Investigation into effect of scratch length and surface condition on Cerchar abrasivity index

Tunnelling and Underground Space Technology 60 (2016) 111–120

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Investigation into effect of scratch length and surface condition on Cerchar abrasivity index O. Yaralı ⇑, H. Duru Bülent Ecevit University, Mining Engineering Department, Zonguldak, Turkey

a r t i c l e

i n f o

Article history: Received 7 May 2015 Received in revised form 29 June 2016 Accepted 12 August 2016

Keywords: Cerchar abrasivity index Tool consumption West apparatus

a b s t r a c t The Cerchar abrasivity index (CAI) obtained from Cerchar abrasivity tests is an indicator of abrasiveness of rocks, and widely used for the estimation of bit/cutting tool life and wear rates in various mining and tunnelling applications. The effect of scratch length on CAI is investigated in this study by use of West apparatus and 6 different steel styluses with Rockwell Hardness of HRC40-42, HRC44-46, HRC48-50, HRC50-52, HRC54-56, and HRC58-60. The tests are carried out on 15 different rock samples (sedimentary, igneous, and metamorphic) on which rough and sawn cut surfaces with scratch lengths varying between 2 mm and 20 mm with an increment of 2 mm, thus resulting in total 27,000 scratches. It is observed that the CAI values between 85% and 93% are reached at the sliding distance of 10 mm while about the final CAI value of 99% is reached at 15 mm. It is also observed that the CAI values on rough surfaces are about 18% higher than those on sawn cut surfaces. Besides, it is determined that the most suitable surface condition in CAI test is sawn cut surfaces according to the coefficient of variation of CAI values in measurement depending on the stylus hardness and the measurement surface condition. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Abrasion is a process that causes removal or displacement of material at a solid surface; and will lead to wear, especially on tools used in mining, drilling, and tunnelling machines (Atkinson, 1993) such as conventional drilling and blasting, TBMs, roadheaders, continuous miners, drum shearers, planers or plows, bucket wheel excavators, dozers, backhoe excavators, or front end loaders. Even in the petroleum industry, extensive drilling is required to penetrate deep seated underground reservoirs (Majeed and Bakar, 2016). In one case study reported by Verhoef (1997), it was sometimes necessary to replace the cutting picks mounted on the cutterhead after every 30 min of use, even though the excavator was cutting a moderately weak calcarenite due to the fact that it gathered substantial quantity of quartz grains up to 1 cm size (Majeed and Bakar, 2016). The Cerchar abrasivity index was originally introduced by the CERCHAR Institute (Laboratoire du Centre d’Etudes et Recherches des Charbonnages de France) in France in the 1970s, and the results were published by Valantin (1973). Procedure of the Cerchar test consists of dragging/scratching a steel stylus of 2000 MPa tensile strength, hardened to Rockwell hardness ⇑ Corresponding author. E-mail address: [email protected] (O. Yaralı). http://dx.doi.org/10.1016/j.tust.2016.08.005 0886-7798/Ó 2016 Elsevier Ltd. All rights reserved.

HRC54-56, and sharpened to a 90° cone angle over a rock surface under a constant load of 70 N. There are mainly two types of Cerchar abrasivity test apparatus in use today (Fig. 1). The first one is the original Cerchar test device (Fig. 1a) which was suggested by the CERCHAR Institute (Valantin, 1973). The other test device was West apparatus (Fig. 1b) which was suggested by West (1989). The test is repeated several times (usually five scratches) either in various directions or in one direction using a fresh stylus for each repetition. The abrasiveness of rock is determined by the resultant wear flat generated at the tip of the stylus, measured in 0.1 mm. The inspection and determination of the wear flat is normally performed by use of a suitable microscope (Fig. 2). The unit of abrasiveness is defined as a wear flat of 0.1 mm which is equal to Cerchar abrasivity index of 1.0 (Plinninger et al., 2003). In the early 1980s, the publications by several authors evaluated the application and the merits of this test (Suana and Peters, 1982; West, 1986, 1989). The Colorado School of Mines (CSM) was the first to use this test in the United States in 1980s (Hamzaban et al., 2014). Other countries and researchers gradually adopted this system, and the years, a sizable data base of the test results was created. The original description of the testing procedure was offered in French Standard NFP94-430-1 (AFNOR, 2000). A standard for Cerchar abrasivity testing was offered by the ASTM (2010). Recently, the ISRM suggested a method for the

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Fig. 1. Outline of the commonly used Cerchar apparatuses. (a) Original Cerchar apparatus: 1, sample vice; 2, hand lever; 3, arm; 4, steel pin; 5, pin chuck; 6, weight. (b) West’s apparatus: 1, sample vice; 2, handcranck; 3, scale; 4, steel pin; 5, holder; 6, weight after (Plinninger et al., 2003).

Horizontal view

Vertical view

Fig. 2. Measurement of wear flat on stylus after testing.

