drillability in hard rock drill and blast tunnelling
TRANSCRIPT
GEOMECHANICS
Felsbau 14 (1996) Nr. 2 1
DRILLABILITY IN HARD ROCK DRILL AND BLASTTUNNELLINGBy Dipl.-Geol. Dr. Kurosch Thuro and o. Univ.-Professor. Dr. phil. Georg Spaun
Usually the main subject in preliminary site investigationsprior to tunnelling projects is the prediction of tunnel stabi-lity. During the last years in conventional drill and blasttunnelling, problems have occured also connected with theaccurate prediction of drillability in hard rock. The drilla-bility is not only decisive for the wear of tools and equip-ment but is - along with the drilling velocity - a standardfactor for the progress of excavation works. The estimationof drillability in predicted rock conditions might bear anextensive risk of costs. Therefore an improoved predictionof drilling velocity and bit wear would be desireable.
The drillability of a rock mass is determined by variousgeological and mechanical parameters. In this report somemajor correlations of specific rock properties as well asgeological factors with measured bit wear and drilling ratesare shown. Apart from conventional mechanical rockproperties (unconfined compressive and tensile strength,Young's modulus) a new property for toughness refering todrillability has been introduced: the specific destructionwork WZ. This new property makes it possible to under-stand better the connection between drilling velocity andthe main mechanical rock character. As well as mechanicalrock properties the influences of geological parameters ondrillabillity have been the main topic of a recently publis-hed dissertation (THURO 1995, 1996).
Parameters of drillability
Drillability is a term used in construction to describe the in-fluence of a number of parameters on the drilling rate(drilling velocity) and the tool wear of the drilling rig. Theinteraction of the main factors is illustrated in Fig. 1.
excavation system & logistics
Working Process
geological parameters
Rock & Rock Mass
rock mass conditions
mechanical rock properties
machine parameters
Drilling Rig
percussive drill hammerpower transfer
drilling bit
operation & maintenanceof the tunnelling rig
Drillability
drilling velocity
drilling bit wear
wear of drilling tools
tunnelling performance
influence on choice
Fig. 1: Illustration of the term „drillability“ and the main influencingparameters.
First of all, the geological parameters generate the specificcharacteristics of rock material and rock mass (Fig. 2). The-se characteristics may be at least partly put into figures withthe help of mechanical rock properties. But rock mass con-
ditions also depend on the geological history, containingweathering, hydrothermal decomposition and the structureof discontinuities. Together they build the basic parametersfor drillability.
mineral
rock
rock mass
mineral compositionmicro fabric
elastic/plastic behaviourmechanical rock properties
rock mass conditionsdiscontinuities
equivalent quartz contentporosity
destruction workcompressive strengthYoung's modulustensile strength
rock density
spacing of discontinuitiesweatheringhydrothermal decomposition
anisotropy
ratio of of /σ σu t
Fig. 2: Geological parameters: General view of the characteristicsof mineral, rock and rock mass.
According to rock conditions the corresponding drilling rigwill be choosen. The machine parameters are depending onthe drilling method: In underground excavation the rotarypercussive drilling is standard, providing maximum per-formance under most circumstances. Parameters are thetechnical specifications of the drill hammer, flushing sy-stem and the design of the drilling bit. Typical tunnellingrigs consist of a diesel-hydraulic tramming carrier, carryingup to three booms with hydraulic drifter feeds and rockdrills. For example the COP 1238 (15 kW impact power)and the COP 1440 (20 kW impact power, both made byAtlas Copco) are the most popular hydraulic rock drills inuse on the marked today. Fig. 3 shows typical button bitsused in underground excavation in rotary percussive drillrigs.
Fig. 3: Typical button drill bits of various manufacturers with 6 - 9buttons and different flushing systems mainly used in hard rock.
The drilling bit is the part of the rig which carries out thecrushing work. The bit consists of a carrier holding theactual drilling tools: buttons of hard metal (wolframcarbidewith a cobalt binder, MOHS´ hardness 9½). Possible sorts of
Both authors are members of Department of General, Applied andEngineering Geology, Technical University of Munich.
THURO & SPAUN: Drillability in hard rock drill and blast tunnelling
Felsbau 14 (1996) Nr. 2 2
button types and their main characteristics are shown inFig. 4.The shape of the button and the design of the bit (geometryand arrangement of buttons, flush holes and draining chan-nels) have a severe influence on bit wear and drilling per-formance. For example, using ballistic 9-button bits, a ma-ximum penetration performance has been obtained inquartzphyllite of the Innsbruck area. In Fig. 5 drilling ratesrelative to the average of the quickest bit type are plottedcomparing 6-, 7-, 8- and 9-button bits.
(semi-)ballistic
spherical
conical(ballistic)
m "non aggressive" shapem minimum drilling ratesm low bit wearm excavation mainly
by impact
m "aggressive" shapem moderate drilling ratesm moderate bit wearm excavation mainly
by shearing / cutting
m "very aggressive" shapem maximum drilling ratesm high bit wearm excavation mainly
by shearing / cutting
Button Types Characteristics
Fig. 4: Button types of drilling bits used for ro-tary percussive drilling and their main characte-ristics.
