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Drillability Assessments in Hard Rock

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Drillability is an important parameter in order to assess the influence that intact rock properties have on performance prediction and cost evaluations in connection with drill-and-blast tunnelling, TBM tunnelling, excavations by roadheaders and hydraulic impact hammers and also rock quarrying. Especially in hard rock conditions, drillability will be of great importance for selection of excavation method and a successful project execution. Unanticipated situations and/or inappropriate assessments can result in considerable delays and great risk of cost overruns. Reliable predictions are therefore required; prediction of net penetration rate and tool wear, time consumption and excavation costs, including risk and assessing risk linked to variation in rock mass boreability, establishing and managing contract price regulation. Several methodologies are available to assess drillability (i.e. rock strength, rock surface hardness, rock brittleness, rock abrasivity or rock petrography). This paper includes a review of the state-of-the-art and discussion of relevant parameters that involves drillability assessments in hard rock conditions.
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3rd Nordic Rock Mechanics Symposium
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DRILLABILITY ASSESSMENTS IN HARD ROCK
Francisco Javier Macias (javier.macias@sintef.no)
SINTEF Building and Infrastructure - Rock and Soil Mechanics Group
Norway
Filip Dahl1, Amund Bruland2, Heiko Käsling3, Kurosch Thuro3
1SINTEF Building and Infrastructure - Rock and Soil Mechanics Group
Norway
2 NTNU Department of Civil and Transport Engineering
Norway
3TUM Chair of Engineering Geology
Germany
ABSTRACT
Drillability is an important parameter in order to assess the influence that intact rock properties have on
performance prediction and cost evaluations in connection with drill-and-blast tunnelling, TBM tunnelling,
excavations by roadheaders and hydraulic impact hammers and also rock quarrying. Especially in hard rock
conditions, drillability will be of great importance for selection of excavation method and a successful project
execution. Unanticipated situations and/or inappropriate assessments can result in considerable delays and
great risk of cost overruns. Reliable predictions are therefore required; prediction of net penetration rate and tool
wear, time consumption and excavation costs, including risk and assessing risk linked to variation in rock mass
boreability, establishing and managing contract price regulation. Several methodologies are available to assess
drillability (i.e. rock strength, rock surface hardness, rock brittleness, rock abrasivity or rock petrography). This
paper includes a review of the state-of-the-art and discussion of relevant parameters that involves drillability
assessments in hard rock conditions.
KEYWORDS
Drillability, Hard rock excavation, Breakability, Abrasivity
INTRODUCTION
Rock properties have a large impact in connection with excavation and tunnelling by use of drill-and-blast, TBMs,
roadheaders, hydraulic impact hammers and also for rock quarrying, especially in hard rock conditions. The term
drillability is commonly used to describe the ability of the rock to be drilled or bored and it will be of great
importance on performance predictions, cost evaluations and selection of excavation method. Unanticipated
situations and/or inappropriate assessments can result in considerable delays and great risk of cost overruns.
Reliable predictions are therefore required for; prediction of net penetration rate and tool wear, time consumption
and excavation costs, including risk and assessing risk linked to variation in rock mass boreability, establishing
and managing contract price regulation. Several methodologies are available to assess drillability (i.e. rock
strength, rock surface hardness, rock brittleness, rock abrasivity or rock petrography). This paper includes a brief
review of the state-of-the-art and discussion of relevant parameters that involves drillability assessments in hard
rock conditions.
1. DRILLABILITY
Drillability can be defined as the ability of the rock to be drilled or bored. Drillability considers the influence that
intact rock properties, breakability and abrasivity, have during drilling and boring in hard rock.
There are several methodologies available to assess the influence of intact rock properties in hard rock
excavation. The main intact rock properties and commonly used test methodology are listed in the following:
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- Strength: Uniaxial Compressive Strength (UCS), Brazilian Tensile Strength (BTS), Point Load Test
(PLT).
- Surface hardness: Sievers’ J miniature drill test (SJ), Vickers hardness (VH).
- Brittleness: brittleness tests (S20), several definitions including: strain, UCS and tensile strengths, stress-
strain relations.
- Abrasivity: Cerchar test (CAI), LCPC test, Abrasion Value Steel test (AVS), abrasive minerals content,
Vickers hardness number of rock (VHNR).
- Rock petrography (rock texture, mineral composition).
The term ‘hard rock’ is not always precisely defined and can be defined by the rock strength. According to the
classification of rock types in terms of Uniaxial Compressive Strength (UCS) presented by the International
Society of Rock Mechanics (ISRM), the term 'hard rock' fall within the categories high, very high and extremely
high strength (UCS > 50 MPa).