Cerchar abrasivity testing was published by Alber et al. (ISRM, 2015). Many researchers investigated the effect of various factors on the CAI which are petrographic properties from thin section analyses (such as grain size, mineralogical composition, degree of cementation, quartz content, the equivalent quartz content, matrix properties), physical and geomechanical properties of rocks (such as P-wave velocity, porosity, density, strength, and young modulus) (Suana and Peters, 1982; Al-Ameen and Waller, 1994; West, 1986; Deketh, 1995; Plinninger et al., 2003; Mathier and Gisiger, 2003; Plinninger et al., 2004; Yaralı, 2005; Yaralı et al., 2008; Lassnig et al., 2008; Thuro and Kasling, 2009; Oparin and Tanaino, 2009; Khandelwal and Ranjith, 2010; Deliormanlı, 2012; Rostami et al., 2014; Hamzaban et al., 2014). Alber (2008) demonstrated that the CAI was definitely stress-dependent. A novel test procedure in Cerchar abrasivity tests was conducted on samples in a triaxial cell showed a stress dependency for various rock types with higher CAI values upon confining pressure. Kahraman et al. (2010) focused on the predictability of UCS and E values of Misis

Fault Breccia from some indirect methods including the CAI using the regression and artificial neural networks analysis. Other researchers conducted experiments with the stylus of different hardness and metallurgical properties. These researchers also emphasized that hardness of stylus effects the results of Cerchar abrasivity tests (Suana and Peters, 1982; CERCHAR, 1986; West, 1989; Al-Ameen and Waller, 1994; Plinninger et al., 2003; Rostami et al., 2005; Michalakopoulos et al., 2006; Yaralı et al., 2008; Stanford and Hagan, 2009; Ghasemi, 2010; Cardu et al., 2012; Yaralı et al., 2013b; Rostami et al., 2014; Duru, 2014). ASTM (2010) and ISRM (2015) are recommended to use a stylus of hardness HRC 55 ± 1. In some cases (e.g. when testing low abrasive rocks), other steel qualities might be preferred (ASTM, 2010). The Cerchar abrasivity index test is a simple test, and can be considered for field applications. However, there are some important differences with respect to testing procedures, type of test device, scratching speed, the surface condition of samples, operator skills, direction of scratch, hardness of stylus, metallurgical properties, and the evaluation of the stylus wear flat (Yaralı

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et al., 2013a). Rostami et al. (2005) sent the exact same set of rock specimens to different laboratories, but obtained different CAI values. They mentioned that differences in the results were mainly related to various equipment types, procedures, stylus and surface types, and finally the measurement methods. Recently, three classification systems are commonly used in research and practice (CERCHAR, 1986; ASTM, 2010; ISRM, 2015). The original CERCHAR (1986) guidelines, ASTM D7625-10 (2010) standard and ISRM suggested methods (ISRM, 2015) recommend only freshly broken rock surfaces for CERCHAR tests, which closely simulate the real field conditions, since cutting tools are always exposed to rough and broken rock surfaces. In the heterogeneous rock types such as conglomerates, coarse grained granite or schistose rock, suitable fresh test surfaces are not achieved by mechanical breaking using a hammer. In these cases, ASTM (2010) suggested that CAI values for smooth surfaces with a diamond saw are acceptable for use, but should be normalized by Eq. (1) before they can be used (Plinninger et al., 2003).

CAI ¼ 0:99CAIs þ 0:48

113

Fig. 3. Tip loss measured at different scratching lengths after (Al-Ameen and Waller, 1994).

ð1Þ

where CAI = Cerchar index for natural surface CAIs = Cerchar index for smooth (saw-cut) surface Some researchers performed their tests on rough surfaces (Al-Ameen and Waller, 1994; Plinninger et al., 2003, 2004; Rostami et al., 2005; Bakar, 2006; Thuro and Kasling, 2009; Gharahbagh et al., 2011; Duru, 2014) while others used sawn surfaces (Suana and Peters, 1982; West, 1989; Alber, 2008; Yaralı et al., 2008; Stanford and Hagan, 2009; Ghasemi, 2010; Rostami et al., 2014; Yaralı et al., 2013b; Duru, 2014). Al-Ameen and Waller (1994) mentioned that the surface finish had a minor effect on the results. In addition, Plinninger et al. (2003) confirmed this finding in the rocks with low CAI values. These researchers indicated that Cerchar tests on rough rock surfaces gave higher CAI values than sawn surfaces, and furthermore this difference was more pronounced when the rock was harder and more abrasive. According to the testing procedures outlined in the original Cerchar document (CERCHAR, 1986), and three standards for the Cerchar abrasivity test, the scratching distance on the rock sample are defined to be 10 mm. Al-Ameen and Waller (1994) performed some tests using various lengths (1, 2, 3, 5, 7 and 10 mm). They concluded that within the initial horizontal movement of the stylus (1 mm), the cone tip tends to deform and shear off due to the dead load and the resistance to horizontal movement, and a flat-ended tip was formed. This flat area formed at the beginning of the test was not dependent on the amount of abrasive material in the host rock but mainly on deformation and shear failure at the tip of the stylus (EN3 with 225 Vickers hardness), and its magnitude which is related to the rock strength and the stylus material. They also observed that the Cerchar index related to the sliding distance of 1 mm was about the two thirds of the final Cerchar index obtained over 10 mm of sliding distance for most of the rock types. Approximately 30% of the Cerchar index could be attributed to the abrasion effect which corresponds to the final the sliding distance of 9 mm (Fig. 3). They mentioned that the tip wear flat diameter generated during the remaining 9 mm of the testing distance could be related to a combination of the abrasive mineral content and the bond strength between the minerals in the rock (i.e. rock strength). Plinninger et al. (2003) performed a series of tests on identical rock samples with various testing lengths only to confirm the observations of Al-Ameen and Waller (1994). The results are illustrated in Fig. 4 in which the wear at the tips of the stylus continues. The similar situation for some rock types was also observed on the rock samples given in Fig. 3. Their observation asserted that about