The third main factor influencing drilla-bility is the working process itself. Firstof all smooth operation and permanentmaintenance of the tunnelling rig is con-tributing to successful drilling perfor-mance. Secondly, a high penetration rateat the tunnel face is not automaticallyleading to a high performance of the hea-ding as will be demonstrated in the fol-lowing case study of the Inntal tunnel. Soit is a matter of understanding the entireexcavation system before applying ex-pertise to the investigation of drillability.
Drilling performance at the Inn-taltunnel/Innsbruck
One of the most striking tunnelling pro-jects in Austria was the 12.7 km longInntaltunnel nearby Innsbruck. Duringrunning excavation works of the Inntal-tunnel, poor drilling and blasting conditi-ons have been recorded over long distan-ces. Drillability of the rock mass has be-en determined by foliation of the Inns-brucker Quarzphyllit and by its geo-technical character.
Often it is a matter of time dependent expenses ratherthan the pure cost of materials that increase the constructi-on costs of a tunnel. So first of all it is of high interest, howdrilling velocity may influence heading performance. Ta-king the Inntaltunnel as an example in Fig. 6 two excavati-on classes have been compared to show how net drillingtime at the tunnel face raises the whole time for drilling oneentire round. In the crown heading a 3-armed Atlas CopcoRocket Boomer H 185 was used with COP 1440 hydraulicrock drills mounted. From excavation class III to class IVbthe drilling time decreases nearly at the same ratio as thenet drilling time. Therefore the share of drilling is reduced
70
75
80
85
90
95
100
105
drill
ing
rate
[%]
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 bits
quartzphyllite
Fig. 5: Drilling rates in quartzphyllite depending on type of buttonand drilling bit. 9 x 45 b = 9 button type, ∅ 45 mm, b - ballistic(s - spherical).
drilling
chargingtransport
supportmiscellaneous 3%
31%27%
15%24%
Excavation Class III
drilling
chargingtransport
supportmiscellaneous
37% 10%
18%
14%21%
Excavation Class IVb
IV
2,3 m/min 3,0 m/min
2,7 m 2,2 m
105 min 68 min
59 min 53 min
6,5 h 6,3 h
3,7 rounds/day 3,8 rounds/day
10,0 m/day 8,4 m/day
48 sec78 sec
3,0 m 2,4 m
IIIComparison of Excavation Class III & IVb
-40 -20 0 20 40
difference in %
drilling velocity
drilling depth
excavation depth
net drilling time
drilling time (round)
charging
round length
rounds per day
heading performance
difference in percentbased on
excavation class IVb
Fig. 6: Excavation class III and IVb in the Inntaltunnel. Effects of decreasing net drilling timeon the entire drilling of one round.
GEOMECHANICS
Felsbau 14 (1996) Nr. 2 3
approximately by one third (27% to 18%, Fig. 6). The ex-tension of drilling time had no real influence on the lengthof one round in class III, because support works were lessextensive than in class IVb.
Drilling rates and mechanical rockproperties
Nevertheless net drilling times are a result of changed dril-ling velocities in different rock types. But what are thechanged drilling velocities based on? To get information onthis point, drilling rates have been measured periodicaly du-ring running excavation works and cores have been takenout of the rock mass to get mechanical rock properties ofrepresentative sections (SPAUN & THURO 1994). In this waydrilling progress could be connected with some main rockparameters. Two of the most frequently used rock proper-ties are the unconfined compressive strength and the tensilestrength. But the correlation between drilling rate and com-pressive strength as well as tensile strength of the testedrock types was rather poor (Fig. 7, Fig. 8). These rockproperties were not likely to describe the toughness of therock material.
To get further on this point it is necessary to get a betterunderstanding of the crushing mechanism at the bottom of aborehole.
COP 1440 - 20 kW
y=a+b·ln x y =0,76 m/min n=20 R =20%σ(n-1)2
very high
high
medium
low
very low
drilling velocity
0,0
1,0
2,0
3,0
4,0
5,0
drill
ing
rate
[m/m
in]
0 20 40 60 80 100 120
unconfined compressive strength UCS [MPa]
Fig. 7: Drilling rate correlated with unconfined compressivestrength. The quality of the correlation is very poor.
COP 1440 - 20 kW
y=a+b·ln x y =0,58 m/min n=20 R =54%σ(n-1)2
very high
high
medium
low
very low
drilling velocity
0
1
2
3
4
5
drill
ing
rate
[m/m
in]
0 2 4 6 8 10 12indirect tensile strength TS [MPa]
Fig. 8: Drilling rate correlated with indirect tensile strength. Thequality of the correlation is rather poor.