2. LABORATORY TEST METHODS TO ASSESS DRILLABILITY
2.1. General
It should be emphasised that the drillability of rock is a complex issue and that it is dependent on different
combinations of various individual intact rock properties. It is due to this not possible to assess the drillability of
rock with certainty by use of a single laboratory test method. A best possible understanding and determination of
the drillability can hence only be achieved based on evaluation of the results from a dedicated set of test
methodologies used to describe both breakability and abrasivity properties of the rock. The information achieved
by the physical testing should ideally also be supported and combined with information on the mineralogical
composition and the petrographic features (grain size, grain bindings, micro cracking, alteration and weathering)
of the rock.
2.2. Laboratory test methods for determination of breakability
The breakability properties of intact rock have a major influence on the drillability of rock. The relatively wide
definition breakability is however including and covering a variety of more specific properties that could be
classified as strength, brittleness and surface hardness properties. There are currently several individual test
methods which are used to determine breakability and some of the most well-known and commonly used are
listed in Table 1.
The Uniaxial compressive test is used to determine the UCS, Poisson's ratio and Young's modulus of the intact
rock. UCS is the primary test method and the most often used to characterize the mechanical behaviour of the
intact rock, strength and deformability. This test uses circular cylinders of rock samples which are compressed
along the longitudinal axes (ISRM, 1979).
The Brazilian Tensile Test is an indirect tensile method to assess the tensile strength of the rock (ISRM, 1978
and ASTM, 2008). The stress failure in the rock sample is a function of the applied load, the sample diameter
and the thickness at the centre of the specimen (Figure 1b) resulting in the Brazilian Tensile Strength (BTS).
The PLT is intended as an index test for the strength classification of rock and it may also be used to predict
other strength parameters as uniaxial tensile and compressive strength (ISRM, 1985). Rock specimens (core,
block or irregular lump) are broken by application of concentrated load through a pair of spherically truncated,
conical platens.
The Brittleness Value (S20) constitutes a measure of the rock brittleness or ability to be crushed by repeated
impacts, and it is determined by use of an impact apparatus. The Brittleness Value (S20) is defined as the
percentage of a pre-sieved fraction that passes through the finer sieve after 20 impacts.
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The punch test, developed by Handewith (1970), has been used by many researchers to evaluate parameters
such as rock drillability, cutting force estimates and TBM penetration rate (Dollinger et al., 1998; Yagiz, 2002,
2009). In this test, a standard conical indenter is pressed into a rock sample that has been cast in a confining
steel ring. The load and displacement of the indenter are recorded with a computer system. The slope of the
force-penetration curve indicates the excavatibility of the rock, i.e., the energy needed for efficient chipping. This
is affected by the stiffness, brittleness, and porosity of the sample.
The LCPC test was originally developed for determination of rock abrasivity and presented by the Laboratorie
Central des Ponts et Chaussées in the 1980s (Normalisation Française P18-579, 1990). The LCPC is also used
to determine an index called the ‘LCPC Breakability Coefficient' (LBC) to quantify the breakability or brittleness
of the sample material (Thuro et al. 2007; Wilfing, 2016). The LCPC Breakability Coefficient LBC is defined as
the fraction below 1.6 mm in the grain size distribution curve after testing.
The Sievers' J- miniature drill test was originally developed by H. Sievers in the 1950s. The Sievers' J-Value (SJ)
constitutes a measure of the rock surface hardness or resistance to indentation.
The SJ value is defined as the mean value of the measured drill hole depths in 1/10 mm, after 200 revolutions of
the 8.5 mm miniature drill bit (Dahl et al., 2012).
Table 1. Some of the most common laboratory test methods for measuring rock breakability in connection with
hard rock drilling and boring.
Test method Index Principle Rock sample Testing tool Reference
Uniaxial
Compressive
Stren
g
th
UCS The specimen is loaded
axially until failure occur
Cut and
grinded rock
core
Hydraulic press
ISRM
(1978)
Brazilian
Tensile
Stren
g
th
BTS The specimen is loaded
until failure occur Cut rock disc
Hydraulic press
equipped with
steel loadin
g
j
aws
ISRM
(1978)
Point Load
Strength PLS (Is50) The specimen is loaded
until failure occur
Rock core
section or
lumps
Hydraulic press
equipped with
spherically-
truncated, conical
platens
ISRM
(1985)
Indentation
Hardness
Testin
g
IHI
Indentor under applied
load penetrates into the
rock surface
Rock sample
with top end
saw-cut
Loading frame
and hard material
conical indente
r
ISRM
(1998)
Punch
penetration
Penetration
index (PI)
Indenter penetrating the
surface of a confined
rock core sample
Rock core
section
Conical tungsten
carbide indenter
Yagiz
(2009)
LCPC
Breakability
LCPC
Breakability
Coefficient
(
LBC
)
Impeller (medium hard
steel) rotating in a vessel
containing crushed rock
Crushed rock
(4-6.3 mm) Steel impeller
Käsling and
Thuro
(2010)
Surface
hardness/
resistance to
penetration
Sievers' J-
Value (SJ)
Penetration depth of
drillings after 200
revolutions
Cut rock
specimen with
parallel sides
8.5 mm tungsten
carbide drill bit
Dahl (2012)
Brittleness
Value
Brittleness
Value
(
S20
)
The prepared sample is
exposed to 20 impacts
Crushed rock
(
11.2
16 mm
)
Mortar with lid and
a 20 k
g
wei
g
ht
Dahl (2012)
In addition to the aforementioned test methods, the rock properties, density and porosity, should also be
included and considered since they may have strong influence on the breakability of the rock. The density is
defined as the mass per volume of a substance (gr/cm3) and porosity is defined as the nonsolid or pore-volume
fraction. Porosity is a volume ratio and thus dimensionless, and is usually reported as a fraction or percent
(ISRM, 1977).