Fig. 4. Effect of testing length on CAI (after Plinninger et al., 2003).

the 70% of the stylus wear occurred during the first millimetre of the testing length. About the 85% of the CAI is achieved after 2 mm of scratch, and only 15% of the change in CAI was achieved on the last 8 mm of the testing path. They also mentioned that the only positive impact of this finding was that deviations in the CAI obtained from the variation of scratch lengths would not be very significant when the variation in the testing length was kept around 10 ± 0.5 mm. Ghasemi, 2010 also mentioned that 10 mm of sliding distance was short and questionable. Yaralı et al. (2013a) carried out Cerchar tests on 6 different rock samples (3 types of sandstone, limestone, granite and quartzite) on rough and sawn cut surfaces in order to determine the effects of sliding length on CAI. They found that the 87% of the final Cerchar index was reached at the sliding distance of 10 mm while the 97% of the final Cerchar index was obtained at the 15 mm. They also suggested that it was suitable to use 15 mm scratch length instead of 10 mm. According to the current standards (CERCHAR, 1986; ASTM, 2010; ISRM, 2015), 10 mm scratch length is accepted in Cerchar tests without any rationale on selection of this scratch length. It is an important question for the Cerchar abrasivity test to predict the actual tool consumption for mechanical excavations. There are two basic aims of this study; the one is to investigate the effect of scratch length on the Cerchar abrasivity tests and to determine whether scratch length of 10 mm in CAI tests is the final sliding distance, or not. The other aim is to find out the effects of specimens surfaces conditions on the CAI value. A series of tests are carried out on 15 different rock types depending on 6 different stylus hardness sorts (HRC40-42,

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HRC44-46, HRC48-50, HRC50-52, HRC54-56, and HRC58-60) and different surface conditions (sawn cut and rough) for a better evaluation of the effect of scratch length and surface conditions on the CAI test values. The tests are repeated at least three times on each rock samples on rough and sawn cut surfaces with a scratch length between 2 mm and 20 mm with an increment of 2 mm. A total of 27,000 scratches are performed in the study. The effect of scratch length and surface condition on the Cerchar abrasivity index is investigated using simple regression analysis method and a coefficient of variation analysis.

types (HRC40-42, HRC44-46, HRC48-50, HRC50-52, HRC54-56, HRC58-60) are used in the tests. The rough surface sample is prepared in a way proposed in the indirect tensile strength by the Brazil test, and fresh surfaces of the residual pieces from the Brazilian test (ISRM, 1978) are used in the Cerchar test. The Cerchar abrasivity tests are carried out according to ISRM (2015). The list of the rock samples and the test parameters depending on stylus hardness and surface condition is given in Table 1. The hardness of all the stylus is checked in a calibrated Rockwell Hardness Tester before the testing procedure (Fig. 6). Prior to each test, the tip of the stylus with a tensile strength of 2000 MPa is sharpened to achieve a conical tip angle of 90°. The

2. Experimental studies In this study, in order to determine effects of scratch distance, the Cerchar abrasivity tests are carried out on 15 different rock samples (sedimentary, igneous, and metamorphic). The tests are performed on both rough and sawn cut rock surfaces with a scratch length between 2 mm and 20 mm, with a stepwise increment of 2 mm. The West apparatus (Fig. 5) and 6 different stylus hardness

Fig. 5. West apparatus used in this study.

Table 1 List of selected rocks and test conditions.

Fig. 6. Rockwell Hardness Tester.

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A total of 27,000 tests (scratches) are performed in this study (stylus with 6 different hardness ⁄ 2 surface conditions ⁄ 3 repetitions for each test ⁄ 5 scratches in each test ⁄ 10 scratching/sliding distances (scratch length between 2 mm and 20 mm with a stepwise increment of 2 mm) ⁄ 15 rock samples). 3. Evaluation of the results The results obtained from these experiments with varying scratch length between 2 mm and 20 mm with an increment of 2 mm are analysed separately depending on various stylus hardness types and surface conditions (sawn cut and rough), and summarized in Fig. 8. As seen in Fig. 8, the wear on testing stylus does not finish in 10 mm sliding path while the wear on testing stylus finalizes at around 16 mm sliding length and levels off. According to Table 2 and Fig. 8, about 60–70% of the final CAI (at a 20 mm sliding distance) is observed at the first 2 mm scratch length, and continues to increase with a declining rate up to 16 mm and levels off. It is also observed that about 85–90% of the final (total) CAI is achieved at 10 mm sliding distance while about 99% of the CAI is achieved at 15 mm sliding distance. Table 3 shows the overall results of the normalized (with respect to 10 mm) CAI values depending on stylus hardness and sample surface condition. Based on these results obtained from experiments, the scratch length in the Cerchar abrasivity test is recommended as 15 mm.