Crushing process underneath a drilling bit
Coming from studies by high-speed photography and ana-lysis of thin sections of rock below the area of disc cuttertools of TBM´s, three main destruction mechanisms couldbe detected (OZDEMIR et al. 1977, WANG et al. 1978,BLINDHEIM 1979). Those results can be generalised andtransfered on the crushing process below the buttons of adrilling bit (Fig. 9).
10 mm
drilling bit
3 detached fragments1 crushed rock powder 2 radial cracks
button button1
22
3
2
3
rotationrotation
pen
etra
tio
n
Fig. 9: Crushing process in rotary percussive drilling. Destructionmechanism under the bit buttons.
Around the contact of the button a new state of stress is in-duced in the rock, where four important destruction mecha-nisms can be distinguished:
1) Under the bit button a crushed zone of fine rock powderis formed (impact).
2) Starting from the crushed powder zone, radial cracks aredeveloped (induced tensile stress).
3) When stress in the rock is high enough (respectively ifenough cracks exist ±parallel to the bottom of the bore-hole), larger fragments of the rock can be sheared offbetween the button grooves (shear stress).
4) In addition to the mechanisms above stress is inducedperiodical (dynamic process).
Looking at the drilling mechanism it is obvious, that besi-des compressive and tensile strength (percussive process)and shear strength (bit rotation) the elastic characteristics ofrock material will be of crucial importance. To be precise,the bit is always drilling through pre-cracked rock (seeMÜLLER-SALZBURG 1963: 104) and we have to devote agreat deal of our time with the post-failure behaviour ofrock to get closer to the crushing mechanism below thedrilling bit.
Monitoring destruction work
With other words, a new property is needed, describingboth brittleness/toughness of rock and the quantity of ener-gy necessary to build new surfaces (cracks) in rock. To getthis newly defined rock property, the deformation processof a rock sample under unconfined compression is studiedin Fig. 10.
THURO & SPAUN: Drillability in hard rock drill and blast tunnelling
Felsbau 14 (1996) Nr. 2 4
Especially in the rock types of the Innsbrucker Quarzphyllit(quartzphyllite, sericite-chlorite-phyllite, carbonate-(quartz)-phyllite) samples showed a distinctive post-failurebehaviour during unconfined compression (class I beha-vior). In contrast to the phyllites the marbles indicated a ty-pical brittle behaviour with no post-failure section at all(class II behavior). Whereas in phyllites drilling conditionswere poor, submitting low drilling rates, the marbles werequite easy to drill, providing high drilling performance. It isobvious, that the area under the envelope of the stress-straindiagram of the phyllite sample is much larger than the areaof the marble sample (Fig. 10) - so why not connect thoseresults? From the physical sight, the envelope curve is not-hing else but the energy (or work), required for destructionof the rock sample.
The newly defined rock property has been determinedas the specific destruction work (in short: destruction work)WZ [kJ/m³] and gives the possibility to compare rock mate-rials refering to brittleness/toughness using drills or cutters.Whereas the YOUNG´s modulus submits the gradient(derivation) of the linear section, the destruction work isestimated out of the area under the stress-strain-envelope(integral, Fig. 11). As a product of both - stress and strain -destruction work represents the work of shape altering in-cludung the post failure section.
post-failure section
DA B
Youngs modulus
C
pre-failure section
destruction work
δε
W = dZ σ ε
δσ
E=δσ/δε
σu
stre
ssσ
strain ε
Fig. 11: Stess-strain curve with a distinctive post-failure behaviour ofthe sample. Determination of Young´s modulus E and specific de-struction work WZ.
The destruction work has proved as a highly significant pa-rameter for the evaluation of net drilling rates in drill andblast tunnelling and therefore is the most important mecha-nical rock property for the investigation of drillability.
Something similar to the destruction work has beenestablished several times in literature: The Coefficient ofRock Strength CRS (PAONE, MADSON & BRUCE 1969),the Rock Impact Hardness Number RIHN (RABIA &BROOK 1980, 1981), and the Swedish Brittleness Test(SELMER-OLSEN & BLINDHEIM 1970), all based on theProtodyakanow impact test (PROTODYAKANOW 1962),perform a method, where a rock sample is crushed by aweigth (represents a definite destruction work) and theobtained grain size is a measure for brittleness. The onlydisadvantage is, that this sort of rock property is not areal physical quantity but an index value lacking a phy-sical unit.
Very similar to the destruction work is the specificenergy es introduced by HUGES (1972). It may bedescribed as the specific work in the stress-strain curveuntil the failure of the sample occures during uncon-fined compression. In this property, however, is missing
the post-failure section of the curve and thus the main pointof the newly defined quantity.
Returning to our example of the Inntaltunnel, the de-struction work WZ proved as a highly significent parameterfor correlation with the drilling performance. Fig. 12 showsthe mean values of drilling rates recorded in 20 differenttunnel sections of the Inntal tunnel compared with the de-struction work of the rock material belonging to them. Thediagramm indicates the close correlation between drillingvelocity and destruction work.
