Evaluation of simple methods for assessing the ...

International Journal of Rock Mechanics & Mining Sciences 38 (2001) 981–994

Evaluation of simple methods for assessing the uniaxial compressive strength of rock S. Kahraman* Faculty of Engineering and Architecture, University of Nigde, 5100 Nigde, Turkey Accepted 13 July 2001

Abstract Published data on 48 different rocks are used to evaluate the correlations between the uniaxial compressive strength (UCS) values and the corresponding results of point load, Schmidt hammer, sound velocity and impact strength tests. The variability of test results for each test and each rock type was evaluated by calculating the coefficient of variation. Using the method of least squares regression, the UCS values were correlated with the other test values. Also, the test methods were evaluated by plotting the estimated values of compressive strength vs. the measured values of compressive strength for each test. The results indicate that the least variability is shown in the impact strength test. So, among the test methods included in this study, the impact strength test is the most reproducible test; but the variability of test results for the other test methods is within acceptable limits for most engineering purposes. Strong linear relations between the point load strength index values and the UCS values were found for the coal measure rocks and the other rocks included in this study. The Schmidt hammer and the sound velocity tests exhibit significant non-linear correlations with the compressive strength of rock. In the sound velocity test, the data points are scattered at higher strength values. There is no clear relation between the impact strength values and the compressive strength values for the coal measure rocks. A weak non-linear correlation was found between the impact strength values and the compressive strength values for the other rocks. All test methods evaluated in this study, except the impact strength, provide reliable estimate of the compressive strength of rock. However, the prediction equations derived by different researchers are dependent on rock types and test conditions, as they are in this study. r 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction Rock engineers widely use the uniaxial compressive strength (UCS) of rock in designing surface and underground structures. The procedure for measuring this rock strength has been standardised by both the American Society for Testing and Materials (ASTM) [1] and the International Society for Rock Mechanics (ISRM) [2]. Although, the method is relatively simple, it is time consuming and expensive; also, it requires well prepared rock cores. Therefore, indirect tests are often used to predict the UCS, such as Schmidt rebound number, point load index, impact strength and sound velocity. These are easier tests to carry out because they necessitate less or no sample preparation and the testing equipment is less sophisticated. Also, they can be used easily in the field. As a result, compared to the uniaxial *Tel.: +90-388-225-0115; fax: +90-388-225-0112. E-mail address: [email protected] (S. Kahraman).

compression test, indirect tests are simpler, faster and more economical. The main objective of this study is to evaluate these simple methods of estimating the UCS of rock. The data in Refs. [3–5] were used to accomplish this objective. The result of uniaxial compression test carried out on 48 different rocks, of which 26 are coal measures rocks, were compared with the corresponding results of the point load, the Schmidt hammer, the sound velocity and the impact strength tests. To determine the correlation coefficient and the variability of results for each test, the data were statistically analysed.

2. Previous investigations 2.1. Point load test The point load test has often been reported as an indirect measure of the compressive or tensile strength

1365-1609/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 5 - 1 6 0 9 ( 0 1 ) 0 0 0 3 9 - 9

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S. Kahraman / International Journal of Rock Mechanics & Mining Sciences 38 (2001) 981–994

of rock [6–9]. It has been used widely in practice due to its testing ease, simplicity of specimen preparation, and field applications. D’Andrea et al. [6] performed uniaxial compression tests and the point load tests. They used a linear regression model to obtain the correlation between two tests. It should be noted that their point load specimen diameter was 25 mm, and that size adjustments must be made to use their relation. Broch and Franklin [8] state that the compressive strength is approximately equal to 24 times the point load index (Is ), referred to a standard size of 50 mm. They also developed a size correction chart so that core of various diameters could be used for strength determination. Bieniawski [9] showed that the compressive strength is nearly 23 times Is : Pells [10] showed that the index-to-strength conversion factor of 24 can lead to 20% error in the prediction of compressive strength for rocks such as dolerite, norite and pyroxenite. Greminger [11] and Forster [12] also showed that the conversion factor of 24 cannot be validly applied to anisotropic rocks. Hassani et al. [13] studied the point load test using an expended database with tests on large specimens and revised the size correlation chart commonly used to reference point load values from cores with differing diameters to the standard size of 50 mm. With this new correction, they found the ratio of compressive strength to Is50 to be approximately 29. Brook [14] emphasised the possible sources of error when using point load test, and proposed an analytical method of ‘‘size correction’’ to a chosen standard size. It is stated by ISRM [15] that on average, the compressive strength is 20–25 times Is : However, it is also reported that in tests on many different rock types the range varied between 15 and 50, especially for anisotropic rocks. So, errors up to 100% should be expected if an arbitrary ratio value is chosen to predict compressive strength from point load tests. Turk . and Dearman [16] have proposed some improvements in the determination of point load strength. They proposed a simple method for determining standard point load strength Is50 ; from test results obtained from a number of irregular, and regular prismatic specimens of different diameter using log–log plots of Is against diameter. This relation is usually linear. Chau and Wong [17] proposed a simple analytical formula for the calculation of the UCS based on the point load strength corrected to a specimen diameter of 50 mm Is50 ). The index-to strength conversion factor (k) relating UCS to Is50 depends on the compressive to tensile strength ratio, the Poisson’s ratio, the length and the diameter of the rock specimen. Their theoretical prediction for k (=14.9) is reasonably close to the experimental observation (k ¼ 12:5) for Hong Kong rocks. Table 1 lists the available equations correlating the UCS to the point load index.

