NEW ROCK ABRASIVITY TEST METHOD BY ROLLING DISC

Abstract

Tunnelling by use of Tunnel Boring Machine (TBM) is gaining a greater presence as a suitable excavation method. Disc cutter consumption has a strong influence on performance and cost when using TBMs and the influence becomes even more relevant in hard rock. Furthermore, good predictions of TBM performance and cost 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 assessments are needed. A new abrasivity test method called Rolling Indentation Abrasion Test was 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 traditionally used methods by introducing wear by rolling contact on intact rock samples.

Full text

NEW ROCK ABRASIVITY TEST METHOD BY ROLLING DISC
*F. J. Macias and A. Bruland
Department of Civil and Transport Engineering
NTNU
Trondheim, Norway
(*Corresponding author: Javier.macias@ntnu.no)
F. E. Dahl
SINTEF, Building and Infrastructure,
Rock Engineering
Trondheim, Norway
ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4

NEW ROCK ABRASIVITY TEST METHOD BY ROLLING DISC
ABSTRACT
Tunnelling by use of Tunnel Boring Machine (TBM) is gaining a greater presence as a suitable
excavation method. Disc cutter consumption has a strong influence on performance and cost when using
TBMs and the influence becomes even more relevant in hard rock. Furthermore, good predictions of TBM
performance and cost 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 assessments are
needed. A new abrasivity test method called Rolling Indentation Abrasion Test was 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 traditionally used methods by introducing wear by rolling contact on intact rock samples.
KEYWORDS
Abrasivity test method, Hard rock tunnel boring, Abrasivity assessment, Rolling contact, Intact rock testing,
Tunnel Boring Machine (TBM) performance
INTRODUCTION
High investments are involved in the tunnelling industry, especially in Tunnel Boring Machines
(TBMs) projects. Performance predictions and cost estimations are crucial for planning and risk
management of TBM projects. Cutter consumption has a large influence in terms of time and cost, and an
even greater effect in case of hard rock projects. Many factors are influencing the amount of cutters
consumed in hard rock TBMs. Normal TBM operation results mainly in abrasive wear on the cutters and
appropriate methods for abrasivity assessment are needed. Rock abrasiveness cannot be considered as an
intrinsic property and the complete tribological system should be considered in addition to the geological
properties of the rock.
In the present paper, a new test method for abrasivity assessment in tunnel boring called Rolling
Indentation Abrasion Test (RIAT) is presented. The traditionally used test methods for determination of
rock abrasiveness uses sliding or impact contact in order to cause wear while the RIAT introduces rolling
contact on intact rock samples. The ambition of the new test device is to have a reliable method to evaluate
the cutter wear in hard rock tunnel boring by reproducing wear behaviour on rolling disc cutters in a more
realistic way. It has been established by many authors that the abrasion of a cutter ring is proportional to its
rolling distance (Rostami, 1997; Bruland, 1998). In the present work, the weight loss of a mini-cutter ring
is measured subsequent to testing in order to evaluate wear in the rolling process and could hence be
related to cutter ring wear. Indentation of the tools in the intact rock sample is considered as an indication
of the surface hardness of the rock or the resistance to indentation by rolling.
The initial results obtained from the presented RIAT are promising and show that it has a great
potential for assessing rock abrasivity on TBM cutters and indentation on hard rock by rolling discs. The
test method is however still under development and further testing should be performed.
The main advantages of the RIAT are, wear caused by rolling contact, testing of intact rock
samples, relatively small samples needed, cost effective way and measurement of the rock indentation
resistance or rock surface hardness in addition to wear.
ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4