COP 1440 - 20 kW
y=a+b·ln x y =0,17 m/min n=20 R =96%σ(n-1)2
marbles
sericite-chlorite-phyllite
quartz phyllites
sericite-chlorite-gneissescarbonite phyllites
very high
high
medium
low
very low
drilling velocity
0
1
2
3
4
5
drill
ing
rate
[m/m
in]
0 50 100 150 200 250destruction work W [kJ/m ]z
3
Fig. 12: Drilling rate correlated with destruction work. The quality ofthe correlation is very good.
Applying expertise to other projects
But not only in foliated rock mass the specific destructionwork has turned out as a suitable rock property for monito-ring drilling rates. As can be seen in Fig. 13 a large varietyof rocks were tested, showing a high significant correlationgraph. The rock material plotted in this diagram includesclay-siltstones, sand- and limestones, conglomerates, marls,marbles, schists and different cristalline rocks derived fromseven tunnel projects in Germany and Austria.
"brittle"marble
stre
ssσ
σ
strain ε
post-failure-sectionpre-failure-section
destruction work
W = dz σ ε
"tough" phyllite
failure
failure point
point
Fig. 10: Stess-strain curve of a brittle marble without and a typical phyllite witha distinctive post-failure behaviour under unconfined compression.
GEOMECHANICS
Felsbau 14 (1996) Nr. 2 5
COP 1440 - 20 kW
0
1
2
3
4
5
0 100 200 300 400 500
destruction work [kJ/m ]3
standard deviationdrill
ing
rate
[m/m
in]
y=a+b·ln x y =0,33 m/min n=64 R =85%σ(n-1)2
very high
high
medium
low
very low
drilling velocity
Fig. 13: Drilling rate plotted against destruction work. The quality ofthe correlation is very good.
As can be seen in Fig. 14, drilling rates are especiallydependent on the impact power of the rock drill. There is adistinct improvement of drilling performance from the COP1238 (15 kW) to the COP 1440 (20 kW) resulting in up to40% higher penetration rates.
COP 1440 - 20 kW
0
1
2
3
4
5
0 100 200 300 400 500
destruction work [kJ/m ]3
drill
ing
rate
[m/m
in]
COP 1238 ME - 15 kW
very high
high
medium
low
very low
drilling velocity
Fig. 14: Drilling rate plotted against destruction work for both rockdrills. There is a distinct improvement of drilling performance fromthe 15 kW to the 20 kW type.
Influence of anisotropy
Of course, rock properties and drilling rates are also highlydependent on the orientation of weakness planes related tothe direction of testing or drilling. In the following figures(Fig. 15 - Fig. 18) the correlations between rock propertiesand the orientation of foliation for a quartzphyllite with flatand smooth discontinuities (continuous line) and a quartz-phyllite with uneven, undulating sericite-chlorite-partings(broken line) are shown.
The striking point is, that whereas the highest values ofdestruction work are gained parallel to foliation, the uncon-fined compressive strength is higher perpenticular to thediscontinuities - and vice versa. In both cases, the minimumis located between 45° and 60°. This is set in advance bythe geometry of the sample (length to diameter ratio 2 : 1).
0
20
40
60
80
100
0
15
30
45
60
7590
dest
ruct
ion
wor
k[%
]
dip angle of
foliation
Fig. 15: Correlation between destruction work and the orientation offoliation for a quartzphyllite with flat and smooth (continuous line) re-spectively uneven, undulating discontinuities (broken line).
100
0
15
30
45
60
7590
0
20
40
60
80
unco
nfin
edco
mpr
essi
vest
reng
th[%
]
dip angle of
foliation
Fig. 16: Correlation between unconfined compressive strength andthe 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 areobtained parallel to foliation, presuming that in this case re-al stresses turn up perpendicular to both foliation and forcedirection (Fig. 17). By an angle of 45°, stresses are pureshear stresses and the test turns out to be a shear test alonga forced gap. It is certainly for this reason that the diagramof drilling rates looks quite the same (Fig. 18) but velocityis high, where tensile strength is low.
In Fig. 19 a mathematical model is given to describe thebehaviour of both drilling and shear (tensile) strength. Assupposed, strength and drilling rate are connected with thegeometry of induced stress.
THURO & SPAUN: Drillability in hard rock drill and blast tunnelling
Felsbau 14 (1996) Nr. 2 6
0
15
30
45
60
7590
0
20
40
60
80
100
tens
ilest
reng
th[%
]
dip angle of
foliation
Fig. 17: Correlation between tensile strength and the orientation offoliation for a quartzphyllite with flat and smooth (continuous line) re-spectively uneven, undulating discontinuities (broken line).
100
0
15
30
45
60
7590
0
20
40
60
80
drill
ing
rate
[%]
dip angle of
foliation
Fig. 18: Correlation between drilling rate and the orientation of folia-tion for a quartzphyllite with flat and smooth (continuous line) re-spectively uneven, undulating discontinuities (broken line).