Table 1 Equations correlating the UCS (qu ) to the point load index (Is )a Reference

Equation

D’Andrea et al. [6] Broch and Franklin [8] Bieniawski [9] Hassani et al. [13] Read et al. [18] (1) Sedimentary rocks (2) Basalts Forster (12) Gunsallus and Kulhawy [19] ISRM [15] Chargill and Shakoor [20] Chou and Wong [17] Grasso et al. [21]

qu qu qu qu

¼ 15:3Is50 þ 16:3 ¼ 24Is50 ¼ 23Is50 ¼ 29Is50

qu qu qu qu qu qu qu qu

¼ 16Is50 ¼ 20Is50 ¼ 14:5Is50 ¼ 16:5Is50þ 51:0 ¼ 20y25Is50 ¼ 23Is54þ 13 ¼ 12:5Is50 ¼ 9:30Is50þ 20:04

a

qu and Is in MPa.

2.2. Schmidt hammer test The Schmidt hammer has been used for testing the quality of concretes and rocks. Schmidt hammer models are designed in different levels of impact energy, but the types L and N are commonly adopted for rock property determinations. The type L has an impact energy of 0.735 Nm which is only one third that of the type N: . Ayday and Goktan [22] found reliable correlations between L and N-type hammer rebound values obtained during field testing. There are different Schmidt hammer recording . techniques in the literature. Ayday and Goktan [23] statistically compared the three most accepted methods (Hucka, Poole and Farmer, and ISRM methods) and concluded that the ISRM method was different from the other methods. While the Schmidt hammer is widely used for the prediction of UCS [24–28], a number of authors have reported its other specific applications. Among these are: the assessment of rock discontinuities [29], mine roof control [30], roadheader and tunnel boring machine performance [31], drilling machine penetration rate [32–35] and joint wall strength [36]. Various empirical equations have been proposed for calculating UCS of rock from Schmidt hammer rebound number. Most researchers have used similar approaches for deriving these equations, four of which are reported by Haramy and DeMarco [27]. Kidybinski (1980) evaluated the use of Schmidt hammer by testing different rock types from Northern Silesia. He observed a correlation between rebound number and UCS for rock and coal, and derived the following equation for estimating the strength of rock: qu ¼ 0:477 eð0:045Rn þrÞ ;

ð1Þ

where qu is the UCS (MPa), Rn is the Schmidt hammer rebound number and r is the rock density (g/cm3).


Aufmuth (1973) acquired Schmidt hammer data from approximately 800 core samples, representing 168 geologic formation and 25 lithologic types. Four rebound readings were taken at different locations along the centre axis of the core. The following equation describes the best-fit approximation relating compressive strength to Schmidt hammer rebound number. qu ¼ 6:910½1:348logðRn rÞ1:325

ð2Þ

where qu is the UCS (MPa), Rn is the Schmidt hammer rebound number and r is the rock density (g/cm3). Deere and Miller (1966) tested 55 mm diameter core from 28 different locations. Twelve rebound readings were recorded along the length of the core for each 901 rotation. The best-fit approximation for compressive strength is as follows: qu ¼ 6:910½0:16þ0:0087ðRn rÞ ;