Fig. 7. Example samples, sawn cut and rough surfaces.

experiments are repeated three times for each rock type with five scratches for each test. An example of rock samples (sample size is 54 mm in diameter, 30 mm in height) used in the Cerchar tests is presented in Fig. 7. The stylus wear surface is examined with a digital microscope (magnification of x35) and the tip wear surface is captured with digital imaging software for the measurement. The stylus tip wear examination is carried both vertically and horizontally to minimize measurement errors, and averaged for the final CAI value (Fig. 2).

HRC=40/42, Smoot

5

5 4

3

CAI

CAI

4

2

3 2

1 0

HRC=40/42, Rough

6

1 0

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22

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HRC=44/46, Smoot

14

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CAI

CAI

12

5

4 3

3

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2

1

1 0

0 0

2

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Scratch Length (mm)

8

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Scratch Length (mm)

HRC=48/50, Smoot

HRC=48/50, Rough

6

6

5

5

4

CAI

4

CAI

10

HRC=44/46, Rough

6

5

3

3

2

2

1

1

0

8

Scratch Length (mm)

Scratch Length (mm)

0

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Scratch Length (mm)

16

18

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22

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0

2

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Scratch Length (mm)

Fig. 8. Summary results of the experiment depending various pin hardness and surface conditions (sawn cut and rough).

20

22

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HRC=50/52, Smoot

4

4

3

3

2

2

1 0

HRC=50/52, Rough

5

CAI

CAI

5

1 0

2

4

6

8

10

12

14

16

18

20

0

22

0

2

4

6

8

Scratch Length (mm) HRC=54/56, Smoot

5

CAI

CAI

16

18

20

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3 2

1

1 0

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Scratch Length (mm)

8

10

12

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20

22

16

18

20

22

Scratch Length (mm)

HRC=58/60, Smoot

5

HRC=58/60, Rough

6 5

4

4

3

CAI

CAI

14

5

3

2

3 2

1 0

12

HRC=54/56, Rough

6

4

0

10

Scratch Length (mm)

1 0

2

4

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12

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16

Scratch Length (mm)

18

20

22

0

0

2

4

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14

Scratch Length (mm)

A1 :Fine Grain Sandstone, A2 :Medium Grain Sandstone, A3 :Coarse Grain Sandstone, A4 :Limestone, A5 :Dolomitic Limestone,A6 :Quartz Sandstone, B1 :Granite, B2 :Diabase, B3:Porphyritic Basalt Andesite, B4 :Porphyritic Andesite, B5 :Basalt, C1 :Quartzite, C2 :Quartz-Schist, C3 :Marble, C4 :Metadiabase Fig. 8 (continued)

A regression analysis is also carried out to investigate the relationship between the CAI values of 10 mm and 15 mm scratch length (Fig. 9). It is indicated that there is a linear relation (Eq. (2)) with very strong correlation coefficient (R2 = 0.982) between the CAI values obtained from 10 mm to 15 mm sliding distances. From this relationship, the equivalent CAI values for the 15 mm scratch length can be estimated from the 10 mm scratch length.

CAI15mm ¼ 1:0442CAI10mm þ 0:11199

ð2Þ

where CAI15mm = equivalent CAI values for 15 mm scratch length CAI10mm = measured CAI values at 10 mm scratch length The test results indicate that the mean CAI values on sawn cut surface are generally lower than those on rough surface (Table 2). A good linear relationship is obtained between the rough and sawn cut surface CAI values. According to these results, the CAI values on rough surface is about 18% higher than the CAI values

on sawn cut surface (Fig. 10) which confirms the observations of Plinninger et al. (2003), Rostami et al. (2005), and Rostami et al. (2014). The coefficient of variation (CV%) of CAI measurements depends on stylus hardness and the surface condition of various samples are shown in Fig. 11. The overall results for all rocks show that the CV for the CAI values on sawn cut surface ranges from 21.22 to 29.30 with a minimum value at stylus hardness HRC 54/56 while for rough surface it ranges from 30.02 to 38.69 with a minimum value at the same stylus hardness (HRC 54/56). This result also indicates that the lowest variation in the measurements of CAI depending on the stylus hardness and surface condition is achieved at the sawn cut samples surface. In statistics, the coefficient of variation (CV), (or relative standard deviation), is a standardized measure of dispersion of data. It is often expressed as a percentage, and is defined as the ratio of the standard deviation to the mean. It is widely used to express the precision and repeatability of an assay.

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O. Yaralı, H. Duru / Tunnelling and Underground Space Technology 60 (2016) 111–120 Table 2 The results of CAI values and the value of percentages abrasiveness for different scratch length for selected rocks. Sample surface

Scratch length (mm) 2

Metamorphic rocks Sawn-cut Rough Smooth and rough Sedimentary rocks Sawn-cut Rough Smooth and rough Igneous rocks Sawn-cut Rough Smooth and rough

15a

10

20

(CAI)

(%)

(CAI)

(%)

(CAI)

(%)

(CAI)

SD Avg. SD Avg. SD Avg.