25
50
75
100
drill
ing
rate
[%]
drilling rate
tensile strength25
50
75
100
indi
rect
tens
ilest
reng
th[%
]
dip angle of foliation
90 75 60 45 30 15 0
high tensile stress low tensile stress
y = a + b·cos xgraph equation
Fig. 19: Drilling rate and tensile strength plotted against the orienta-tion of foliation.
To understand the connection between stress field and dril-ling rates one has to study the destruction process down theborehole once again (Fig. 20 and Fig. 21).
compressive/tensile stress
shear stress
UCS TS UCS TS
testingarrangements
shear stress
Fig. 20: Drilling process according to different orientations of dis-continuities (foliation).
When the direction of drilling is right-angled to the orien-tation of foliation, rock material is compressed right-angledbut sheared parallel to it (Fig. 21/1). Although cracks willdevelop radial to compression, the cracks parallel to thebottom of the borehole will be used for chipping. Usuallyin this case the highest drilling velocities are obtained be-cause of the favourable schist orientation. Drilling is con-trolled by the shear strength of the foliated rock material.The minimum destruction work causes large sized chipsand a maximum drilling performance.
If the drilling axis is oriented parallel to foliation, com-pression also is parallel but shear stress is right-angled (Fig.21/3). It should be clear, that less cracks will develop forreasons of higher strength right-angled to the weakness pla-nes. Drilling is controlled by the tensile strength parallel tothe foliation producing small sized fragments and minimumdrilling performance.
Generally, drilling is controlled by the dip angle of fo-liation (Fig. 21/2), submitting medium sized fragments du-ring the crushing process. Drilling performance is - bygeometrical reason - mainly a cosine function of the dipangle.
Anyway, it is for sure, that in the parallel case, rockproperties are the highest and drilling rates are low. In ad-dition blasting conditions are often related with drilling. Soif the tunnel axis is parallel to the main foliation, drillingand blasting conditions suppose to be very poor. In the caseof the Inntaltunnel the consequences were a lower headingperformance and higher drilling expenses than expected.
In comparison with the crushing process under a TBMdisk cutter (WANNER 1975), the process underneath a but-ton drill bit is completely different in foliated rock. In con-trast to rotary percussive drilling, TBM penetration ratesare a maximum when foliation is ± diagonal to the tunnelaxis.
GEOMECHANICS
Felsbau 14 (1996) Nr. 2 7
As a further result of anisotropy, problems may occurewhen 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 offoliation, if obtuse-angled, into the normal direction of fo-liation. In any case, drill tracks may be seen as curvaturesand produce distinct borehole deviation.
Structure and texture of rock material and their influ-ence on rock properties are discussed in detail byHOWARTH & ROWLANDS (1987). Though the effect on de-struction work is much greater than supposed. Mechanicalrock properties of the Innsbrucker Quarzphyllit are severelydominated by the elastic-plastic behaviour of rock material.For example high values of destruction work have beenmeasured in rock material showing a tight and laminatedmicro fabric and high grade of interlocking between quartz-feldspar-mouldings and mica layers. In addition, mica con-sisted mainly of biotite, giving evidence of a higher gradeof metamorphism within the low grade (greenschist) zone.
Low values of destruction work have beenobtained in rock where quartz-feldspar-mouldings often were cut through or dislo-cated by internal folding. The result is aspongy fabric with the possibility to absorbhigh distortions but not as tough as thestructure described above.
Of course there are a lot of other geolo-gical parameters strongly influencing dril-ling performance - not possible to bediscussed in this paper - such as spacing ofdiscontinuities, hydrothermal decompositi-on, status of weathering and porosity of themicro fabric.
Drilling bit wear
In the second place, the wear of drillingequipment may be a severe factor of costs intunnelling. As a leading parameter, the wearof drilling bits has been examined in diffe-rent rock types. Other tools such as drifterrods, couplings and shank adapters have alife-span in average ten times the one ofbutton bits.
An important hint for surveying abrasi-vity of rock is the analysis of worn-out dril-ling bits. Bit wear occures in six basicforms, generally combined according torock mass conditions:
1) Button wear. Wear of the hard metalbuttons according to high abrasivity ofthe rock, such as granite, gneiss or am-phibolite.
2) Steel wear: Wear of the steel calibre indiameter as a result of chip grinding inweak and moderate strong rock withhigh abrasivity, such as sandstones,schists, weathered and decomposedrock.
3) Button damage: Breaking of drill but-tons because of high shear stress. If the
drifter rod becomes stuck or fixed according to jointing,hard components or steel support, severe damage of thebuttons may result.
4) Total button removal: By the same reasons of buttondamage buttons may break out as a whole.
5) Total wear out: When parts of hard metal or entirebuttons are chipped off, frequently those rotating pieces- too big to be removed by flushing - rip out more but-tons leading to a total wear-out of the button bit.
6) Steel shaft damage: Damage of the steel shaft belowthe buttons by reasons of steel quality or severe force.
In consequence it is possible to make a statistical analysisof worn out tools to gain an impresson of the grinding ef-fect of rock fragmentation.