ð3Þ

where qu is the UCS (MPa), Rn is the Schmidt hammer rebound number and r is the rock density (g/cm3). Beverly et al. (1979) used the same test procedures as Deere and Miller to obtain additional Schmidt hammer data from 20 new locations. They combined their data with that of Deere and Miller and derived the following relation: qu ¼ 12:74 e½0:0185ðRn rÞ ;

ð4Þ

where qu is the UCS (MPa), Rn is the Schmidt hammer rebound number and r is the rock density (g/cm3). Haramy and DeMarco [27] conducted Schmidt hammer (L-type) test using large coal blocks acquired from 10 different US locations. They obtained the following best-fit equation: qu ¼ 0:094Rn 0:383;

ð5Þ

where qu is the UCS (MPa) and Rn is the Schmidt hammer rebound number. Sheorey et al. [28] found a reasonable correlation between the large-scale in situ crushing strength of 0.3 m cubes of coal and the lower mean of rebound values (N-type). The following equation is proposed for the in situ crushing strength of coal: qu ¼ 0:4RLM 3:6;

ð6Þ

where qu is the in situ crushing strength of coal (MPa) and RLM is the lower mean of rebound values. Cargill and Shakoor [20] performed the Schmidt hammer (L type) tests on rock cores (NX) and derived following equations.

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2.3. Sound velocity test Seismic surveys have been carried out in site and laboratory investigations. Attempts have been made to assess grouting, rockbolt reinforcement and blasting efficiencies in the rock mass by the seismic velocity determination [37–39]. Researchers [40–46] have examined the relation between rock properties and sound velocity; they found that sound velocity is closely related with rock properties. Inoue and Ohomi [47] tested many soft rocks in order to confirm the relations among uniaxial compressive strength, propagation velocity of elastic waves and density. They expressed the following general formula: qu ¼ krVp2 þ A;

ð9Þ

where qu is the UCS (kg/cm2), r is the rock density (g/cm3) and Vp is the p-wave velocity (km/s). . Goktan [48] derived the following equation for coal measure rocks: qu ¼ 0:036Vp 31:18;

ð10Þ

where qu is the UCS (MPa) and Vp is the p-wave velocity (m/s). 2.4. Impact strength test The impact strength test was first developed by Protodyakonov, and then it was used by Evans and Pomeroy [49] for the classification of coal seams in the UK. The test was then modified by Paone et al. [50], Tandanand and Unger [51], and Rabia and Brook [52]. Tandanand and Unger obtained simple relations between the strength coefficient and compressive strength. Rabia and Brook used the modified test apparatus to determine the rock impact hardness number and developed an empirical equation for predicting drilling rates for both DTH and drifter drills. Hobbs [53] applied this test to various rocks and found the following equation: qu ¼ 53ISI 2509;

ð11Þ

ln qu ¼ 4:3102 ðRn rd Þ þ 1:2

for sandstones;

ð7Þ

where qu is the UCS (kg/cm2) and ISI is the impact strength index. To estimate the compressive strength from the impact . strength index Goktan [48] derived the following expression:

ln qu ¼ 1:8102 ðRn rd Þ þ 2:9

for carbonates;

ð8Þ

ln qu ¼ 0:095ISI 3:667;

where qu is the UCS (MPa), Rn is the Schmidt hammer rebound number and rd is the dry density (g/cm3).

ð12Þ

where qu is the UCS (MPa) and ISI is the impact strength index.

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3. Statistical analysis The coefficients of variation (CoV) were determined to evaluate the variability of test results for each test and each rock type. The CoV is calculated by dividing the standard deviation by the population mean and expressing it as a percentage. The higher

the CoV, the more variable are the results of a given test. The USC values were correlated with the other test values using the method of least squares regression. The equation of the best-fit line, the 95% confidence limits, and the correlation coefficient (r) were determined for each regression.

Table 2 Results of the uniaxial compression test [3,4] Location/panel

Rock type

Compressive strength (MPa)

Standard deviation (MPa)

Coefficient of variation (%)

Osmaniye/Bahçe Osmaniye/Bahçe Osmaniye/Bahçe Osmaniye/Bahçe Gaziantep/Erikli Gaziantep/Erikli Gaziantep/Erikli Gaziantep/Erikli Pozanti Pozanti Yahyali Yahyali Konya Adana Misis Emet Emet Tarsus Mersin Ceyhan Ceyhan Yumurtalik

