasons of higher
strength at right angles to foliation. Drilling is controlled by the tensile strength parallel to the foliation producing small-sized fragments and a minimum drilling
performance (Fig. 16).
It is certain, that in the parallel case, rock properties are the highest and drilling rates are low. In addition, blasting conditions are often related to drilling.
Thus, if the tunnel axis is parallel to the main foliation, drilling and blasting conditions are supposed to be
very poor.
As a further result of anisotropy, problems may occure when drilling direction is diagonal to the tunnel
axis: When the angle between drilling and tunnel axis
is acute-angled, drifter rods are deviated into the dip
direction of foliation, if obtuse-angled, into the normal
direction of foliation. In any case, drill tracks may be
seen as curves and produce distinct borehole deviation
and a “geologically caused” overbreak.
shear stress
shear stress
Although mechanical properties allow prediction of
drilling performance to be more precise, geological influences are even more decisive for drilling velocity as
well as for the bit life. There are several geological influences though only some can be mentioned here:
1. anisotropy - orientation of discontinuities related to
the direction of testing or drilling
2. spacing of discontinuities
3. mineral composition - equivalent quartz content
4. pore volume - porosity of the micro fabric
Hydrothermal decomposition of rock material very
often shows the same effects as the status of weathering. Some of the possibly connected problems have
already been discussed in this paper.
tensile strength
25
60
45
30
dip angle of foliation
15
0
Fig. 16 Drilling rate and tensile strength plotted against the
orientation of foliation
Spacing of discontinuities
Of course, drilling rates are also dependent on spacing
of discontinuities in rock mass. Discontinuities are, as
a law, weakness planes in rock mass - thus MüllerSalzburg (1963) talks about rock mass as "broken
rock". The spacing of joints could also be described as
"joints per meter" and is another parameter for the precracking of rock.
In the chart of Fig. 17 the influence of discontinuities is not visible, if the spacing is large against the
dimensions of the borehole. When the joints get closer, the drilling velocity increases up to the double.
But the connected problem is borehole instability, causing hole collapses and timeconsuming scaling of the
established blasthole. By this means, the efforts of fast
drilling, especially in fault zones, may be rendered useless very soon.
433
limestone (middle Muschelkalk)
%
5
200
spacing large
against dimension
of borehole
equivalent quartz content
140
120
collapse of
boreholes
common
equ =
100
Σ A ⋅R
i
i
i=1
80
20 cm
63 cm
200 cm
A - mineral amount [%]
R - Rosiwal abrasiveness [%]
n - number of minerals
very
widely
6,3 cm
widely
2 cm
medium
extreme
closely
fault
zone
0,6 cm
closely
2
160
n
3
very
closely
drilling rate [m/min]
180
4
the Mohs hardness is known, the abrasiveness of minerals can be estimated by this chart with satisfactory
accuracy (within a half degree of Mohs hardness).
joint spacing
Formula 2 Determination of the equivalent quartz content
Fig. 17 Correlation between drilling rate and joint spacing in limestone of the middle Muschelkalk
9
8
quartz
Mohs hardness
7
6
5
4
3
2
y = 2.12 + 1.05·ln x yσ(n-1)= ½
1
n=24 R2=95%
0
1
10
100
1000
Rosiwal abrasiveness
Fig. 18 Correlation between Rosiwal abrasiveness and Mohs hardness, enclosing 24 different minerals (excluding diamond)
Bit Wear
2500
very low
2000
bit life-span [m/bit]
Equivalent quartz content
Having discussed some factors influencing drilling rates, parameters for predicting the drilling bit wear are
now mentioned. As a leading parameter, the wear of
drilling bits has been examined in different rock types.
Other tools such as drifter rods, couplings and shank
adapters have a life-span on average ten times the one
of button bits and thus are not suitable.
Technical parameters are not really suitable for
drillability studies though there are about 200 hardness
tests for rock characterization (Atkinson 1993, West
1989, Brook 1993, Nelson 1993). Much of them have
been introduced for a special purpose and have not
been developed further. Only few have gained international attention such as the drilling rate index DRI
(Selmer-Olsen and Blindheim 1970) or the Cerchar abrasivity index CAI (Valantin 1973, Suana and Peters
1982).
The point is, there is no single physical property in
existence to quantify and describe ´´hardness’’ as if it
is the uniaxial compressive strength for stress. Also a
lot of petrographic parameters such as rock texture and
mineral fabric have been discussed to be used for predicting tool wear and drillability (Howarth and Rowlands 1987). But the performed structural methods are
very time consuming and thus have not been applied in
practice.
It is clear, that tool wear is predominantly a result
of the mineral content harder than steel (Mohs hardness ca. 5.5), especially quartz (Mohs hardness of 7).