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Figure 1 Some of the most common laboratory test methods for measuring rock breakability in connection with
hard rock drilling and boring., a) UCS, b) BTS, c) PLT, d) S20 (Dahl et al., 2012), e) Punch test (Yagiz, 2009), f)
LCPC (Thuro et al., 2007) and g) SJ (Dahl et al., 2012)
There are in addition some methods that adopt approaches that are different from model testing. The
measurement of mineralogical parameters such as quartz content, ‘Equivalent Quartz Content’ (EQC) or the
‘Vickers Hardness Number Rock (VHNR)’ are commonly also used to assess breakability of hard rock. The
‘Equivalent Quartz Content’ parameter encompasses the influence of the entire mineral content of the rock on
abrasiveness relative to quartz, while the ‘Vickers Hardness Number’ is used as a measure of the hardness of
each individual component mineral. Individual Vickers hardness values, combined with the percentage of each
mineral found in a rock, can be used to calculate the so-called Vickers Hardness Number Rock, or VHNR
(Salminen and Viitala, 1985).
2.3. Laboratory test methods for rock abrasivity
During recent decades, many test methods have been employed to measure rock abrasivity. Several
researchers, e.g. Plinninger and Restner (2008), Macias (2016) …, have summarized and discussed abrasivity
testing. Table 2 provides a list of the most commonly used methods in connection with tunnel boring.
Table 2. The most common laboratory test methods for measuring rock abrasivity in connection with hard rock.
Test method Index Principle Rock sample Testing tool
CERCHAR test
(1986)
CERCHAR
Abrasivity Index
(
CAI
)
Indenter (hard steel)
moves over a rock
surface
Intact rock Steel stylus
LCPC test (1990) LCPC abrasivity
Coefficient (LAC)
Impeller (medium hard
steel) rotating in a vessel
containin
g
crushed rock
Crushed rock
(4-6.3 mm) Steel impeller
NTNU/SINTEF
abrasivit
y
(
1960
)
Abrasion Value
(
AV
)
Tungsten carbide piece
slidin
g
over crushed rock
Crushed rock
(
<1mm
)
Tungsten
carbide piece
NTNU/SINTEF
abrasivity (1983)
Abrasion Value
cutter Steel
(
AVS
)
Cutter ring steel piece
sliding over crushed rock
Crushed rock
(<1mm)
Cutter ring steel
piece
The content of quartz and other hard and abrasive minerals found in the rock is normally significantly influencing
rock abrasiveness. As previously discussed, the measurement of mineralogical parameters such as quartz
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content, ‘Equivalent Quartz Content’ (EQC) or the ‘Vickers Hardness Number Rock (VHNR)’ involve methods
that adopt approaches that are different from model testing.
Figure 2. The most common laboratory test methods for measuring rock abrasivity in connection with hard rock
a) Outline of the CERCHAR test apparatus (West, 1989)., b) The LCPC abrasivity testing device (Thuro et al.,
2007) and c) Outline of the Abrasion Value (AV) and Abrasion Value Cutter Steel (AVS) test (Dahl et al., 2012).
The CERCHAR abrasivity test is used to determine the CERCHAR Abrasivity Index (CAI) and was originally
developed and presented by the Centre d'Études et Recherches des Charbonnages de France in the 1970s
(Valantin, 1974). The test measures the wear on the tip of a steel stylus having a Rockwell Hardness of HRC 55
(ISRM, 2014) or HRC 40 (ASTM, 2010). A rock specimen is held firmly in the test apparatus while a sharpened
steel stylus under a normal force of 70 N is moved a total distance of 10.0 mm across the rock. (Figure 2a). The
CAI is a dimensionless unit and is calculated by multiplying the wear surface on the steel stylus in units of 0.01
mm by 10.
The LCPC test is used to determine an index called the ‘LCPC Abrasivity Coefficient’ (LAC) for classifying the
abrasivity of the rock. As aforementioned, the testing principle was originally developed and presented by the
Laboratorie Central des Ponts et Chaussées in the 1980s (Normalisation Française P18-579, 1990).