Test methods for rock abrasivity: State of the art
Brief descriptions of some of the most commonly used laboratory test methods for TBM cutter
life assessment follow. The CERCHAR abrasivity test is a method to determine the CERCHAR Abrasivity
Index (CAI) for classifying the abrasivity of the rock. The testing principle was originally developed and
introduced by Centre d' Études et Rescherches des Charbonnages de France in the 1980s (Cerchar, 1986).
A rock specimen is firmly held in the test apparatus (Figure 1). A normal force of 70 N is applied
while the stylus is moved a total distance of 10.0 mm across the rock. The duration of the movement of the
stylus should be completed within 1 ± 0.5 s with Type 1 apparatus and 10 ± 2 s with Type 2 apparatus. The
test measures the wear on the tip of a steel stylus having a Rockwell Hardness of HRC 55.
Figure 1 – Two main types of CERCHAR test apparatus commonly in use. Left Type 1, original design
CERCHAR-type apparatus. Right Type 2, the modified CERCHAR apparatus as reported by West (1989).
1 = mass, 2 = pin chuck/guide, 3 = stylus, 4 = specimen, 5 = vice, 6 = lever/hand crank (Alber et al., 2014).
The CAI is a dimensionless unit value and is calculated by multiplying the wear surface stated in
units of 0.01 mm by 10. Table 1 shows the abrasivity classification system for the CAI.
Table 1 – Classification of CAI (Alber et al., 2014).
Mean CAI
Classification
0.1
–
0.4
Extremely low
0.5
–
0.9
Very low
1.0
–
1.9
Low
2.0
–
2.9
Medium
3.0
–
3.9
High
4.0
–
4.9
Very high
≥5
Extremely high
ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4

The LCPC test is a method used to determine an index called the LCPC Abrasivity Coefficient
(LAC) for classifying the abrasivity of the rock. The testing principle was originally developed and
introduced by Laboratorie Central des Ponts et Chaussées in the 1980s (Normalisation Française P18-579,
1990).
An outline of the test apparatus is given in Figure 2. The impeller is a rectangular metal plate with
size (50 × 25 × 5 mm) and it is made of standardized steel with a Rockwell hardness B 60–75. The
impeller rotates for 5 minutes at a speed of 4,500 rpm in the cylindrical container filled with the sample
material which is a crushed, sieved and air-dried specimen of 500 ± 2 g of the fraction 4–6.3 mm. The
metal impeller is weighed before and subsequent to testing and the mass loss of the impeller constitutes
quantity of the rock abrasivity.
Figure 2 – LCPC abrasivity testing device (Thuro et al., 2007). 1 = motor, 2 = metal impeller, 3 = sample
container (diameter 93 mm × 100 mm), 4 = funnel tube.
The LCPC Abrasivity Coefficient (LAC) is calculated (equation 1) as the mass loss of the
impeller divided by the sample mass (500 g).
LAC = (m0 – m)/ M (1)
Where m0 is the mass of the steel impeller before LCPC test (g); m is the mass of the steel impeller after
LCPC test (g); and M is the mass of the sample material (0.0005 t).
The LAC varies between 0 and 2,000 g/t for natural rocks and soil samples. A close linear
relationship between LAC and CAI was reported by Thuro et al (2007) and the classification and terms for
LAC shown in Table 2 were introduced.
Table 2 – Classification of LAC (Thuro et al., 2007).
LAC (g/t
)
Classification
0
–
50
Not abrasive
50
–
100
Not very abrasive
100
–
250
Slightly
abrasive
250
–
500
Medium abrasive
500
–
1
,
250
Very abrasive
1
,
250
–
2
,
000
Extremely ab
ra
sive
ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4

The test method often referred to as the "Norwegian abrasion test method" is amongst other used
to determine Abrasion Value Cutter Steel (AVS). The AVS, which was developed and introduced by
NTNU in the beginning of the 1980s, constitutes a measure of the rock abrasion or ability to induce wear
on cutter ring steel. The AVS represents time dependent abrasion of cutter steel caused by crushed rock
powder. Figure 3 shows the outline of the abrasion test method (Dahl et.al. 2012).
Figure 3 – Outline of the Abrasion Value Cutter Steel (AVS) test (Dahl et al., 2012).
The AVS is defined as the measured weight loss of the test piece in milligrams after 1 minute (i.e.
20 revolutions of testing). The values of AVS in the NTNU/SINTEF database for 2621 recorded values are
ranging from 0.0 (limestone) to 68.5 (quartzite) according to Dahl et al. (2012). The classification of rock
abrasion on cutter steel is given in Table 3.
Table 3 – Classification of rock abrasion on cutter steel (Dahl et al., 2012).
AVS (mg)
Abrasion on cutter steel
≥44.0
Extremely high
36.0
–
44.0
Very high
26.0
–
35.9
High
13.0
–
25.9
Medium
4.0
–
12.9
Low
1.1
–
3.9
Very low
≤1.0
Extremely low
Figure 4 gives the AVS ranges for a selection of common metamorphic, igneous and sedimentary
rock types.
ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4