0.70 2.27 0.93 2.14 0.81 2.21

7.69 78.42 6.44 65.37 6.57 71.80

0.93 2.75 1.45 3.09 1.18 2.92

3.29 93.50 1.00 93.81 1.54 93.74

1.03 2.93 1.52 3.26 1.27 3.10

0.63 99.10 0.22 99.19 0.38 99.16

1.05 2.96 1.53 3.29 1.29 3.13

SD Avg. SD Avg. SD Avg.

0.26 1.45 0.40 1.45 0.32 1.45

6.17 63.01 8.48 58.60 7.21 60.63

0.49 2.01 0.72 2.16 0.59 2.09

4.18 86.37 4.28 84.99 4.04 85.62

0.56 2.28 0.83 2.48 0.68 2.38

0.54 97.79 0.45 97.70 0.31 97.72

0.57 2.33 0.86 2.54 0.70 2.43

SD Avg. SD Avg. SD Avg.

0.17 2.38 0.46 2.37 0.81 2.20

6.83 73.57 6.53 64.44 6.57 71.80

0.34 3.06 0.38 3.32 1.18 2.92

0.65 94.23 2.32 90.83 1.54 93.73

0.37 3.22 0.42 3.60 1.27 3.10

0.20 99.26 0.62 98.44 0.38 99.16

0.37 3.25 0.43 3.66 1.28 3.12

SD: Standard Deviation; Avg.: Average. a Average of 14 and 16 mm scratch length.

Table 3 Normalized CAI with respect to CAI values (in percent) obtained in 10 mm scratch length. Scratch length (mm)

2 4 6 8 10 12 14 15 16 18 20 Scratch length (mm)

2 4 6 8 10 12 14 15 16 18 20

HRC40/42

HRC44/46

HRC48/50

Sawn cut

Rough

Sawn cut

Rough

Sawn cut

Rough

77.70 84.81 90.13 95.35 100.00 104.14 107.18 108.41 109.65 109.65 109.65

67.23 77.58 87.41 93.52 100.00 104.93 107.89 110.00 112.11 112.11 112.11

77.90 85.25 91.20 95.69 100.00 104.09 106.83 108.15 109.46 109.46 109.46

68.89 79.83 87.54 94.92 100.00 104.08 107.85 109.27 110.69 110.69 110.69

78.25 85.68 90.75 96.08 100.00 104.06 106.57 108.04 109.52 109.52 109.52

67.11 77.85 88.23 94.57 100.00 104.39 107.85 109.74 111.62 111.62 111.62

Sawn cut

Rough

Sawn cut

Rough

Sawn cut

Rough

75.84 84.02 90.14 94.76 100.00 103.76 106.47 107.75 109.03 109.03 109.03

71.11 81.31 90.47 95.18 100.00 104.63 108.69 109.94 111.19 111.19 111.19

76.98 83.65 88.78 94.25 100.00 104.54 107.76 109.58 111.39 111.39 111.39

71.24 81.34 89.32 95.09 100.00 103.26 105.66 107.60 109.55 109.55 109.55

77.82 85.01 90.55 95.37 100.00 103.93 107.18 108.66 110.14 110.14 110.14

71.09 80.02 87.62 93.60 100.00 105.08 108.98 111.17 113.36 113.36 113.36

HRC50/52

HRC54/56

4. Discussion In this study, it is observed that the wear at the tip of the stylus in CAI with 10 mm sliding length does not finish due to the interaction with the rock surface and the tip of the stylus. This outcome is very important for mechanized excavation projects in mining and civil construction industries, especially in hard and very abrasive rocks. Since a huge portion of excavation budget is spent on repair and costly replacement of rock cutting tools (Fowell and Bakar, 2007; Hamzaban et al., 2014). It is also noted

HRC58/60

that not only is tool wear a problem, but also other machine components in contact with rock during excavation have a tendency to experience wear, which results in expensive component replacements and time loss (Fowell and Bakar, 2007). This is very important for researchers and practisers to predict real tool consumption. Since, small increase in the CAI value of the rock will lead to larger tool consumption in the tunnel excavation. Information on pick consumption is given below by using a model accounting for the specific pick consumption as function of UCS and CAI (see Fig. 12).

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6.0 5.0

y = 1.0442x + 0.1199 R2 = 0.9825

CAI -15 mm

4.0 3.0 2.0 1.0 0.0 0.0

1.0

2.0

3.0

4.0

5.0

6.0

CAI -10 mm Fig. 9. The relationship between CAI values of 10 mm and 15 m scratch length.

ALL ROCK SAMPLES

6

CAI rough surface

5

y = 1.1683x - 0.2186 R2 = 0.8096

4 3 2 1 0

0

0.5

1

1.5

2

2.5

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3.5

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CAI sawn-cut surface

45 38.69 31.24

30 25

30.75

27.95

26.74

24.72

22.97

32.75

30.20

30.02

29.30

HRC58/60 Sawn-cut

35

HRC54/56 Rough

40

21.22

20 15 10

HRC58/60 Rough

HRC54/56 Sawn-cut

HRC50/52 Rough

HRC50/52 Sawn-cut

HRC48/50 Rough

HRC48/50 Sawn-cut

HRC44/46 Rough

HRC44/46 Sawn-cut

0

HRC40/42 Rough

5 HRC40/42 Sawn-cut

Coefficient of Variation (CV, %)

Fig. 10. Relationship between CAI values of sawn-cut surfaces and CAI values of rough surfaces.