An outline of the test apparatus is given in Figure 2b. The impeller is a rectangular metal plate of dimensions 50
× 25 × 5 mm made of standardised steel with a Rockwell hardness of B 60–75. It rotates for 5 minutes at a
speed of 4,500 rpm inside a cylindrical container filled with the rock sample material, which consists of 500 ± 2
grams of a crushed, sieved (fraction 4–6.3 mm) and air-dried rock specimen. The impeller is weighed before and
after testing and its weight loss constitutes a measure of rock abrasivity.
The Abrasion Value (AV) index was originally developed and presented by NTNU at the beginning of the 1960s
while the Abrasion Value Cutter Steel was introduced at the beginnings of the 1980s. The test methods are
often referred to as the "Norwegian abrasion test methods" (Figure 2c). The AV and AVS indices represent a
measure of rock abrasion or the ability of a rock to induce wear on tungsten carbide and cutter ring steel
respectively. It is a time-dependent parameter determined by measuring the abrasion of tungsten carbide and
cutter steel caused by crushed and sieved (< 1.0 mm) rock powder. Figure 2 is an illustration of the method
(Dahl et al., 2012).
The AV index is defined as the weight loss of the test piece after 5 min testing (100 revolutions) of testing while
the AVS index is defined as the measured weight loss of the test piece in milligrams after 1 minute (20
revolutions) of testing.
Recently a new rock abrasivity test method for cutter life assessments in hard rock tunnel boring: the Rolling
Indentation Abrasion Test (RIAT) has been developed (Macias et al., 2016; Macias, 2016; Macias 2017). The
Rolling Indentation Abrasion Test method (RIAT) involves the use of miniature rolling discs that penetrate the
surface of an intact rock sample (Figure 3). A suitable drive unit provides the rotation, torque and vertical thrust
of the tool. The RIAT Abrasivity Index (RIATa) is defined as the weight loss (in mg) incurred by a miniature cutter
ring during a RIAT test. The RIAT Indentation Index (RIATi) is defined as the average value of ten evenly
distributed measurements of cutter penetration depth (measured in 1/100 mm).
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Figure 3. Outline and photograph illustrating the components of the Rolling Indentation Abrasion Test (RIAT)
method (Macias et al., 2016).
3. DRILLABILITY ASSESSMENTS IN HARD ROCK
3.1. General
Several excavation methods for hard rock are widely used in mining and construction and the Quarrying and
underground construction industry has been experiencing a strong development with enormous technological
improvement. In underground hard rock projects, drill and blast excavation (D&B), tunnel boring machine (TBM),
roadheader and hydraulic impact hammer methods are currently widely used with great success.
In hard rock tunnelling selection of the most suitable excavation method is however not a simple issue. It may,
as experience has shown, result in undesired situations. The choice is more complex than a simple economic
issue and it is rarely clear in the early stages of the projects. It is necessary to have an entire overview of the
parameters involved in the excavation method choice (Macias and Bruland, 2014).
Many parameters are involved with different roles in every project case; project characteristics and purpose,
environmental aspects or even social issues are involved. Every project is unique and a comprehensive and
detailed study should be carried out.
3.2. Assessments for drilling in hard rock
Drilling rate and drill bit wear for drill-and-blast tunnelling and rock quarrying is dependent of several parameters.
The net penetration rate of any drilling equipment (top hammer, coprod, DTH, rotary) is very dependent upon the
rock drillability (Olsen, 2009). According to Thuro (1997), the principal parameters influencing drilling velocity
may be jointing of rock mass, orientation of schistosity (rock anysotropy), degree of interlocking of
microstructures, porosity and quality of cementation of clastic rock, degree of hydrothermal decomposition and
weathering of a rock mass. The abrasive minerals (equivalent quartz content), porosity or the quality of the
cementation may influence drilling bit wear. Figure 4 shows an illustration of the main factors, machine and
geological parameters, influencing drilling in tunnelling.
Figure 4. Illustration of the main factors influencing hard rock drilling (Thuro, 1997).
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Several models to predict the net penetration rate and tool consumption in drilling equipment have been
developed. It exists a wide variability of drilling applications.
Table 3 summarises only some of the existing performance and tool life prediction models for drilling equipment.
Table 3. Some of the existing performance and tool life prediction models for drilling.
Model Basic input parameters Output
parameters
Reference
Rock parameters Machine parameters
Drill-and-blast
tunnelling
UCS (Destruction work)
Porosity, Joint spacing,
EQC
Power of the
percussive drill
Drilling velocity,
tool life (m/bit)
Thuro (1997)
Drilling DRI Drill bit diameter Drilling velocity Sandvik Tamrock
(1999)
Rock Abrasivity
Index (CAI) - Drill-
and-blast tunnelling
UCS, EQC Drill bit lifetime
(m/bit)
Plinninger et al.
(2002).
Plinninger (2010)
Drill-and-blast
tunnelling
DRI, VHNR Power of the
percussive drill, drill
hole diameter
Drilling velocity,
tool life
Zare (2007)
Quarrying DRI, VHNR Drill bit/hole diameter,
hole depth, percussive
drill type and capacity
Drilling velocity,
tool life
Olsen (2009)
3.3. Assessments for tunnel boring in hard rock
Several prediction models for estimates of performance and cutter wear in hard rock tunnel boring have been
developed in recent decades. Table 4 summarises the main basic input and output parameters of the common
performance prediction models.