Figure 4 – Recorded range of AVS for a selection of common rock types. (Dahl et al., 2012).
THE ROLLING INDENTATION ABRASION TEST (RIAT)
The RIAT uses a tool that is fitted with two replaceable miniature cutter rings. The miniature
discs are rolling and penetrating the surface of an intact rock sample. The rotation, torque and vertical
thrust of the tool are provided through a suitable drive unit. Figure 5 shows the new test device and the
mini cutters used by the RIAT test method.
Figure 5 – The RIAT tool and the mini-cutter rings.
Preliminary test procedure
The RIAT test is performed on an intact rock sample. The recommended rock sample should be
cut and grinded in order to have an optimal levering of the surface with minimum size equivalent to a 100
mm diameter core sample. Slight deviations can however be absorbed by the design of the tool.
ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4

Rolling velocity is defined as 40 revolutions per minute (rpm) with a normal thrust of 1250 N. The
values have been defined by considering real cutter parameters in hard rock TBMs and previous evaluation
approaches. The mini cutters have a constant tip width and are made of a commercial steel type with
Rockwell Hardness HRC 50±1.
An outline of the initial RIAT tool dimensions is given in Figure 6 while the main parameters for
the preliminary test procedure are given in Table 2. It is important to note that the testing method still is
under development and that further adjustments may be considered.
Figure 6 – Outline of the RIAT method.
Table 2 – Main parameters for the RIAT test method.
Parameter
Value
Thrust (N)
1,
25
0
Rolling velocity (rpm)
40
Testing t
ime (min)
30
The RIAT Abrasivity Index is defined as the weight loss of the mini-cutter rings measured in (mg)
subsequent to testing. It is recommended to perform several measurements in order to achieve a
representative average value. Dust and debris should be removed from the track during testing in order to
ensure that the tools are constantly in contact with intact rock.
In addition to the weight loss, penetration of the mini-cutters into the intact rock can also be
measured after testing. The penetration value of the RIAT test does hence provide an indication of the
indentation resistance or rock surface hardness. The RIAT Indentation Index is defined as the average
value of the ten measures evenly distributed of the cutter penetration depths in the rock in 1/100 mm.
ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4

RESULTS
Preliminary results
A comprehensive evaluation approach and trial testing was initially developed in order to define
appropriate and capable testing parameters for the final purpose of the testing method. Real TBM
parameters have hence also been included in the evaluation approaches.
Four rock types, limestone, basalt, granite and quartzite, from lowest to highest abrasivity and covering the
whole range of hard rock abrasivity were chosen. Three parallel tests were performed for every rock type.
Figure 7 shows rock samples after testing and Table 3 presents the RIAT Abrasivity (mg) and
RIAT Indentation (1/100 mm) values for the rock types tested.
Figure 7 – Rock samples after testing by RIAT. a limestone, b basalt, c Iddefjord granite and d quartzite.
Table 3 – Preliminary results achieved by the RIAT.
RIAT Abrasivity and RIAT Indentation.
RIAT indexes
Limestone
Ba
salt
Iddefjord
granite
Quartzite
RIAT Abrasivity (mg) 3 10 39 104
RIAT Indentation (1/100 mm) 380 116 50 5*
*Not measurable
The lowest and the highest RIAT abrasivity of the test performed are 3 (limestone) and 104 (quartzite)
while for the RIAT indentation are 5 (quartzite) and 380 (limestone).
Figure 8 shows the results obtained for the tested rock types.
(a) (b)
(c) (d)
ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4