To include all minerals of a rock sample, the equivalent quartz content has been determined in thin sections by modal analysis - meaning the entire mineral
content refering to the abrasiveness or hardness of
quartz (Formula 2). Therefore each mineral amount is
multiplied with its relative Rosiwal abrasiveness to
quartz (with quartz being 100%, Rosiwal 1896, 1916).
An appropriate correlation between Mohs hardness
and Rosiwal abrasiveness is given in Fig. 18. When
defects of binder,
porosity
hydrothermal
decomposition
low
1500
moderate
1000
high
main graph
500
very high
extremely h.
0
0
20
40
60
80
100
equivalent quartz content [%]
sandstone
fanglomerate &
conglomerate
limestone & marl
crystalline rock
phyllite & gneiss
marble
hydrothermaly decomposed
Fig. 19 Bit life of different rock types correlated with the equivalent quartz content enclosing 42 case studies in 8 tunnel projects
The method of determining the equivalent quartz
content is wide-spread among tool manufacturers, engineers and engineering geologists for preliminary site
investigations prior to tool wear problems.
In Fig. 19 the bit life of different rock types is correlated with its equivalent quartz contents. It is visible
434
3
bit life-span [m/bit]
dry density [g/cm ]
that bit wear raises mainly with increasing equivalent
Bit Wear
2
2,1
2,2
2,3
2,4
2,5
2,6
quartz content. But obviously some kinds of rock have
2000
their own curves: (a) sandstones, especially those with
hydrothermally
low
decomposed
higher porosity, often corresponding with a defect in
1500
the silicic cementation; and (b) hydrothermally demoderate
composed crystalline rock.
defect binder
In each of those special rock types the interlocking
1000
inc
of the grains in the microfabric is "disturbed". Therer
high
por easin
g
osi
ty
fore, for purposes of prediction, each rock type must
compact
500
be discussed individually. In Fig. 20 a rock family very high
something like a "normal facies" - of limestone, marl,
y=174+60?x yσ =136m/bit n=8 R =90%
extremely h.
0
conglomerates, together with phyllites and marbles has
25
20
15
10
5
0
been built to be described by a logarithmic regression
porosity [%]
curve.
For the chosen rock family the relation is very clo- Fig. 21 Correlation of bit life-span and porosity (dry density) in
sandstones
se and may be used for a forecast of bit wear, when the
equivalent quartz content is determined by a thin secdry density [g/cm ]
Drilling velocity
tion modal analysis.
2
2,1
2,2
2,3
2,4
2,5
2,6
2
(n-1)
3
7-button bits
limestone, marl, conglomerates, phyllites, marbles
Bit Wear
6
very low
5
clay-silt-stone
y=3131-624·ln x yσ(n-1)=144m/bit n=22 R2 =95%
bit life-span [m/bit]
2000
low
1500
standard deviation
moderate
1000
high
500
drilling rate [m/min]
2500
COP 1440 - 20 kW
very high
4
high
3
moderate
2
low
1
very high
extremely h.
0
0
20
40
60
80
100
y=1.83+0.12·x
yσ(n-1)=0.12m/bit
n=8
R2=98%
very low
0
25
20
15
10
porosity [%]
5
0
equivalent quartz content [%]
Fig. 20 Bit life-span of limestone, marl, conglomerates, together
with phyllites and marbles and corresponding equivalent quartz
content
Porosity and binder defects
For sandstones and decomposed rock other relationships must be discussed. The expected connection is
also detected when plotting the porosity of sandstones
instead of the equivalent quartz content into the diagram (Fig. 21). Porosity is measured here as a function
of dry density of rock material and ranges from a compact (dense) to a totally decomposed silicic binder-free
fabric.
There seems to be a correlation between the porosity of the rock and technical parameters, such as bit
wear (Fig. 21), drilling rates (Fig. 22) and - naturally mechanical rock properties such as unconfined compressive strength (Fig. 29) and destruction work (Fig.
30). Although the number of cases in each chart is
quite low, the good correlation coefficient suggests a
close connection. The data were collected in the
Schönrain Tunnel near Würzburg, where mainly rock
of the middle und upper Bunter sandstone has been encountered and in the Achberg Tunnel nearby Unken in
the Werfen sandstone formation.
