1997 gr86 prediction

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Geol Rundsch (1997) 86: 426 - 438, Offprint 426

with Contributions to "Prediction in geology", Vrije Universiteit Amsterdam, February, 22nd-24th 1996

Original Paper

K. Thuro

Drillability prediction - geological influences in hard rock drill andblast tunnelling

Abstract*

Usually the main subject in preliminary si-

te investigations prior to tunnelling projects is the pre-

diction of tunnel stability. During the last years in

conventional drill and blast tunnelling, problems have

occured also connected to the accurate prediction ofdrillability in hard rock. The drillability is not only de-

cisive for the wear of tools and equipment but is - a-

long 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 pre-

diction of drilling velocity and bit wear would be de-

sireable. 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 mea-

sured bit wear and drilling velocity are shown.

Drilling velocity is dependent on a lot of geological

parameters: Those principal parameters include join-

ting of rock mass, orientation of schistosity (rock an-

isotropy), degree of interlocking of microstructures,

porosity and quality of cementation in clastic rock, de-

gree of hydrothermal decomposition and weathering of

a rock mass. Drilling bit wear increases with the equi-

valent quartz content. The equivalent quartz content

builds the main property for the content of wear-rele-

vant minerals. For various groups of rock types dif-

ferent connections with the equivalent quartz contentcould be detected. In sandstone bit wear is also depen-

dent on porosity or the quality of the cementation. Fi-

nally an investigation program is submitted, which

helps to improve the estimation of rock drillability in

planning future tunnel projects.

K. Thuro

Lehrstuhl fr Allgemeine, Angewandte und Ingenieur-Geologie,Technische Universitt Mnchen

D-85747 Garching, Germany

Fax: +49 89 289 14382

e-mail: thuro@ mineral.min.chemie.tu-muenchen.de

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 un-

der most circumstances (Cohrs 1988). The hydraulic

drill hammer is a combination of a rotary drilling ma-

chine 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 ro-

tation - comparable to rotary drilling. By means of

high feed pressure (12 - 20 kN), lying more than a de-

cade 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 machi-

ne parameters

Regarding just the procedure, the rotary percussive

drilling is superior to the rotary drilling and the per-

cussive drilling (Feistkorn 1987). The hydraulics faci-

litate an optimum energy transfer from the percussivemechanism to the drilling rod. Parameters are the


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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 se-

lection of of different carriers, booms, feeds, and rock

drills (Fig. 2).

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.

Fig. 4 Typical button drill bits with six, seven, eight and nine but-

tons 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 9). Possible

sorts of button types and their main characteristics are

shown in Fig. 5.

(semi-)ballistic

spherical

conical(ballistic)

! "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

Button Types Characteristics

Fig. 5 Button types of drilling bits used for rotary percussive dril-

ling and their main characteristics

The shape of the button and the design of the bit (ge-

ometry and arrangement of buttons, flush holes and

draining channels) have a strong influence on bit wearand drilling performance. In Fig. 6 drilling rates rela-

tive 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 phyl-

lite 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 u-

sing an 8-button bit, giving an optimum between but-

ton stress and button area in brittle rock.


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428

70

75

80

85

90

95

100

105

drillin

grate[%]

6 x 45 s 7 x 45 s 8 x 45 s 8 x 45 b 9 x 45 s 9 x 45 b

button bit type

quartz phyllite (Innsbrucker Quarzphyllit)

70

75

80

85

90

95

100

105

6 x 45 s 7 x 45 s 8 x 45 s 8 x 45 b 9 x 45 s 9 x 45 b

limestone (Muschelkalk)

drillingrate[%]

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 o f Drillabili ty

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 in-teraction of the main factors is illustrated in Fig. 7.

Apart from technical parameters, especially the ge-

ological parameters will basically influence the dril-

ling 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 histo-

ry, weathering conditions, hydrothermal decom-

position and the structure of discontinuities. Therefo-

re, one has to go through three levels of investigation:

mineral - rock type - and rock mass - meaning also

three levels of dimension!