Dolomite Sandstone-1 Sandstone-2 Altered sandstone Limestone Marl Diabase Serpentine Limestone Clayed limestone Hematite Metasandstone Serpentine Limestone Limestone Sandstone Limestone Dolomite Limestone Limestone Gravelled limestone Limestone

68.0 149.2 45.2 20.1 51.3 39.5 110.9 69.1 123.8 45.1 61.8 25.7 54.3 15.7 85.2 70.5 42.1 96.3 49.9 76.1 36.1 68.4

6.01 1.52 2.33 0.92 3.03 0.75 6.04 2.20 3.81 1.71 3.52 0.90 2.41 0.53 2.10 1.12 2.63 1.10 5.44 1.03 0.72 0.80

8.91 1.02 5.10 4.62 5.90 1.73 5.41 3.20 3.10 3.71 5.73 3.41 4.40 3.32 2.51 1.60 6.23 1.13 10.81 1.30 1.92 1.13

Marl Limestone Marl Marl Marl Marl Limestone Marl Altered marl Marl Marl Marl Marl Siliceous limestone Marl Marl Marl Marl Tuff Sandy marl banded with tuff Clayed marl Marl-limestone Limestone Siliceous marl Clayed marl Marl

64.9 77.5 82.4 80.2 69.2 52.1 66.6 17.9 8.0 21.6 22.0 13.5 49.3 152.7 41.8 38.7 21.4 45.5 10.1 40.4

0.82 2.93 0.81 3.11 0.92 1.24 1.12 0.40 1.62 0.41 0.12 1.01 0.73 2.21 0.32 0.70 1.22 0.43 0.51 2.10

1.21 3.70 1.01 3.92 1.32 2.43 1.70 2.51 2.02 1.73 0.71 7.10 1.52 1.53 0.65 1.94 5.63 0.84 5.01 5.12

10.5 61.5 91.2 4.4 7.9 10.5

0.22 1.51 0.43 0.52 0.51 0.52

2.23 2.51 0.40 12.41 6.22 4.73

Coal measure rocks Soma/Isiklar Soma/Isiklar Soma/Kisrakdere Soma/Elmali Soma/Sarikaya Tinaz/Bagyaka Tinaz/Bagyaka Eskihisar Eskihisar Milas/Sekky Milas/Ikizky Tunçbilek/Beke Tunçbilek/12A Tunçbilek/12A Tunçbilek/merler 4CD Tunçbilek/37 Tunçbilek/36 Orhaneli Orhaneli Orhaneli Keles Keles Keles Seyitmer Seyitmer Seyitmer


3.1. Uniaxial compressive strength test The average values of the UCS are listed in Table 2. It is reported by both Kahraman [3] and Eskikaya and Bilgin [4] that uniaxial compression tests were performed on trimmed core samples, which had a diameter of 33 mm and a length-to-diameter ratio of 2. The UCS . values range from 4.4 MPa for the Seyitomer siliceous marl to 152.7 MPa for the Tunçbilek/12A siliceous

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limestone. The CoV ranges from 0.40% for the Keles . limestone to 12.41% for the Seyitomer siliceous marl with an overall average of 3.46%. 3.2. Point load test The point load strength values are given in Table 3. It is reported by Kahraman [3] that the diametral point load test was carried out on the cores having a diameter

Table 3 Results of the point load test [3,4] Location/Panel

Rock type

Point load strength (MPa)





4.32 13.83 4.57 1.32 5.61 3.35 12.66 7.14 6.65 5.73 8.26 5.25 16.21 1.40 9.80 7.75 5.44 12.01 3.31 8.82 3.11 7.00

0.68 0.73 1.17 0.31 0.59 0.45 0.85 0.76 1.02 0.76 0.42 0.54 0.82 0.38 1.25 0.65 0.57 0.43 0.40 0.79 0.62 0.90

15.75 5.29 21.04 17.93 10.45 13.65 6.74 10.64 15.39 13.25 5.14 10.49 5.06 21.59 12.70 8.43 10.60 3.61 12.09 8.98 19.78 12.89

Marl Limestone Marl Marl Marl Marl Limestone Marl Altered marl Marl Marl Marl Marl Siiceous limestone Marl Marl Marl Marl Tuff Sandy marl banded with tuff Clayed marl Marl-limestone Limestone Siliceous marl Clayed marl Marl

3.60 2.77 3.73 4.11 2.95 2.09 2.06 0.84 0.31 1.01 1.07 2.13 1.98 5.66 1.73 1.05 1.73 1.66 1.43 1.68