Figure 8 – Preliminary results achieved by the RIAT. Error bars show standard deviation.
Initial results obtained by use of the preliminary RIAT procedure are promising for abrasivity and
indentation assessments in hard rock tunnel boring. The initially achieved results show a capable definition
of the abrasivity and indentation on TBM cutters. Further testing, including several rock types, will hence
be performed in order to provide more data on the capability and reproducibility of the RIAT.
DISCUSSION
The main issue in rock abrasivity assessment is to have a test method which reproduces as
faithfully as possible the real cutter wear behaviour in hard rock tunnel boring.
Many of the existing abrasivity test methods have been developed for different purposes than hard
rock tunnel boring. The current abrasivity testing methods, CERCHAR, LCPC or AVS discussed
previously make different assumptions. LCPC and AVS uses crushed rock specimens; grain size 4 – 6.3
mm in the LCPC and < 1 mm in the AVS test. The test piece used in the LCPC is an impeller with defined
hardness that rotates in a container filled with the specimen. The test piece used in the AVS is made of
cutter ring steel and runs on the specimen with an applied weight. Both of them define wear by measuring
the weight loss of the test pieces subsequent to completed testing. The use of crushed rock will however
disregard the relevance of the rock textural influence on the wear. The CERCHAR test uses intact rock
samples and a stylus sliding under a weight. The size of the obtained wear surface on the tip of the stylus
subsequent to testing is used as a measure of the abrasiveness. The stylus sliding contact can however not
reproduce the real wear behaviour experienced in tunnel boring. It is commonly experiencing problems
associated with testing of very hard rock types, where the tip of the stylus is not able to fully penetrate the
rock resulting in an underestimation of the wear.
ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4

CONCLUSIONS
Cutter wear and therefore cutter consumption have a large impact on planning and risk
management for TBM projects. Abrasivity testing reproducing the cutter wear behaviour as realistic as
possible is hence needed for good abrasivity assessments.
None of the current laboratory test methods have been developed for wear assessment with TBMs
and are hence not reproducing the wear behaviour encountered during tunnel boring. The uses of crushed
rock or sliding tool-rock contact are the main weaknesses of the established methods.
The obtained initial results from the presented Rolling Indentation Abrasivity Test (RIAT) are
promising and show that it has a great potential for performing capable testing to evaluate rock abrasivity
on TBM cutters and indentation on hard rock by rolling discs. The test method is however still under
development and further testing should be performed.
The main advantages of the RIAT are:
- Wear caused by rolling contact.
- Testing of intact rock samples.
- Can be performed on relatively small samples.
- Straightforward procedure which allows testing of several samples in a cost effective
way.
- Provides measurement of rock indentation resistance or rock surface hardness in addition
to wear.
FURTHER WORK
Further work is being carried out in order to improve the capability and reproducibility of the
RIAT method:
- Characterize abrasivity including more rock types.
- Analysis of the test surface influence.
- Evaluation of the capability of the test for cutter life prediction on hard rock TBMs.
Cutter life data of ongoing and recently finished projects is being analysed with RIAT
results.
ACKNOWLEDGMENTS
The authors would like to thank the research project “Future Advanced Steel Technology for
Tunnelling” (FAST-Tunn). This project is managed by SINTEF, and funded by the Research Council of
Norway. The Robbins Company, BASF Construction Chemicals, the Norwegian Railroad Authorities,
Scana Steel Stavanger, BMS steel, the LNS Group and Babendererde Engineers are industrial partners and
co-founders. NTNU is a researcher partner in the project responsible for three PhDs.
The authors would also like to acknowledge Øystein G. Hagemo at the Faculty of Natural
Sciences and Technology’s workshop (NTNU) for his great work in the design and manufacture of the tool,
Professor Nuria Espallargas at Department of Engineering Design and Materials (NTNU) for her helpful
advices, as well as the laboratory team for their support during testing.
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ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics - ISBN: 978-1-926872-25-4