Working Processexcavation system & logistics,

operation & maintenance of the tunnelling rig

Geological Parameters

Rock & Rock Mass

mechanical

rock properties,

rock mass

conditions

Machine Parameters

Drilling Rig

percussive

drill hammer,

power transfer,

drilling bit

Drillability

tunnelling

performance

drilling velocity

wear of drilling tools

drilling bit wear

Fig. 7 Illustration of the term "drillability" and the main influen-

cing parameters.

mineral

rock

rock mass

mineral composition

micro fabric

elastic/plastic behaviourmechanical rock properties

rock mass conditionsdiscontinuities

equivalent quartz content

porosity / cementation

destruction workcompressive strengthYoung's modulustensile strength

rock density

spacing of discontinuitiesstatus of weatheringhydrothermal decomposition

anisotropy

ratio of compressive and

tensile strength

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 con-

tributes to a successful drilling performance. Second-

ly, 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 drilla-

bility.

The necessity of drillabili ty 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 dia-

grams 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 timefor charging of the explosives during construction has

increased five times as compared with calculation. Fi-

nally, the time for excavating one entire round has be-


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been doubled, as can bee seen by the bigger diameter

of the construction circle, and heading performance

has been cut in half.

drilling

charging

mucking

support

Calculation

Construction

67 min

90 min

62 min

20 min

27%

36% 25%

8%

167 min

102 min

79 min

157 min

33%

drilling

charging

mucking

support

31%

33%

20%

16%

net drilling time

round length

heading performance

Construction

8.4 h

7.6 m/day

121 sec

Calculation

60 sec

4.2 h

13.3 m/day

Comparison:

1.3 m/min2.5 m/mindrilling rate

Fig. 9 Working round in the Altenberg Tunnel in calculation andfinal construction. Effects of increased drilling time on the perfor-

mance 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

Hunsrck range and volcanic rock of the Idar-Ober-

stein volcanic area. But about one half of the volcanic

rock has already been deeply weathered and decom-

posed to a clay-siltstone with swelling minerals ran-

ging 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 flu-

shing, collapsed boreholes and - above all - bad

drilling and blasting conditions during running

excavation works.That is why 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 riskof costs. Therefore an improved prediction of drilling

velocity and bit wear would be desireable.

Fanglomerate composition

volcanic rock

weatheredvolcanic

rock

quartzite

vein quartz

schist

40%18%

22%

10%10%

0 100 200

compressive strength [MPa]

quartzite

vein quartz

volcanic rock

swellability

very high

moderate

high

no swell-ability

low5%

10%

20%

30%

0%equivalentCa-montmorillonitcontent

swellability of the weathered volcanic rock

0

10

20

30

40

swelling[%]

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 bitwear

To get information on the correlation between drilling

rate, bit wear, mechanical rock properties and geologi-

cal parameters, extensive field studies and laboratorywork was carried out. Until now, nine tunnel projects

in Germany, Austria and North India have been follo-

wed 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 map-

ping 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 para-

meters.Before going into a detailed analysis of drillability

parameters, a classification of drillability is given,

contributing up-to-date experience. Firstly, a drill-


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ability classification should rely on values easily ob-

tained on the site. Secondly, the parameters should be

expressive and provide a good resolution of drilling ra-

te and wear characteristic. The system proposed here

is based on net drilling velocity, measured at the tun-

nel face and drilling bit wear recorded as the bit life-

span.

drilling rate =borehole depth

net drilling time

meters

minutes

drilling bit wear

drilling velocity

total boremeters

number of drill bits

meters

bitsbit life-span =

Formula 1 Determination of drilling velocity and drilling bit wear

The drilling performance is taken as the drilling ve-

locity 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 diffe-

rent rock types or homogeneous areas derived from 25

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.

Mechanical rock properties

The most frequently used rock properties are the un-

confined compressive strength, the Youngs modulus

and the tensile strength. As a derived rock property,

the ratio of unconfined compressive strength and ten-

sile strength often is designated as toughness (or britt-

leness) 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, Movin-

kel & Johannessen 1986). Thus extensive rock testing

has been carried out based on the ISRM suggested me-

thods (Brown 1981, ISRM 1985) to gain re-

presentative mean values of the properties of the dril-

led rock types.