0.50 1.01 0.50 0.33 0.91 0.29 0.49 0.11 0.02 0.33 0.29 0.31 0.27 1.12 0.10 0.03 0.10 0.49 0.13 0.54

13.68 31.59 13.40 8.03 23.53 13.75 18.95 13.26 7.43 27.99 22.16 14.38 13.47 19.72 6.05 2.91 5.88 24.58 8.73 24.76

0.39 3.25 3.79 0.23 0.42 0.57

0.05 0.34 0.78 0.02 0.01 0.18

13.32 9.90 20.48 10.00 2.92 26.40

Coal measure rocks Soma/Isiklar Soma/Isiklar Soma/Kisrakdere Soma/Elmali Soma/Sarikaya Tinaz/Bagyaka Tinaz/Bagyaka Eskihisar Eskihisar . Milas/Sekkoy . Milas/Ikizkoy Tunçbilek/Beke Tunçbilek/12A Tunçbilek/12A . Tunçbilek/Omerler 4CD Tunçbilek/37 Tunçbilek/36 Orhaneli Orhaneli Orhaneli Keles Keles Keles . Seyitomer . Seyitomer . Seyitomer

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trends, one for the coal measure rocks and another for the other rocks, are found. The coal measure rocks follow a more steeply sloped line than the other rocks. Because the value of Konya serpentine was in an anomalous position, it was omitted from the graph of the other rocks. The equations of the two lines are: For coal measure rocks: qu ¼ 23:62IS50 2:69;

ð13Þ

r ¼ 0:93: For the other rocks: qu ¼ 8:41IS50 þ 9:51; Fig. 1. Point load strength vs. UCS.

ð14Þ

r ¼ 0:85;

of 33 mm and a length of 66 mm. corrected to a specimen diameter of 50 mm. Eskikaya and Bilgin [4] reported that they used rectangular samples having a thickness of 50 mm. The point load strength index . values range from 0.23 MPa for the Seyitomer siliceous marl to 16.21 MPa for the Konya Serpentine. The CoV ranges from 2.91% for the Tunçbilek/37 marl to 31.59% for the Soma/Isiklar limestone with an overall average of 13.52%. According to Broch and Franklin [8], the point load strength test results are less scattered than the UCS test results. Bieniawski [9] states just the opposite. In this study, the UCS test results are less scattered than the point load strength test results, encouraging the Bieniawski’s statement. An approximately linear relation between the point load strength index values and the UCS values was found (Fig. 1). As it is shown in Fig. 1, two separate

where qu is the UCS (MPa) and Is50 is the point load index (MPa). 3.3. Schmidt hammer test N-type Schmidt hammer rebound number values are given in Table 4. It is reported by both Kahraman [3] and Eskikaya and Bilgin [4] that the Schmidt hammer tests were conducted in the field. The Schmidt hammer was held in a downward position and 10 impacts were carried out at each point, and the peak rebound value was recorded. The average Schmidt hammer rebound number ranges from 15 for the Keles clayed marl to 70 for the Osmaniye/Bahçe Sandstone-1. The CoV ranges from 0.82% for the Osmaniye/Bahçe Sandstone-1 to 24.78% for the Orhaneli/sandy marl banded with tuff with an overall average of 5.96% (Table 4).

Table 4 Results of the Schmidt hammer (N-type) test [3,4] Location/Panel

Rock type

Rebound number


Osmaniye/Bahçe Osmaniye/Bahçe Osmaniye/Bahçe Osmaniye/Bahçe Gaziantep/Erikli Gaziantep/Erikli Gaziantep/Erikli Gaziantep/Erikli Pozanti Pozanti Yahyali Yahyali Konya Adana Misis Emet Emet Tarsus Mersin

Dolomite Sandstone-1 Sandstone-2 Altered sandstone Limestone Marl Diabase Serpentine Limestone Clayed limestone Hematite Metasandstone Serpentine Limestone Limestone Sandstone Limestone Dolomite Limestone

59 70 53 36 55 56 64 62 61 58 44 54 59 42 68 38 58 55 51

2.08 0.58 3.05 0.58 0.58 1.73 1.00 2.52 1.00 1.15 3.09 4.32 4.79 2.08 2.08 1.82 3.09 0.58 1.82

Coefficient of variation (%) 3.51 0.82 5.80 1.62 1.06 3.09 1.56 4.08 1.64 1.97 6.69 8.00 8.06 4.91 2.99 4.80 5.36 1.04 3.58