Of course, the range of tool wear can be measured byrecording the quantity of worn-out button bits refering totheir total drilling length - the so called "life-span" of thebit. Life-span is reported in boremeters per bit [m/bit].
2 Drilling is controlledby the dip angle of foliation¦medium sized fragments¦ drilling performance is a
cos-function of the dip angle
3 Drilling is controlledby tensile strength¦maximum destruction work¦ small sized fragments¦minimum drilling performance
1 Drilling is controlledby shear strength¦minimum destruction work¦ large sized fragments¦maximum drilling performance
1 crushed rock powder2 cracks, rigth-angled to foliation
1
2
1
button button
drilling bit rotationrotation
pen
etra
tion
buttonbutton
1
2
1
drilling bit rotationrotation
pen
etra
tion
1
2
1
button button
drilling bit rotationrotation
pen
etra
tio
n
0 10 mm5
Fig. 21: Physical destruction process in foliated rock. Crushing mechanism below the bitbuttons depending on the dip angle.
THURO & SPAUN: Drillability in hard rock drill and blast tunnelling
Felsbau 14 (1996) Nr. 2 8
Drilling bit wear in the "InnsbruckerQuarzphyllit"
Fig. 22 gives an impression of the wide variety of the rocktypes contained in the "Innsbrucker Quarzphyllit". Com-monly quartzphyllites, sericite-chlorite-phyllites and -gneisses are put together under the term "quartzphyllite".
quartzphyllitesericite-chlorite-phyllite
carbonate-quartzphyllite
quartz marble marblecarbonate-phyllite
greenschist
sericite-chlorite-gneiss
25%27%
15%
10% 4%4%
10%
6%
"Innsbrucker Quarzphyllit"
Fig. 22: Rock types contained in the "Innsbrucker Quarzphyllit" andthe composition of samples taken from the Inntal tunnel.
In Fig. 23 an example for a statistical analysis is givenshowing the wear characteristic of drill bits used in rockbelonging to the "Innsbrucker Quarzphyllit". In fact, thiswear statistic is something like a fingerprint of the ex-amined rock. In quartzphyllites and associated rock typessteel wear is dominating compared to the marbles, wheretotal button removal comes first.
(1) button wear
(2) steelwear
(3) button damage
(4) total buttonremoval
(5) total wear-out
(6) steel shaftdamage (2%)
6%
24% 46%
12%
10%
quartzphyllites, serizite-chlorite-phyllites & gneisses
(1) button wear
(2) steelwear
(3) buttondamage
(4) totalbuttonremoval
(5) totalwear-out
(6) steel shaftdamage (1%)
12%
36%
15%
16%
20%
carbonate pyllites, marbles& quartz marbles
Fig. 23: Wear characteristic of drill bits used in rock belonging to the"Innsbrucker Quarzphyllit".
It is clear, that tool wear is a result of the mineral con-tent harder than steel, especially quartz (MOHS´ hardness =7). To include all minerals, the equivalent quartz content isdetermined - meaning the entire mineral content refering tothe abrasiveness/hardness (after ROSIWAL 1896, 1916) ofquartz (ROSIWAL abrasiveness = 100). Therefore each mi-neral share is multiplied with its relative abrasive-ness/hardness to quartz (quartz = 100%). An appropriatecorrelation between MOHS hardness and ROSIWAL abrasi-veness is given in Fig. 24. Taking the average mineral con-tent of quartzphyllites (Fig. 25), the equivalent quartz con-tent is approximately 46% and thus is slightly higher thanthe pure quartz content of 42%.
graph equation y=a+b·ln x
standard deviation = ½
n=24 R =95%2
0
1
2
3
4
5
6
7
8
9
Moh
sha
rdne
ss
1 10 100 1000
Rosiwal abrasiveness
quartz
Fig. 24: Correlation between MOHS hardness and ROSIWAL abrasi-veness.
equivalent quartz content equ=45,5%
quartz
feldspar
sericite
biotite 2%carbonate 3%
chlorite
42%
24%
22%
7%
"quartzphyllite"
Fig. 25: Average mineral content of quartzphyllites, sericite-chlorite-phyllites and -gneisses ("quartzphyllite") and derived equivalentquartz content.
In Fig. 26 the bit life-spans of rock types contained in the"Innsbrucker Quarzphyllit" are correlated with their equi-valent quartz contents. It is obvious that bit wear raiseswith increasing equivalent quartz content. The expectedrelation is also detected when plotting the properties ofother rock material into the diagramm (Fig. 27). For sand-stones and decomposed rock other correlations thandiscussed here have been found (THURO 1995, 1996).
GEOMECHANICS
Felsbau 14 (1996) Nr. 2 9
Fig. 26: Bit life-span of "Innsbrucker Quarzphyllit" rock types andcorresponding equivalent quartz content.
Classification of drillability
Finally a classification of drillability is given,contributing up-to-now experience. First of all, adrillability classification should rely on valueseasyly obtained on the site. Secondly, the para-meters should be expressive and provide a goodresolution of drilling rate and wear characteristic.The system proposed here is based on net drillingvelocity, measured at the tunnel face and drillingbit wear recorded as the bit life-span.