Regarding the drilling rig, the drilling process is

fundamental for the choice of the investigation pa-

very high

high

med miu

low

very low

DrillingVelocity

extremelyhigh

high

moderate

veryhigh

low

verylow

Bit Wear

0

1

2

3

4

5

0 500 1000 1500 2000 2500

normal

poor

verypoor

easy

poor

extremely

Drilla

bility

not

btaine

d yet

o

percussive drill COP 1440 - 20 kW

[m/bit]

drillingrate

[m/min

]

sandstones phyllites & gneiss

marble

limestone & marl

conglomerate &fanglomerate

quartz-mica-schistquartzite

amphibolite

not o

btaine

d yet

Fig. 11 Classification diagram enclosing 25 case studies of different rock types or homogeneous areas derived from 9 tunnel projects


6/12


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432

Geological parameters

Although mechanical properties allow prediction of

drilling performance to be more precise, geological in-

fluences are even more decisive for drilling velocity as

well as for the bit life. There are several geological in-

fluences though only some can be mentioned here:

1. anisotropy - orientation of discontinuities related tothe 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 weathe-

ring. Some of the possibly connected problems have

already been discussed in this paper.

AnisotropyOf course, rock properties and drilling rates are also

highly dependent on the orientation of weakness pla-nes 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 pa-

rallel to it. Although cracks will develop radial to com-

pression, the cracks parallel to the bottom of the

borehole will be used for chipping. Usually in this ca-

se the highest drilling velocities are obtained, because

of the favourable schist orientation. Drilling is control-led 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 control-

led by the tensile strength parallel to the foliation pro-

ducing small-sized fragments and a minimum drilling

performance (Fig. 16).

It is certain, that in the parallel case, rock proper-ties are the highest and drilling rates are low. In addi-

tion, blasting conditions are often related to drilling.

Thus, if the tunnel axis is parallel to the main foliati-

on, drilling and blasting conditions are supposed to be

very poor.

As a further result of anisotropy, problems may oc-

cure 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 beseen as curves and produce distinct borehole deviation

and a geologically caused overbreak.

compressive/tensile stress

shear stress

UCS TS UCS TS

testing

arrangements

shear stress

Fig. 15 Drilling process according to different orientations of folia-

tion (after Spaun and Thuro 1994).

25

50

75

100

drillingrate[%]

drilling rate

tensile strength

25

50

75

100

indirecttensilestrength[%]

dip angle of foliation

90 75 60 45 30 15 0

high tensile stress low tensile stress

y = a + bcos xgraph equation

Fig. 16 Drilling rate and tensile strength plotted against the

orientation of foliation

Spacing of discontinuitiesOf course, drilling rates are also dependent on spacing

of discontinuities in rock mass. Discontinuities are, as

a law, weakness planes in rock mass - thus Mller-

Salzburg (1963) talks about rock mass as "brokenrock". The spacing of joints could also be described as

"joints per meter" and is another parameter for the pre-

cracking of rock.

In the chart of Fig. 17 the influence of discontinui-

ties is not visible, if the spacing is large against the

dimensions of the borehole. When the joints get clo-

ser, the drilling velocity increases up to the double.

But the connected problem is borehole instability, cau-

sing hole collapses and timeconsuming scaling of the

established blasthole. By this means, the efforts of fast

drilling, especially in fault zones, may be rendered u-seless very soon.


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spacing large

against dimensionof borehole

collapse of

boreholescommon

2

3

4

5

drilling

rate[m/min]

fault

zone

extreme

closely

very

closely

closely

medium

widely

very

widely

joint spacing

80

100

120

140

160

180

200

0,6 cm 2 cm 20 cm 200 cm6,3 cm 63 cm

%limestone (middle Muschelkalk)

Fig. 17 Correlation between drilling rate and joint spacing in li-

mestone of the middle Muschelkalk

Equivalent quartz content

Having discussed some factors influencing drilling ra-tes, 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 notbeen developed further. Only few have gained interna-

tional attention such as the drilling rate index DRI

(Selmer-Olsen and Blindheim 1970) or the Cerchar ab-

rasivity 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 pre-

dicting tool wear and drillability (Howarth and Row-

lands 1987). But the performed structural methods arevery 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 hard-

ness ca. 5.5), especially quartz (Mohs hardness of 7).