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Table 4. Continued Location/Panel

Rock type

Rebound number


Ceyhan Ceyhan Yumurtalik

Limestone Gravelled limestone Limestone

58 47 50

1.73 1.15 2.06

3.01 2.47 4.10


60 62 53 56 59 54 62 42 27 46 45 52 54 69 46 47 53 34 35 34

2.06 1.41 6.52 7.02 1.50 1.41 2.12 6.07 0.71 6.53 6.43 3.60 4.95 0.58 1.91 1.73 1.53 3.24 5.41 8.65

3.42 2.28 12.39 12.62 2.55 2.57 3.51 14.58 2.67 14.30 14.39 6.90 9.25 0.84 4.21 3.68 2.90 9.53 15.20 24.78

15 34 F F 29 42

0.71 3.46 F F 3.00 4.04

4.88 10.19 F F 10.34 9.8


The values of the Schmidt hammer rebound number were multiplied with the respective density values (Table 5) and then correlated with the corresponding values of the UCS (Fig. 2). Multiplying the rebound number by the density improves the correlation with the UCS. The UCS increases exponentially with the produce of the rebound number and the density. The equation of the curve is qu ¼ 6:97 e0:014Rn r ;

ð15Þ

r ¼ 0:78; where qu is the UCS (MPa), Rn is the rebound number and r is the rock density (g/cm3).


. range from 1.0 km/s for the Seyitomer marl to 6.3 km/s for the Osmaniye/Bahçe dolomite. The CoV ranges from 1.79% for the Tarsus dolomite to 12.91% for the Gaziantep/Erikli Serpentine with an overall average of 6.21%. There is a non-linear relation between the p-wave velocity and the UCS (Fig. 3). The higher the strength the more scattered the data points. The equation of the curve is qu ¼ 9:95Vp1:21 ;

ð16Þ

r ¼ 0:83; where qu is the UCS (MPa) and Vp is the p-wave velocity (km/s).

3.4. Sound velocity test 3.5. Impact strength test The average values of the sound velocity (p-wave velocity) are given in Table 6. It is reported by Kahraman [3] that p-wave velocities were measured on the rock blocks having an approximate dimension of 13 20 12 cm3. The transducers used in the tests had a frequency of 54 kHz. The sound velocity values

The impact strength test values are listed in Table 7. It is reported by both Kahraman [3] and Eskikaya and Bilgin [4] that the device designed by Evans and Pomeroy [49] was used in the test The impact strength index range from 51 for the Eskihisar altered marl to

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Table 5 Density values for the rock tested [3,5] Location/Panel

Rock type

Density (g/cm3)



2.92 3.00 2.77 2.55 2.74 2.20 2.96 2.88 2.73 2.42 3.61 2.73 2.63 1.86 2.71 2.56 2.71 2.98 2.66 2.96 2.61 2.81


2.45 F 2.46 2.42 F F F 1.66 F F F 2.03 F F 1.93 F 1.91 1.92 1.85 F


Fig. 2. N-Type Schmidt hammer value X density vs. UCS.

Fig. 3. Sound velocity vs. UCS.

F F F F F 1.83 Fig. 4. Impact strength vs. UCS.

90.3 for the Konya serpentine. The CoV ranges from 0.14% for the Gaziantep/Erikli limestone to 8.70% for the Tunçbilek/12A marl with an overall average of 1.98%. The plot of the UCS as a function of the impact strength index is shown in Fig. 4. There is a non-linear relation between the UCS and the impact strength index

for the other rocks. A weak correlation (r ¼ 0:45) was found between the UCS and the impact strength index for the coal measure rocks. The weak correlation is probably due to the lower elastic modulus of the coal measure rocks. The rocks with lower elastic modulus absorb impact energy. The fact that


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Table 6 Results of the sound velocity test [3] Location/Panel

Rock type

p-wave velocity (km/s)





6.3 4.6 4.5 2.0 5.4 3.1 5.2 2.9 5.3 3.3 2.8 5.2 5.0 2.2 5.5 3.7 4.7 5.6 4.1 5.6 3.3 5.0

0.21 0.21 0.12 0.20 0.38 0.06 0.11 0.38 0.29 0.40 0.36 0.49 0.21 0.25 0.25 0.11 0.21 0.10 0.11 0.21 0.30 0.20