Fig. 22 Correlation of drilling rates and and porosity (dry density)
in sandstones
435
Fig. 23 Hard, quartzitic Bunter sandstone with a very dense and
compact fabric. No pores can be seen and the fracture runs through
each individual quartz grain ("intragranular failure"; picture length
approx. 1 mm)
Fig. 26 Small hexahedric granules of silicic cement growing on
quartz grains (picture length approx. 0.1 mm)
Fig. 24 Hard Bunter sandstone with a less dense fabric. Fracturing
is dominated by intergranular (grain-to-grain) failure. Larger hexahedric quartz crystals growing on grains are developing out of
small granules of silicic binder (picture length approx. 1 mm)
Fig. 27 Clayey binder of the decomposed Bunter sandstone showing kaolinite crystals growing in the twinning lamellae of a plagioclase crystal (picture length approx. 0.1 mm)
Fig. 25 Hydrothermally decomposed Bunter sandstone, characterised by a porous fabric with a clayey binder in replacement of the
original, silicic cement (picture length approx. 5 mm)
Fig. 28 In the grain gaps, small calcite rhombohedrons are growing
as secondary binder (picture length approx. 0.14 mm)
436
dry density [g/cm3]
2
2,1
2,2
2,3
2,4
2,5
2,6
compressive
strength
after ISRM
unconfined compressive strength [MPa]
120
y=168-53⋅ln x
yσ(n-1)=8,1MPa
n=8
R2=95%
very high
100
80
high
60
40
20
moderate
clay-silt-stone
low
very low
0
25
20
15
10
porosity [%]
5
0
Fig. 29 Correlation of unconfined compressive strength and porosity (dry density) in sandstones
dry density [g/cm3]
2
2,1
2,2
2,3
2,4
2,5
2,6
250
destruction work [kJ/m3]
y=327-103⋅ln x
yσ(n-1)=30kJ/m3
n=8
R2=83%
200
150
100
50
clay-silt-stone
0
25
20
15
10
porosity [%]
5
shown where fracture is characterized by intragranular
failure.In Fig. 24, a hard Bunter sandstone with a higher porosity is visible, suggesting a fabric less dense
than before. The silicic cement does not fill every gap
between the quartz grains but the cementation is more
than just a grain-to-grain binding (intergranular failure). The small granules of silicic cement are also hexahedric, as can bee seen by increased enlargement in
Fig. 26. The hydrothermally decomposed Bunter sandstone of Fig. 25 is characterised by a porous fabric
with a clayey binder. Clay has replaced the original
silicic cement. The rock has changed its colour from
originally red to a flat whitish-grey, thus indicating
hydrothermal activity dating from a fault zone
("Harrbacher Sprung") in the Schönberg Tunnel. The
contact of the grains is not solid anymore but only weakly cemented and the surface of the grains looks
"dirty".
In Fig. 27 the clayey binder of the decomposed
Bunter sandstone is visible, showing kaolinite crystals
growing in the twinning lamellae of a plagioclase crystal. The small flakes probably are fed into the grain
gaps by circulating ground water.
In the grain gaps, small calcite rhombohedrons
grow as secondary binder (Fig. 28). It looks like the silicic binder has been removed from the sandstone together with the red colour, leaving behind some clayey
material and calcitic cement.
0
Conclusion
Fig. 30: Correlation of destruction work and porosity (dry density)
in sandstones
The fabric of the different stages of porosity (or dry
density) can be visualized by raster electron microscope photography. In Fig. 23 a very dense and compact fabric of a hard, quartzitic Bunter sandstone is
After all these observations, it is clear, that neither laboratory and field testing alone, geology alone, nor
experience alone and equipment design and operation
expertise alone can lead to the point where drillability
is anything like a clearly defined formula.
Firstly, with the discovered correlation charts for
Investigation Program
preliminary site investigations
engineering geological mapping
rock & soil description and classification
quantitative description of discontinuities
on basis of IAEG and ISRM standardization
anisotropy
spacing of discontinuities
status of weathering
hydrothermal decomposition
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 compressive /
tensile strength
rock density / porosity
" influence of anisotropy
or other factors
petrographic description
mineral composition
micro fabric
equivalent quartz content
degree of interlocking
Fig. 31 Proposal of an investigation program for preliminary site investigations
437
mechanical and petrographic rock properties, it should
be possible to predict drilling rates and bit wear for the
examined rock types in a satisfactory manner. But besides rock properties, the main problem is the variety
of geological phenomena, which cannot be put into
figures and rock properties.
Nevertheless in preliminary site investigation the
most important thing to do is simple and basic geological mapping. This sounds simple. But it is extremely
necessary to keep in mind all the parameters possibly
influencing drilling performance. Secondly, it is very
important to prepare all rock and soil descriptions in a
way engineers are able to understand. Only in such a
manner is it possible to raise the level of geological
contribution to underground construction, and the entire excavation system must be understood before
applying geological expertise to the solution of expected or developing drillability problems
In Fig. 31 an investigation program for preliminary
site investigations is presented, which should help to
improve the estimation of rock drillability in planning
future tunnel projects, trying to integrate all discussed
knowledge bases.
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