To include all minerals of a rock sample, the equiva-

lent quartz content has been determined in thin secti-

ons 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 toquartz (with quartz being 100%, Rosiwal 1896, 1916).

An appropriate correlation between Mohs hardness

and Rosiwal abrasiveness is given in Fig. 18. When

the Mohs hardness is known, the abrasiveness of mine-

rals can be estimated by this chart with satisfactory

accuracy (within a half degree of Mohs hardness).

equ =


n

i=1

A Ri i

A - mineral amount [%]R - Rosiwal abrasiveness [%]n - number of minerals

Formula 2 Determination of the equivalent quartz content

0

1

2

3

4

5

6

7

8

9

Mohshardness

1 10 100 1000

Rosiwal abrasiveness

quartz

y = 2.12 + 1.05ln x y = n=24 R =95%(n-1) 2

Fig. 18 Correlation between Rosiwal abrasiveness and Mohs hard-

ness, enclosing 24 different minerals (excluding diamond)

hydrothermaly decomposed

0

500

1000

1500

2000

2500

0 20 40 60 80 100

0

500

1000

1500

2000

2500

0 20 40 60 80 100

0

500

1000

1500

2000

2500

0 20 40 60 80 100

0

500

1000

1500

2000

2500

0 20 40 60 80 100

0

500

1000

1500

2000

2500

0 20 40 60 80 100

low

moderate

high

very highextremely h.

Bit Wear

very low

defects of binder,porosityhydrothermal

decomposition

main graphbitlife-span[m/bit]

equivalent quartz content [%]

sandstone phyllite & gneiss

marble

limestone & marl

fanglomerate &conglomerate

crystalline rock

Fig. 19 Bit life of different rock types correlated with the equiva-

lent quartz content enclosing 42 case studies in 8 tunnel projects

The method of determining the equivalent quartz

content is wide-spread among tool manufacturers, en-

gineers and engineering geologists for preliminary site

investigations prior to tool wear problems.

In Fig. 19 the bit life of different rock types is cor-

related with its equivalent quartz contents. It is visible


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434

that bit wear raises mainly with increasing equivalent

quartz content. But obviously some kinds of rock have

their own curves: (a) sandstones, especially those with

higher porosity, often corresponding with a defect in

the silicic cementation; and (b) hydrothermally de-

composed crystalline rock.

In each of those special rock types the interlocking

of the grains in the microfabric is "disturbed". There-

fore, for purposes of prediction, each rock type must

be discussed individually. In Fig. 20 a rock family -

something like a "normal facies" - of limestone, marl,

conglomerates, together with phyllites and marbles has

been built to be described by a logarithmic regression

curve.

For the chosen rock family the relation is very clo-

se and may be used for a forecast of bit wear, when the

equivalent quartz content is determined by a thin sec-

tion modal analysis.

0

500

1000

1500

2000

2500

0 20 40 60 80 100

limestone, marl, conglomerates, phyllites, marbles

standard deviation

y=3131-624ln x y =144m/bit n=22 R =95%(n-1)2

bitlife-span[m/bit]

equivalent quartz content [%]

low

moderate

high

very high

extremely h.

Bit Wear

very low

Fig. 20 Bit life-span of limestone, marl, conglomerates, together

with phyllites and marbles and corresponding equivalent quartz

content

Porosity and binder defectsFor sandstones and decomposed rock other relation-

ships must be discussed. The expected connection is

also detected when plotting the porosity of sandstones

instead of the equivalent quartz content into the dia-

gram (Fig. 21). Porosity is measured here as a function

of dry density of rock material and ranges from a com-pact (dense) to a totally decomposed silicic binder-free

fabric.

There seems to be a correlation between the porosi-

ty of the rock and technical parameters, such as bit

wear (Fig. 21), drilling rates (Fig. 22) and - naturally -

mechanical rock properties such as unconfined com-

pressive 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

Schnrain Tunnel near Wrzburg, where mainly rock

of the middle und upper Bunter sandstone has been en-countered and in the Achberg Tunnel nearby Unken in

the Werfen sandstone formation.

increasing

low

moderate

high

very high

extremely h.