3.29 4.49 2.55 10.00 6.97 1.84 2.21 12.91 5.48 12.12 12.88 9.42 4.11 11.61 4.60 3.15 4.46 1.79 2.84 3.74 9.16 4.00


3.4 F F F F F F F F F F 1.5 F F F F 1.9 F 1.2 F



F F F F F 1.0

F F F F F 0.10

F F F F F 10.00


the impact strength test was originally developed for coal testing explains this situation. The equations of the two trends are: For coal measure rocks: qu ¼ 1:82ISI 74:21; r ¼ 0:45:

ð17Þ

For the other rocks: qu ¼ 41010 ISI5:87 ;

ð18Þ

r ¼ 0:65; where qu is the UCS (MPa) and ISI is the impact strength index.

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Table 7 Results of the Impact strength test [3,4] Location/Panel

Rock type

Impact strength index



Dolomite Sandstone-1 Sandstone-2 Altered sandstone Limestone Marl Diabase Serpentine Limestone Clayed limestone Hematite Metasandstone Serpentine Limestone Limestone Sandstone Limestone Dolomite Limestone limestone Gravelled limestone Limestone

83.4 87.8 80.3 70.4 82.2 76.1 89.5 81.2 82.9 80.5 84.3 85.0 90.3 72.5 84.1 75.8 82.0 80.6 78.9 81.5 75.9 83.6

0.66 0.32 0.32 0.72 0.11 0.79 0.60 0.20 0.17 0.75 0.32 0.43 0.38 0.65 0.11 1.07 1.40 0.87 1.22 1.20 0.85 0.45

0.79 0.37 0.40 1.03 0.14 1.04 0.67 0.25 0.21 0.93 0.38 0.51 0.42 0.90 0.15 1.41 1.71 1.08 1.54 1.47 1.12 0.54

Marl Limestone Marl Marl Marl Marl Limestone Marl Altered marl Marl Marl Marl Marl Siliceous limestone Marl Marl marl Marl Tuff Sandy marl banded with tuff Clayed marl Marl-limestone Limestone Siliceous marl Clayed marl Marl

75.2 65.0 74.0 79.0 76.0 73.0 71.0 F 51.0 59.0 57.0 70.4 66.0 76.0 62.0 65.0 69.9 54.0 69.3 55.0

0.30 2.52 4.04 1.15 5.30 2.52 1.53 F 4.04 0.58 0.58 0.81 5.68 1.53 1.52 0.58 0.81 5.57 0.64 1.53

0.41 3.85 5.44 1.45 6.96 3.46 2.14 F 7.87 0.97 1.01 1.16 8.70 2.03 2.48 0.89 1.16 10.31 0.93 2.79

F F F F F 78.0

F F F F F 1.73

F F F F F 2.22


4. Evaluation of the test methods The coefficient of variation values of each rock type and test method are summarised in Table 8. The impact strength test yields the most consistent results of the five methods. Although the other four methods are not as reproducible as the impact strength test, the variability


of their results is still within acceptable limits for most engineering purposes. The point load test has the highest average value of coefficient of variation. The coefficient of variation for both the Schmidt hammer and sound velocity tests are rather close that of the UCS test. The empirical methods used in this study were evaluated by comparing their results with each other.


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Table 8 The average coefficients of variation for each rock type and test method Location/Panel

Osmaniye/Bahçe Osmaniye/Bahçe Osmaniye/Bahçe Osmaniye/Bahçe Gaziantep/Erikli Gaziantep/Erikli Gaziantep/Erikli Gaziantep/Erikli Pozanti Pozanti Yahyali Yahyali Konya Adana Misis Emet Emet Tarsus Mersin Ceyhan Ceyhan Yumurtalik Coal measure rocks Soma/Isiklar Soma/Isiklar Soma/Kisrakdere Soma/Elmali Soma/Sarikaya Tinaz/Bagyaka Tinaz/Bagyaka Eskihisar Eskihisar . Milas/Sekkoy . Milas/Ikizkoy Tunçbilek/Beke Tunçbilek/12A Tunçbilek/12A . Tunçbilek/Omerler 4CD Tunçbilek/37 Tunçbilek/36 Orhaneli Orhaneli Orhaneli Keles Keles Keles . Seyitomer . Seyitomer . Seyitomer Overall average

Rock type

Coefficient of variation (%) UCS (MPa)

Point load strength (MPa)