Table 4. Commonly used performance prediction models for hard rock tunnel boring including their input and
output parameters.
Penetration rate
model
Basic input parameters Output
parameters
Reference
Rock parameters Machine parameters
Gehring model UCS Penetration rate Gehring (1995)
Wilfing (2016)
CSM Model UCS, BTS, Rock mass
fracturing
Cutter diameter, cutter
tip width, average
cutter spacing, number
of cutters, cutterhead
rpm, thrust force
Penetration rate Rostami (1997)
Yagiz (2014)
NTNU model DRI, porosity, degree of
fracturing, orientation,
CLI, quartz content
TBM diameter, Cutter
diameter, number of
cutters, gross average
cutter thrust, rpm
Penetration rate,
advance rate
Macias (2016)
QTBM Q-value (with RQDo),
rock mass strength,
CLI, quartz content,
induced biaxial stress
on tunnel face, porosity
Average cutter load,
TBM diameter
Penetration rate,
advance rate
Barton (2000)
RME RMR parameters, DRI TBM diameter, type of
machine
Penetration rate,
advance rate
Bieniawski et al.
(2006)
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In addition, recent methods and models have been developed to make cutter life assessments. Table 5 shows
commonly used cutter life models.
Table 5. Common cutter life models for hard rock tunnel boring including their input and output parameters.
Cutter life
model
Basic input parameters
Output parameters Reference
Rock parameters Machine parameters
Gehring CAI, UCS Wear rate, cutter life Gehring (1995)
CSM CAI Cutterhead rpm Cutter life, delays,
total cost Rostami (1997)
NTNU CLI, quartz content
(%)
Cutterhead diameter (m),
cutter diameter (mm),
number of cutters,
cutterhead rpm
Cutter life, delays,
total cost Macias (2016)
Maidl CAI, UCS Average cutter ring
life Maidl et al. (2008)
RME CAI, UCS Cutter life Bieniawski et al.
(2009)
Frenzel CAI Cutterhead diameter (m)
Cutter life, delays,
total cost Frenzel (2011)
The CERCHAR Abrasivity Index (CAI), as defined by the ASTM (2010) and/or the ISRM (2014) is used by the
CSM model and the Gehring model (Gehring, 1995), Maidl model (Maidl et al., 2008), Bieniawski (Bieniawski et
al., 2009) and Frenzel (Frenzel (2011). The drillabity parameters that influence cutter wear in the NTNU model
are the Cutter Life Index (CLI) and rock quartz content (%). As aforementioned, the Cutter Life Index (CLI) is
evaluated based on the Sievers’ J-value and the Abrasion Value Cutter Steel (AVS) as defined by Bruland
(2000). Hassanpour et al. (2014) have proposed a new empirical TBM cutter wear prediction model where
mineral hardness (quantified using the VHNR) and UCS are the drillability parameters.
3.4. Assessments for roadheader and hydraulic impact hammer excavation
The use of roadheaders is normally not recommended for rocks with high strength (Bilgin et al., 2005) and
therefore, the method might result on limited application in hard rock conditions. However, Bilgin et al. (2005)
reported an instantaneous cutting rate of 5 m3/h in high strength rock formation (> 100 MPa) but with a high tool
consumption resulting in a low machine utilization and low advance rate.
Several models exist to assess performance predictions for roadheaders. Table 6 summarises the main basic
input and output parameters of the common performance prediction models for roadheaders.
Table 6. Commonly used performance prediction models for roadheaders including their input and output
parameters (modified from Balci et al., 2004).
Penetration
rate model
Basic input parameters Output parameters Reference
Rock parameters Machine parameters
Bilgin et al. UCS, RQD* Machine power Instantaneous cutting rate Bilgin et al.
(
1988
,
1990
)
Gehring UCS Instantaneous cutting rate Gehring (1989)
Thuro UCS Instantaneous cutting rate Thuro and Plininger
(
1999
)
Balci et al. UCS Machine power Instantaneous cutting rate Balci et al. (2004)
*RQD: Rock Quality Designation
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Regarding to estimation of tool consumption of roadheaders, the CAI test is used according to Johnson and
Fowell (1986).
The main rock parameters used to predict the impact hammer excavation are UCS and RQD according to the
equation proposed by Bilgin et al. (1997; 2002). Later, the influence the thickness of dikes with high rock
strength was introduced by Ocak and Bilgin (2010).
4. CONCLUSIVE REMARKS
Several methods for excavations in hard rock conditions have been generally used with success but in too many
cases, due to unanticipated situations and/or inappropriate assessments, with undesirable consequences. Some
of the most commonly used are drill-and-blast tunnelling, TBM tunnelling, roadheaders and rock quarrying.