Pin Hardness (HRC) Fig. 11. Coefficient of variation of CAI values measurement depends on pin hardness and surface condition of various samples.

For a development gate of 4 m  4 m around a longwall of dimension 3000 m  300 m a volume of 105,600 m3 have to be excavated. Pick consumption doubles to (see Fig. 12): 0.026 picks/solid m3 CAI = 1.0 Pick consumption is 2746 0.050 picks/solid m3 CAI = 1.3 Pick consumption is 5280 Pick consumption is 2746 instead of assumed 5280 (€ 14 per pick) Wrong estimate: € 35, 76 The abrasiveness classification systems are suggested with the help of Eq. (1) for both sawn-cut surface and rough surface as listed

in Table 4 and in Table 5. These classification systems are based on the measured CAI at 10 mm sliding length or the equivalent CAI calculated for 15 mm of sliding length based on stylus having a Rockwell Hardness HRC 55 ± 1. Based on the results from the experiments, the scratch distance should be considered to increase from 10 mm to 15 mm in the Cerchar abrasivity test especially for the abrasiveness classification for harder and more abrasive rocks to predict tool consumption. However, since the CAI10mm and CAI15mm are highly correlated, and all the cutting tool prediction databases are based on CAI10mm, the abrasivity classification given in this study can only be used after collecting a large data of tool wear and CAI15mm test results.

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Fig. 12. Specific pick consumption as function of UCS and CAI (Restner and Pichler, 2007).

Table 4 Classification of CAI for smooth or sawn-cut surface condition. Classification

CAI (10 mm) (after CERCHAR, 1986)

CAI (15 mm)

Not very abrasive Slightly abrasive Medium abrasiveness to abrasive Very abrasive Extremely abrasive

0.3–0.5 0.5–1.0 1.0–2.0 2.0–4.0 4.0–6.0

0.30–0.65 0.66–1.20 1.21–2.25 2.26–4.40 4.41–6.50

Table 5 Classification of CAI for rough surface condition. Classification

CAI (10 mm) (after ISRM, 2015)

CAI (15 mm)

Extremely low Very low Low Medium High Very high Extremely high

0.1–0.4 0.5–0.9 1.0–1.9 2.0–2.9 3.0–3.9 4.0–4.9 P5

0.10–0.60 0.61–1.10 1.11–2.30 2.31–3.40 3.41–4.50 4.51–5.50 P5.51

5. Conclusions In the first section of this study, the influence of scratch distance and rock surface conditions was examined on the results of Cerchar abrasivity test to validate the earlier research investigations. The tests were conducted on fifteen rock samples using West apparatus depending on 6 different styluses with a Rockwell Hardness of HRC40-42, HRC44-46, HRC48-50, HRC50-52, HRC54-56, and HRC58-60, and different surface conditions (sawn cut and rough). The tip wear surface values for both sawn cut and broken rock surfaces according to Brazilian test were measured by using a digital microscope (magnification of 35), and the stylus tip wear examination was carried both vertically and horizontally to minimize measurement errors, and averaged for the final CAI. In the second part of this research, the results obtained from the experiments

were investigated using the simple regression analysis method and the coefficient of variation analysis. The following are the main conclusions obtained from the research: 1. Experimental studies indicate that about 60–70% of the final Cerchar abrasivity index (at a 20 mm sliding) is observed at the first 2 mm scratch and continues to increase up to 16 mm, and then it levels off. This premature high tip wear may partially depend on the deformation and shear failure at the tip of stylus due to high stress concentration. It is also observed that 85–93% of stylus wear is obtained at the sliding distance of 10 mm while about 99% of the CAI is reached at the 15 mm. It is found out that there is a linear relation and a good fit (R2 = 0.982) between the CAI values obtained from 10 mm to 15 mm sliding distances. The new abrasiveness classification systems for CAI obtained at the 15 mm scratch distance is suggested for both sawn-cut surface and rough surface. 2. The effects of surface condition on the CAI values were investigated and found a linear relation between the rough and sawn cut surface CAI values. It is observed that the CAI values on rough surface were about 18% higher than those on sawn cut surface. These results are in good agreement with the conclusions of earlier studies carried out by Plinninger et al. (2003), Rostami et al. (2005), Ghasemi (2010), Yaralı et al. (2013b), Rostami et al. (2014), Duru (2014). It is also observed that the CAI values on rough surface were about 18% higher than those on sawn cut surface. The overall results for all rocks showed that the CAI values on sawn cut surface varied between 1.66 and 4.33 with a coefficient of variation (CV%) between 21.22 and 29.30 with a minimum value at stylus hardness HRC54/56. On the other hand, the CAI values on rough surface ranged from 0.62 to 4.68 with a coefficient of variation (CV%) between 30.02 and 38.69 with a minimum value at stylus hardness HRC54/56. This result also indicates that the lowest dispersion in measurements of CAI depending on stylus hardness and surface condition is obtained at sawn cut surfaces and stylus hardness of HRC 54 ± 1.