To get an impression how wide values of bitwear and drilling rates may vary, mean values ofdifferent rock types or homogeneous areas deri-ved from seven tunnel projects have been takenfor the diagrams of Fig. 28. The investigationshave been carried out using 15 kW and 20 kWborehammers (Atlas Copco COP 1238 ME andCOP 1440). The matrix is based on the experi-ence, that high drilling rates (3 - 4 m/min) andlow bit wear (1500 - 2000 m/bit) should bedescribed as "fair" drillability.
Conclusion
An investigation program is submitted, whichshould help to improve the estimation of rockdrillability in planning future tunnel projects.
First of all, with the discovered correlationcharts for mechanical and petrographic rockproperties, it should be possible to predict dril-ling rates and bit wear for the examined rock ty-pes in a satisfactory manor.
Besides rock properties - the main thing inpreliminary site investigation is - first of all -simple and basic geological mapping. This so-unds rather simple. But it is extremely necessaryto keep in mind all the parameters possibly influ-encing drilling performance. Therefore it is veryimportant to prepare all rock and soil descripti-ons in a suitable way, engineers are able to un-derstand.
0
500
1000
1500
2000
2500
0 20 40 60 80 100
limestone, marl, conglomerates, phyllites, marbles
standard deviation
y=a+b·ln x y =144 m/bit n=22 R =95%σ(n-1)2
low
moderate
high
very high
extreme high
Bit Wear
very low
bitl
ife-s
pan
[m/b
it]
equivalent quartz content [%]
Fig. 27: Bit life-span of limestone, marl, conglomerates, together withphyllites and marbles and corresponding equivalent quartz content.Not suitable for sandstones and decomposed or weathered rock.
Fig. 28: Classification diagram for two rock drills (COP 1238 - 15 kW and COP 1440 -20kW) enclosing drilling rate and bit wear.
THURO & SPAUN: Drillability in hard rock drill and blast tunnelling
Felsbau 14 (1996) Nr. 2 10
For that reason we would like to finish this paper with thewords of Priscilla P. NELSON (1993: 261):
„Whatever the reasons, it is clear, that neither geologyalone, laboratory and field testing alone, experience alonenor equipment design and operation expertise alone canget an engineer to the point where underground excavationis a clearly defined engineering process. Integration of allthese knowledge bases is required to raise the level of en-gineering contribution to underground construction, andthe entire excavation system must be understood beforeapplying engineering expertise to the solution of expectedor developing problems.“
ReferencesBLINDHEIM, O.T. (1979): Drillability predictions in hard rock tunnel-ling. - Tunnelling 1979, London, Inst. Min. Metall., 284-289.
HOWARTH, D.F. & ROWLANDS, J.C. (1987): Quantitative assesse-ment of rock texture and correlation with drillability and strengthproperties. - Rock Mech. & Rock Eng., 20., 57-85.
HUGHES, H.M.(1972): Some aspects of rock machining. - Int. Journalof 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 torock properties. - in: HUDSON, J. (ed.-in-chief): Comprehensive rockengineering. Principles, practice & projects. Vol. 4. Excavation, Sup-port and Monitoring. - 849 S., Oxford, New York, etc. (Pergamon),261-291.
OZDEMIR, L., MILLER, R. & WANG, F.-D. (1977): Mechanical tunnelboring, 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 drillabilityof rocks. - Proc. 5th Symp. on Rock Mech. Univ. Minnesota, May,103-118.
RABIA, H. & BROOK, N. (1980): An empirical equation for drill perfor-mance prediction. - 21st Symp. on Rock Mech. Univ. Missouri-Rolla,May, 103-111.
RABIA, H. & BROOK, N. (1981): The effects of apparatus size andsurface area of charge on the impact strength of rock. - Int. J. RockMech. Min. Sci. & Geomech. Abstr., 18., 211-219.
ROSIWAL, A. (1896): Neue Untersuchungsergebnisse über die Härtevon Mineralien und Gesteinen. - Verhandlg. d. k.k. geol. R.-A. Wien,475-491.
ROSIWAL, A. (1916): Neuere Ergebnisse der Härtebestimmung vonMineralien und Gesteinen. Ein absolutes Maß für die Härte spröderKörper. - Verhandlg. d. k.k. geol. R.-A. Wien, 117-147.
SELMER-OLSEN, R. & BLINDHEIM, O.T. (1970): On the drillability ofrock by percussive drilling. - Proc. 2nd Cong. of the Int. Soc. forRock Mech., Belgrade, 65-70.
SPAUN, G. & THURO, K. (1994): Untersuchungen zur Bohrbarkeit undZähigkeit des Innsbrucker Quarzphyllits. - Felsbau, 12., 2, 111-122.
THURO, K. (1995): Geologisch-felsmechanische Untersuchungen zurBohrbarkeit von Festgesteinen beim konventionellen Bohr- undSprengvortrieb anhand ausgewählter Tunnelprojekte. - 156 S. Dis-sertation TU München.