Especially in hard rock conditions, intact rock properties play a significant role in excavation. Drillability is
commonly used to describe the ability of the rock to be drilled or bored and it is used to assess the influence that
intact rock properties have on performance prediction and cost evaluations in drill-and-blast tunnelling, TBM
tunnelling, roadheaders and rock quarrying.
The drillability of rock is however not representing one clearly defined property determined by a single laboratory
test method. Drillability can basically be defined as a combination between the breakability and the abrasivity of
the rock. Breakability and abrasivity are again dependent on combinations of several individual properties and a
best possible determination of the drillability of intact rock can hence only be achieved by evaluating the
information gained from a dedicated set of laboratory test methods used to determine both the breakability and
abrasivity. Each dedicated test method will represent a certain defined rock property and it is important to
achieve a good understanding of the principle of the test method in order to be able to evaluate and use the
information which can be gained from the test results.
There are currently several performance prediction models available for estimation of time and cost in
connection with excavation of hard rock by use of the various excavation methods. The common thread for most
of them are that they are using several rock properties along with different machine parameters as input
parameters in order to obtain a best possible estimate. Drillability plays a significant role in the prediction models
for performances, breakability, and for tool life assessments, abrasivity. More effort should be emphasized on
understanding the rock breaking process and tool wear tribological system as well as continuously extending
and improving the existing prediction models for the different excavation methods.
REFERENCES
ASTM (2008). D3967-08: standard test method for splitting tensile strength of intact rock core specimens. ASTM
International, West Conshohocken.
ASTM (2010). Standard test method for laboratory determination of abrasiveness of rock using the CERCHAR
method. Designation: D7625-10.
Balci, C., Demircin, M.A., Copur, H., Tuncdemir, H. (2004). Estimation of optimum specific energy based on rock
properties for assessment of roadheader performance. The Journal of The South African Institute of Mining
and Metallurgy, December 2004, pp 633-641.
Barton, N. (2000). TBM tunnelling in jointed and faulted rock. A.A. Balkema, Rotterdam (2000). ISBN 90 5809
341 7.
Bieniawski, Z.T., Celada, C. B., Galera, J. M. and Tardaguila, I. G. (2009). Prediction of Cutter Wear using RME.
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... Unanticipated situations and/or inappropriate assessments can result in considerable delays and great risk of cost overruns. 2 Rock abrasivity and hardness are two of crucial parameters of all the mechanical properties, and the related research was initiated nearly forty years ago. 3 In recent years, the research target is not only focused on measuring them precisely, but also focused on figuring out the internal relationship between various properties and obtaining accurate prediction value. ...
Article
Rock abrasivity and hardness are two of the crucial mechanical properties in geological exploration and petroleum engineering. To figure out how the rock mineral composition determines the rock mechanical properties, ninety-six samples from ten provinces of China were collected to carry out tests including mineral contents, mineral particle size, abrasivity and hardness, and testing results indicated there is strong relationship between them. Through data processing of normalization, correlation analysis, and grouping, the raw testing data were used to establish a prediction function with Back-Propagation Artificial Neural Network (short as BP-ANN). With this prediction function, rock abrasivity and hardness can be accurately calculated from input parameters including rock type, mineral contents, and particle size. Besides, the calculation results from this prediction function also revealed the changing trend of abrasivity and hardness on how to be affected by mineral contents and particle size.
... Numerous equations and models have been proposed by various authors to quantify the rock brittleness. Some of these equations have been summarized previously (Yagiz and Gokceoglu 2010;Meng et al. 2015;Macias et al. 2017). Most of the equations introduced in the literature are based on the strength ratios (such as; Hucka and Das 1974;Kahraman and Altindag 2004); however, the strength ratios by itself do not represent the brittleness of rock since the brittleness is combination of rock properties rather than only one parameter. ...
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Determining the rock brittleness is often needed in a wide range of rock engineering projects; however, direct measurement of the brittleness are expensive, time consuming and also the test devices is not available in every laboratory. Due to that, assessing the brittleness of rock as a function of some rock properties such as uniaxial compressive strength, Brazilian tensile strength and density of rock is unavoidable. The aim of this paper is to develop predictive models for estimating the rock brittleness using two techniques, genetic algorithm (GA) and particle swarm optimization (PSO). For this aim, four different models including linear and non-linear were developed using GA and PSO techniques. Further, in order to validate the accuracy of proposed models, various statistical indices including the root mean square error (RMSE), the variance account for (VAF), the coefficient of determination (R²) and performance index (PI) were computed and utilized herein. The values RMSE, VAF, R² and PI ranged between 2.64–5.25, 82.58–93.06%, 0.851–0.932 and 1.480–1.708, respectively; with the quadratic form of the GA approach indicating the best performance. It is concluded that both the GA and PSO techniques could be utilized for predicting the rock brittleness; however, GA-quadratic model is superior.