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3. Based on the results obtained from the experiments, the scratch distance should be considered to increase from 10 mm to 15 mm in Cerchar abrasivity test especially for the abrasiveness classification for harder and more abrasive rocks to predict tool consumption. However, since the CAI10 and CAI15 are highly correlated and all the cutting tool prediction databases are based on CAI10, the abrasivity classification given in this study can only be used after collecting a large data of tool wear and CAI15 test results. It is also strongly recommended to employ only styli tempered to Rockwell Hardness HRC 54 ± 1 as suggested by the standards ASTM (2010) and ISRM (2015), and the test should be carried out on sawn cut samples surface to obtain more reliable results.

Acknowledgements This study was supported by TUBITAK (The Scientific and Technological Research Council of Turkey) under the project number of 110M579. The authors also gratefully acknowledge Bülent Ecevit Üniversitesi (the project number of 2012-17-11-04). We acknowledge the useful and constructive comments of Assoc. Prof. Hamit Aydın (Bülent Ecevit University, Turkey), Prof. Sair Kahraman _ (Hacettepe University, Turkey), Prof. Hanifi Çopur (Istanbul Technical University, Turkey), and Assoc. Prof. Çetin Mekik (Bülent Ecevit University, Turkey). Further, we would like to extend our thanks to the reviewers of this article for their valuable observations and suggestions to enhance the quality of this work. References Al-Ameen, S.I., Waller, M.D., 1994. The influence of rock strength and abrasive mineral content on the Cerchar abrasive index. Eng. Geol. 4 (36), 293–301. Alber, M., 2008. Stress dependency of the Cerchar abrasivity index (CAI) and its effects on wear of selected rock cutting tools. Tunn. Undergr. Space Technol. 23 (4), 351–359. AFNOR, 2000. Roches D’etermination du pouvoir abrasive d’uneroche Partie 1: Essai de rayure avec une pointe. (NF P 94-430-1). Paris. Atkinson, R.H., 1993. Hardness tests for rock characterization. In: Hudson, J.A. (Ed.), Comprehensive Rock Engineering, vol. 3. Pergamon Press, New York (Chapter 5). ASTM D7625-10 ASTM, 2010. Standard Test Method for Laboratory Determination of Abrasiveness of Rock Using the CERCHAR Method. ASTM International, United State. Bakar, A.M.Z., 2006. A Critical Review of Rock Abrasivity Measurement Methods M. Sc. Dissertation. University of Leeds. Cardu, M., Giraudi, A., Rocca, V., Verga, F., 2012. Experimental laboratory tests focused on rock characterisation for mechanical excavation. Int. J. Min. Reclam. Environ. 26 (3), 199–216. CERCHAR, 1986. Centre d’ Etudes et Recherches de Charbonnages de France. The Cerchar abrasiveness index. Deketh, H.J.R., 1995. Wear of Rock Cutting Tools. Laboratory Experiments on the Abrasivity of Rock, first ed. A.A. Balkema, Rotterdam. Duru, H., 2014. Evaluation of Effect of Parameters on the Cerchar Abrasivity Indeks Test MSc thesis. Institute of Science and Technology, Department of Mining Engineering, Bülent Ecevit University, Zonguldak, Turkey. Deliormanlı, A.H., 2012. Cerchar abrasivity index (CAI) and its relation to strength and abrasion test methods for marble stones. Constr. Build. Mater. 30, 16–21. Fowell, R.J., Bakar, A.M.Z., 2007. A review of the Cerchar and LCPC rock abrasivity measurement methods. In: Proceeding of the 11th Congress of the International Society for Rock Mechanics, pp. 155–160. Gharahbagh, E.A., Rostami, J., Ghasemi, A.R., Tonon, F., 2011. Review of rock abrasion testing. In: Proceeding of the 45th US Rock Mechanics/Geomechanics Symposium, pp. 11–141. Ghasemi, A., 2010. Study of Cerchar Abrasivity Index and Potential Modifications for more Consistent Measurement of Rock Abrasion MS A Thesis in Energy and Mineral Engineering. The Pennsylvania State University. Department of Energy and Mineral Engineering, USA.