Bit Wear2 2,1 2,2 2,3 2,4 2,5 2,6

dry density [g/cm ]3

0

500

1000

1500

2000

bitlife-span[m/bit]

25 20 15 10 5 0

porosity [%]

hydrothermallydecomposed

compact

porosity

defect binder

y =136m/bity=174+60?x n=8 R =90%(n-1)2

Fig. 21 Correlation of bit life-span and porosity (dry density) in

sandstones

0

1

2

3

4

5

6

drillingrate[m/min]

25 20 15 10 5 0

porosity [%]

very high

high

moderate

low

very low

Drilling velocity

COP 1440 - 20 kW

7-button bits2 2,1 2,2 2,3 2,4 2,5 2,6

clay-silt-stone


y =0.12m/bity=1.83+0.12x n=8 R =98%(n-1)2

Fig. 22 Correlation of drilling rates and and porosity (dry density)

in sandstones


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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. 24 Hard Bunter sandstone with a less dense fabric. Fracturing

is dominated by intergranular (grain-to-grain) failure. Larger hexa-

hedric quartz crystals growing on grains are developing out of

small granules of silicic binder (picture length approx. 1 mm)

Fig. 25 Hydrothermally decomposed Bunter sandstone, characteri-

sed by a porous fabric with a clayey binder in replacement of the

original, silicic cement (picture length approx. 5 mm)

Fig. 26 Small hexahedric granules of silicic cement growing on

quartz grains (picture length approx. 0.1 mm)

Fig. 27 Clayey binder of the decomposed Bunter sandstone sho-

wing kaolinite crystals growing in the twinning lamellae of a plagi-

oclase crystal (picture length approx. 0.1 mm)

Fig. 28 In the grain gaps, small calcite rhombohedrons are growing

as secondary binder (picture length approx. 0.14 mm)


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0

20

40

60

80

100

120

unconfinedcompressivestrength[MPa]

25 20 15 10 5 0

porosity [%]

y =8,1MPay=168-53 ln x n=8 R =95% (n-1)2

2 2,1 2,2 2,3 2,4 2,5 2,6

clay-silt-stone

very high

high

moderate

low

compressivestrength

very low

after ISRM


Fig. 29 Correlation of unconfined compressive strength and poro-

sity (dry density) in sandstones

0

50

100

150

200

250

destructionwork[kJ/m

]3

25 20 15 10 5 0

porosity [%]

2 2,1 2,2 2,3 2,4 2,5 2,6


clay-silt-stone

y =30y=327-103 ln x n=8 R =83% (n-1) kJ/m3 2

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 micro-

scope photography. In Fig. 23 a very dense and com-

pact fabric of a hard, quartzitic Bunter sandstone is

shown where fracture is characterized by intragranular

failure.In Fig. 24, a hard Bunter sandstone with a hig-

her 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 failu-

re). The small granules of silicic cement are also he-

xahedric, as can bee seen by increased enlargement in

Fig. 26. The hydrothermally decomposed Bunter sand-

stone 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 Schnberg Tunnel. The

contact of the grains is not solid anymore but only we-

akly cemented and the surface of the grains looks

"dirty".

In Fig. 27 the clayey binder of the decomposedBunter sandstone is visible, showing kaolinite crystals

growing in the twinning lamellae of a plagioclase crys-

tal. 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 si-

licic binder has been removed from the sandstone to-

gether with the red colour, leaving behind some clayey

material and calcitic cement.

ConclusionAfter all these observations, it is clear, that neither la-

boratory 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

destruction workcompressive strengthYoung's modulustensile strengthratio of compressive /

tensile strengthrock density / porosity" influence of anisotropy

or other factors

Investigation Program

preliminary site investigations engineering geological mapping

rock & soil description and classificationquantitative description of discontinuities

on basis of IAEG and ISRM standardization

mechanical rock properties

mineral composition

micro fabric

sampling out of drilling cores

if possible, out of an investigation tunnel


degree of interlocking

anisotropy

spacing of discontinuitiesstatus of weathering

hydrothermal decomposition

petrographic description

Fig. 31 Proposal of an investigation program for preliminary site investigations


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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 be-

sides 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 geolo-

gical 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 ent-

ire excavation system must be understood before

applying geological expertise to the solution of expec-

ted or developing drillability problems

In Fig. 31 an investigation program for preliminarysite 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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1997 gr86 prediction

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