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Disc cutter consumption is of great importance when using the Tunnel Boring Machine (TBM) method for tunnelling in hard rock conditions. Abrasive wear is the most common process affecting cutter consumption; therefore good laboratory tests for rock abrasivity assessments are needed to carry out reliable cutter consumption assessments that enable project planning and control of the risk. A new rock abrasivity test method named Rolling Indentation Abrasion Test (RIAT) has recently been developed. It reproduces wear behaviour on hard rock tunnel boring in a more realistic way than the traditionally used methods by introducing wear by rolling contact on intact rock samples. Detailed cutter consumption data, operational machine parameters and laboratory tests results from a recently finished hard rock TBM project has been analysed in order to evaluate the applicability of the RIAT method to assess cutter consumption in hard rock tunnel boring.
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The tunnel boring machine (TBM) method has become widely used and is currently an important presence within the tunnelling industry. Large investments and high geological risk are involved using TBMs, and disc cutter consumption has a great influence on performance and cost, especially in hard rock conditions. Furthermore, reliable cutter life assessments facilitate the control of risk as well as avoiding delays and budget overruns. Since abrasive wear is the most common process affecting cutter consumption, good laboratory tests for rock abrasivity assessments are needed. A new abrasivity test method by rolling disc named Rolling Indentation Abrasion Test (RIAT) has been developed. The goal of the new test design and procedure is to reproduce wear behaviour on hard rock tunnel boring in a more realistic way than the traditionally used methods. Wear by rolling contact on intact rock samples is introduced and several rock types, covering a wide rock abrasiveness range, have been tested by RIAT. The RIAT procedure indicates a great ability of the testing method to assess abrasive wear on rolling discs. In addition and in order to evaluate the newly developed RIAT test method, a comprehensive laboratory testing program including the most commonly used abrasivity test methods and the mineral composition were carried out. Relationships between the achieved results from conventional testing and RIAT results have been analysed.
Thesis
Performance prediction is one of the crucial issues for estimating excavation costs and construction time of tunnel projects. In mechanized tunneling, TBM performance highly depends on an achieved penetration rate and cutter wear. The aim of this thesis is to illustrate the improvement of the GEHRING (1995) penetration prediction model by investigating two parameters that significantly influence the penetration. These are namely the toughness of rocks and the discontinuity pattern in rock mass. Analysis is done by performing an extensive laboratory program (uniaxial compression tests, Brazilian tensile tests, point load tests, Cerchar abrasivity tests, LCPC abrasivity tests, thin sections) and on-site penetration tests. Laboratory testing aims to obtain a deep understanding of the deformation behavior of rocks under load. In addition, several characterization and classification methods for rock toughness are analyzed since neither have yet to gain complete acceptance. The commonly used toughness index, described by the ratio of uniaxial compressive strength and Brazilian tensile strength shows inappropriate results. Conversely, the ratio of uniaxial compressive strength and point load index yields best results regarding accuracy of laboratory parameters and applicability in practice. Therefore, this index seems suitable for an implementation into penetration prediction models. In order to investigate the influence of discontinuities on TBM penetration, estimated parameters by two existing prediction models (Gehring model, Colorado School of Mines model) are compared with the results of 30 penetration tests and geological mapping at two tunnel projects. Penetration tests are a common tool to determine the performance of a TBM in certain geological environments. During a test, the TBM is operated under defined conditions that allow the comparison of different tunnel projects and machine types in analogous geological conditions. Results show that only for a narrow scope, considered prediction models reveal appropriate fitting. Once the rock mass is fractured or the stress level within the rock mass changes, existing models are not applicable. Based on this fact, the necessity to update penetration prediction models by implementing a correction factor for discontinuities can be clearly shown. The correction factor suggested by GEHRING does not reflect reality since only one discontinuity system is considered and the enhanced effect of intersecting systems on the penetration is neglected. Hence, this factor has to be revised. For this purpose, a combination with the fracturing factor originating from the prediction model of NTNU (BRULAND 2000) seems to be suitable, since it yields a good correlation with the obtained data. The rock fracturing index incorporates the parameter of rock mass fabric into the CSM prediction model. However, analyses reveal that results are not satisfying and the applicability of this index is additional limiting factor. The general relation between force and resulting penetration is also in the focus of this thesis. It has been proven that force-penetration graphs can be described best by a linear function with certain y-axis offset. Consequently, CSM and Gehring prediction models are based on an inappropriate mathematical equation. The offset is characterized by the point of subcritical penetration and depends highly on the Brazilian tensile strength and the LCPC breakability coefficient. Incorporation of the critical y-axis offset, as well as of correction factors for rock toughness and discontinuity pattern result in a new prognosis tool called the “Alpine model” which is based on the existing Gehring model.