THURO, K. (1996): Bohrbarkeit beim konventionellen Sprengvortrieb.Geologisch-felsmechanische Untersuchungen an sieben ausge-wählten Tunnelprojekten. - Münchner Geologische Hefte, Reihe B:Angewandte Geologie, 1., 1 - 152.
WANG, F.-D., OZDEMIR, L. & SNYDER, L. (1978): Prediction and ex-perimental verification of disk cutter forces in hard rock. - in: Euro-tunnel '78 conference, Basle, Switzerland (Basle: Congress Centre,1978), 1st day, March 1st, 1978, pap. 4, 44 S.
destruction workcompressive strengthYoung's modulustensile strengthratio of of /σ σu trock density / porosityð influence of anisotropy
or other factors
Investigation Program
1. preliminary site investigation engineering geological mappingrock & soil description and classificationquantitative description of discontinuitieson basis of IAEG and ISRM standardization
2. mechanical rock properties
mineral compositionmicro fabric
sampling out of drilling coresif possible, out of an investigation tunnel
equivalent quartz contentdegree of interlocking
anisotropyspacing of discontinuitiesweatheringhydrothermal decomposition
3. petrographic description
Fig. 29: Proposal of an investigation program, which should help to improve the estimation of rock drillability in planning future tunnel pro-jects.
GEOMECHANICS
Felsbau 14 (1996) Nr. 2 11
Abstract
THURO & SPAUN: DRILLABILITY IN HARD ROCK DRILLAND BLAST TUNNELLING
The drillability of a rock mass is determined by variousgeological and mechanical parameters. In this report somemajor correlations of specific rock properties as well asgeological factors with measured bit wear and drilling ve-locity are shown. Apart from conventional mechanical rockproperties (compressive and tensile strength, Young's mo-dulus) a new property for toughness/brittleness referring todrillability has been introduced: the specific destructionwork WZ. This new method makes it possible to understandbetter the connection between drilling velocity and the mainmechanical rock character.
During running excavation works of the Inntaltunnel,poor drilling and blasting conditions have been recordedover long distances. Drillability of the rock mass has beendetermined by foliation of the Innsbrucker Quarzphyllit andby its geotechnical character. In this paper bit wear anddrilling velocity in correlation with main geological andmechanical properties are discussed. Rock properties andand drilling datas of the Innsbrucker Quarzphyllit areshown in correlation of shist foliation. Besides, the im-portance of the criystalline microstructure could be proved.
Authors
Kurosch Thuro, Dipl.-Geol. Dr.rer.natGeorg Spaun, o.Univ.-Prof. Dr.phil.
Lehrstuhl für Allgemeine, Angewandte und Ingenieur-Geologie, Technische Universität MünchenLichtenbergstraße 4, D-85747 Garchinge-mail: [email protected]
Thuro & Spaun: Bohrbarkeit von Festgesteinen beimBohr- und Sprengvortrieb.
Die Bohrbarkeit des Gebirges wird durch unterschiedlichegeologische und felsmechanische Parameter bestimmt. Indiesem Beitrag werden die wesentlichen Abhängigkeitenzwischen den spezifischen Materialeigenschaften von Ge-stein und Gebirge und den meßbaren Parametern Bohrkro-nenverschleiß und Bohrgeschwindigkeit aufgezeigt. Nebenden konventionellen felsmechanischen Kennwerten (Druck-, Zugfestigkeit und Elastizitätsmodul) wurde ein neues Maßfür die Zähigkeit bezüglich der Bohrbarkeit von Gesteineneingeführt: die spezifische Zerstörungsarbeit Wz. Die neueAuswertemethode ermöglicht es, den ursächlichen Zusam-menhang zwischen der Netto-Bohrgeschwindigkeit und denfelsmechanischen Eigenschaften eines Gesteins besser alsbisher nachzuvollziehen. Im Verlauf des bergmännischenTunnelvortriebs erwies sich das Gebirge des Inntaltunnelsüber weite Strecken als schwer bohrbar und ebenso alsschwer sprengbar. Die Bohrbarkeit des Gebirges wurdezum einen durch die Schieferung des Innsbrucker Quarz-phyllits bestimmt, zum anderen durch seine geotechnischenEigenschaften. In diesem Beitrag werden Bohrkronenver-schleiß und Bohrgeschwindigkeit in Abhängigkeit der we-sentlichen geologischen und felsmechanischen Parameterdiskutiert. Es werden felsmechanische Kennwerte des Inns-brucker Quarzphyllits und bohrtechnische Daten in Abhän-gigkeit von der Schieferungsrichtung vorgestellt und dieBedeutung des Mikrogefüges nachgewiesen. Abschließendwird ein Vorschlag für ein Untersuchungsprogramm unter-breitet, welches bei künftigen Vorerkundungen für Tunnel-und Stollenprojekte helfen soll, Gestein und Gebirge imHinblick auf die Bohrbarkeit besser zu erfassen.