Hamzaban, M.T., Memarian, H., Rostami, J., 2014. Continuous monitoring of pin tip wear and penetration into rock surface using a new Cerchar abrasivity testing device. Rock Mech. Rock Eng. 47 (2), 689–701. ISRM, 1978. Suggested method for determining tensile strength of rock materials. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 15, 99–103. ISRM, 2015. Alber. M., Yaralı, O., Dahl, F., Bruland, A., Kasling, H., Michalakopoulos, T.N., Cardu, M., Hagan, P., Aydın, H., Özaraslan, A. ISRM Suggested method for determining the abrasivity of rock by the Cerchar abrasivity test. The ISRM Suggested Methods for Rock Characterization, Testing and Monitoring. 20072014, PART I. 101-107 Kahraman, S., Alber, M., Fener, M., Günaydin, O., 2010. The usability of Cerchar abrasivity index for the prediction of UCS and E of Misis Fault Breccia: Regression and artificial neural networks analysis. Expert Syst. Appl. 37 (12), 8750–8756. Khandelwal, M., Ranjith, P.G., 2010. Correlating index properties of rocks with Pwave measurements. J. Appl. Geophys. 71 (1), 1–5. Lassnig, K., Latal, C., Klima, K., 2008. Impact of grain size on the Cerchar abrasiveness test. Geomech. and Tunn. 1 (1), 71–76. Majeed, Y., Bakar, A.M.Z., 2016. Statistical evaluation of CERCHAR Abrasivity Index (CAI) measurement methods and dependence on petrographic and mechanical properties of selected rocks of Pakistan. Bull. Eng. Geol. Environ. http://dx.doi. org/10.1007/s10064-015-0799-5. Mathier, J.F., Gisiger, J.P., 2003. Abrasivity of Icelandic basalts. In: Proceeding of ISRM 2003-Technology Roadmap for Mechanics. South African Institute of Mining and Metallurgy, pp. 809–812. Michalakopoulos, T.N., Anonostou, V.G., Bassanou, M.E., Panagiotou, G.N., 2006. The influence of steel stylus hardness on Cerchar abrasiveness index value. Int. J. Rock Mech. Min. Sci. 43, 321–328. Oparin, V.N., Tanaino, A.S., 2009. Assessment of abrasivity by physico-mechanical properties of rocks. J. Min. Sci. 45 (3), 240–249. Plinninger, R., Kasling, H., Thuro, K., Spaun, G., 2003. Testing conditions and geomechanical properties influencing the Cerchar abrasiveness index (CAI) value Technical Note. Int. J. Rock Mech. Min. Sci. 40, 259–263. Plinninger, R., Kasling, H., Thuro, K., 2004. Wear prediction in hardrock excavation using the Cerchar abrasiveness index (CAI) value. In: Proceeding of the ISRM Regional Symposium EUROCK 2004 & 53rd Geomechanics Colloquium. Essen. Germany, pp. 599–604. Restner, U., Pichler, J., 2007. Mont Cenis Tunnel modification project-lowering of tunnel invert with SANDVIK tunnel miner MT620. In: Bilgin, N., Çopur, H., Balcı, C., Yüce, E. (Eds.), Proceedings of the 2th Symposium on Underground _ Excavations for Transportation, Istanbul, Turkey, pp. 191–197. Rostami, J., Özdemir, L., Bruland, A., Dahl, F., 2005. Review of issues related to Cerchar abrasivity testing and their implications on geotechnical investigations and cutter cost estimates. In: Proceeding of Rapid Excavation and Tunnelling Conference (RETC). Seattle, pp. 15–29. Rostami, J., Ghasemi, A., Gharahbagh, E.A., Dogruoz, C., Dahl, F., 2014. Study of dominant factors affecting Cerchar abrasivity index. Rock Mech. Rock Eng. 47, 1905–1919. Stanford, J., Hagan, P., 2009. An assessment of the impact of stylus metallurgy on the Cerchar abrasiveness index value. In: Coal Operators’ Conference. University of Wollongong, Autralia, pp. 347–355. Suana, M., Peters, T., 1982. The Cerchar abrasivity index and its relation to rock mineralogy and petrography. Rock Mech. 15 (1), 1–6. Thuro, K., Kasling, H., 2009. Classification of the abrasiveness of soil and rock. Geomech. Tunn. 2 (2), 179–188. Valantin, A., 1973. Examen des differens procedes classiques de determination de la nocivite des roches vis-à-vis de l’attaque mecaniqe, pp. 133–140. Verhoef, P.N.W., 1997. Wear of Rock Cutting Tools – Implications for the Site Investigation of Rock Dredging Projects. A.A. Balkema, Rotterdam. West, G., 1986. A relation between abrasiveness and quartz content for some coal measures sediments. Int. J. Min. Geol. Eng. 4, 73–78. West, G., 1989. Rock abrasiveness testing for tunneling Technical Note. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 26 (2), 151–160. Yaralı, O. 2005. Zonguldak Tasßkömür Havzası kömür çevre kayaçlarinin asßindiricilik özelliklerinin arasßtirilmasi (in Turkish). Türkiye Uluslararası 19. Madencilik _ Kongresi Kitabı. TMMOB Maden Müh. Odası Yayını: Izmir. Turkey:243-251. (The 19th International Mining Congress and Fair of Turkey, June 9–12, 2005, _ Izmir, Turkey, Chamber of Mining Engineers of Turkey) Yaralı, O., Yasar, E., Bacak, G., Ranjith, P.G., 2008. A study of rock abrasivity and tool wear in coal measures rocks. Int. J. Coal Geol. 74 (1), 53–66. Yaralı, O., Aydın, H., Duru, H., Özaraslan, A., 2013a. Effect of scratch length on the Cerchar abrasivity index. In: Kwasniewski, M., Lydzba, D. (Eds.), Proceeding of EUROCK 2013 the 2013 ISRM International Symposium, Wroclaw, Poland, pp. 369–378. Yaralı, O., Aydın, H., Duru, H., Özaraslan, A., 2013b. Development of standard test method for Cerchar abrasıvıty index Final Report. TUBITAK Project, Zonguldak, Turkey.