Thesis
The use of hard rock tunnel boring machines (TBMs) has become widely and generally used with success but in too many cases, due to unanticipated situations and/or inappropriate assessments, with catastrophic consequences. Hard rock tunnel boring involves major investments and high levels of geological risk, which require reliable performance predictions and cutter life assessments. Hard rock tunnel boring leads the interaction between the rock mass and the machine, which is a process of great complexity. The tunnelling system around the excavation process has a great relevance in the final goal of performance predictions for hard rock TBMs, which is the estimation of time and cost. The overall aim of this thesis is to build on the existing knowledge of tunnel boring and wear processes, technology and capacity, thus enhancing performance prediction and cutter life assessments in hard rock tunnel boring projects. The prediction model for hard rock TBMs developed by the Norwegian University of Science and Technology (NTNU) is based on a combination of field performance data, engineering geological back-mapping, and laboratory testing. The development of TBM technology during recent decades, and the possible influence of parameters not considered in previous versions, has made revision of the model to improve prediction accuracy an essential requirement. TBM specifications, the penetration rate and cutter life models have been revised and extended to adapt to the current technology. In addition, it has been incorporated a new definition of the rock mass fracturing, the influence of the cutterhead velocity (rpm) on penetration rate on the in response to the results of in-situ trials 'RPM tests' and cutter thrust on cutter life. The tunnel length effect on time consumption for the tunnelling activities has been introduced for the estimation of the machine utilization and therefore in the advance rate model. A revised and extended version of the current version of the NTNU prediction model for TBM performance and cutter life has been published included in this thesis. Cutter consumption and parameters such as cutter ring wear play a significant role in performance and cost predictions, especially in hard rock conditions. Cutter wear involves a complex tribological system in interaction with the geological properties of the rock mass. Existing laboratory test methods fail to reproduce wear behaviour encountered during tunnel boring. Because of this, and the importance of cutter wear, it was considered of interest to develop a new rock abrasivity test method for tool life assessments in hard rock tunnel boring: The Rolling Indentation Abrasion Test (RIAT). Understanding the processes and failure mechanisms during cutter wear enabled new knowledge to be applied as part of the development of the new rock abrasivity test method for laboratory use. In addition, cutter wear mechanisms affecting cutter rings during tunnel boring might lead to better cutter consumption predictions and future improvements in cutter ring development.
Article
Depending on rock excavatability and rock abrasivity disc cutters can exhibit significant wear. Detailed data on 7 tunneling projects comprising a total length of more than 127 km and more than 12,000 replaced disc cutters have been collected in a custom-made data base. A detailed analysis has been carried out to characterize failure modes and wear patterns of disc cutters. A prediction model has been established that includes cutter life and cutter costs.
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This book covers the fundamentals of tunneling machine technology: drilling, tunneling, waste removal and securing. It treats methods of rock classification for the machinery concerned as well as legal issues, using numerous example projects to reflect the state of technology, as well as problematic cases and solutions. The work is structured such that readers are led from the basics via the main functional elements of tunneling machinery to the different types of machine, together with their areas of application and equipment. The result is an overview of current developments. Close cooperation among the authors involved has created a book of equal interest to experienced tunnelers and newcomers. © 2008 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH und Co.KG, Berlin.
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Specific energy is defined as the amount of work required to break a unit volume of rock and is used to predict the performance of mechanical miners. It is obtained from full-scale rock cutting experiments, which require large block samples, experienced personnel and expensive equipment found in only a few research centres in the world. Estimation of the optimum specific energy, at which a given geological formation is excavated at optimum cutting geometry in the most energy efficient manner, from mechanical rock properties to predict the performance/efficiency of roadheaders is the basic aim of this study. The mechanical rock property tests require only core samples, which are easier to obtain and to test. In this context, full-scale rock cutting tests are performed on 23 different rock, mineral and ore samples including sandstone, claystone, tuffite, chromite, trona and copper collected from some operating mines in Turkey. The optimum specific energy values are obtained for each sample. Physical and mechanical property tests are performed on the core samples obtained from the same samples to determine uniaxial compressive strength, indirect (Brazilian) tensile strength, static and dynamic elasticity moduli, Schmidt hammer rebound values, density, and Cerchar abrasivity index. The relationships between the optimum specific energy and the mechanical rock properties are analysed using statistical methods. The results indicate strong relationships between the optimum specific energy and the mechanical rock properties. The strongest relationships are found by using uniaxial compressive strength and tensile strength. The performance models developed in this study are in good agreement with the empirical models previously developed for roadheaders and can be used reliably for prediction purposes.
Article
Since the early days of roadheader technology, two different types of cutterhead have been used, and individual manufacturers claim advantages for each type. During the last five or six years, some manufacturers have designed their machines to allow application with both types of cutterheads, thus meeting the demand and to some extent accepting the philosophy of their customers, who want to get the most appropriate cutterhead for their conditions. This article is intended to put the discussion on a basis of rational findings and data. For both types the most advanced designs of cutterhead and their use on up-to-date machines are considered. Subjects covered include the cutting process, cutting energy, site tests, pick consumption, and other aspects of the subject.