Dolomite Sandstone-1 Sandstone-2 Altered sandstone Limestone Marl diabase Serpentine Limestone Clayed limestone Hematite Metasandstone Serpentine LIMESTONE Limestone Sandstone Limestone Dolomite Limestone Limestone Gravelled limestone Limestone

8.91 1.02 5.10 4.62 5.90 1.73 5.41 3.20 3.10 3.71 5.73 3.41 4.40 3.32 2.51 1.60 6.23 1.13 10.81 1.30 1.92 1.13

15.75 5.29 21.04 17.93 10.45 13.65 6.74 10.64 15.39 13.25 5.14 10.49 5.06 21.59 12.70 8.43 10.60 3.61 12.09 8.98 19.78 12.89

3.51 0.82 5.80 1.62 1.06 3.09 1.56 4.08 1.64 1.97 6.69 8.00 8.06 4.91 2.99 4.80 5.36 1.04 3.58 3.01 2.47 4.10

3.29 4.49 2.55 10.00 6.97 1.84 2.21 12.91 5.48 12.12 12.88 9.42 4.11 11.61 4.60 3.15 4.46 1.79 2.84 3.74 9.16 4.00

0.79 0.37 0.40 1.03 0.14 1.04 0.67 0.25 0.21 0.93 0.38 0.51 0.42 0.90 0.15 1.41 1.71 1.08 1.54 1.47 1.12 0.54


1.21 3.70 1.01 3.92 1.32 2.43 1.70 2.51 2.02 1.73 0.71 7.10 1.52 1.53 0.65 1.94 5.63 0.84 5.01 5.12

13.68 31.59 13.40 8.03 23.53 13.75 18.95 13.26 7.43 27.99 22.16 14.38 13.47 19.72 6.05 2.91 5.88 24.58 8.73 24.76

3.42 2.28 12.39 12.62 2.55 2.57 3.51 14.58 2.67 14.30 14.39 6.90 9.25 0.84 4.21 3.68 2.90 9.53 15.20 24.78


0.41 3.85 5.44 1.45 6.96 3.46 2.14 F 7.87 0.97 1.01 1.16 8.70 2.03 2.48 0.89 1.16 10.31 0.93 2.79

2.23 2.51 0.40 12.41 6.22 4.73 3.46

13.32 9.90 20.48 10.00 2.92 26.40 13.52

4.88 10.19 F F 10.34 9.80 5.96

F F F F F 10.00 6.21

F F F F F 2.22 1.98

Data from each test were used in the respective empirical equation to calculate the estimated UCS. The estimated values of compressive strength were then plotted against the measured values of compressive strength for each test, respectively (Figs. 5–8). The error in the estimated

Schmidt hammer

Sound velocity (km/s)

Impact strength

value is represented by the distance that each data point plots from the 1 : 1 diagonal line. A point lying on the line indicates an exact estimation. As it is shown in Figs. 5–7, the point load, the Schmidt hammer and the sound velocity tests are reliable methods for the

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Fig. 5. Estimated UCS vs. measured UCS for the point load test.

Fig. 7. Estimated UCS vs. measured UCS for the sound velocity test.

Fig. 6. Estimated UCS vs. measured UCS for the Schmidt hammer test.

Fig. 8. Estimated UCS vs. measured UCS for the impact strength test.

estimation of the UCS of rocks. For the sound velocity test, the data points fall closer to the line at low strength values but become more scattered at higher strength values. This suggest that the ability to estimate the UCS of rocks using the sound velocity test is the best at low strength values, and is less reliable at higher strength values. Fig. 8 shows that, the impact strength test for coal measure rocks is not reliable for the prediction of compressive strength. The impact strength test for the other rocks is only reliable for at low strength values.

and the other rocks included in this study, respectively. Significant non-linear correlation exists between the compressive strength of rock and the values produced by the Schmidt hammer rebound number and density values. The results of the sound velocity test show strong non-linear correlation with those of the uniaxial compression test. The data points are scattered at higher strength values. There is no relation between the impact strength values and the compressive strength values for the coal measure rocks. A quite weak correlation exists between the impact strength values and the compressive strength values for the other rocks. All empirical methods evaluated in this study, except the impact strength, can be used to predict the compressive strength of rock. However, the prediction equations derived by different researchers are dependent on rock types and test conditions. One who wants to use the prediction equations must not forget this reality. Further study is required to see how varying the rock type affects correlations. Additional work is needed to

5. Conclusions The indirect test methods that may be used to predict the compressive strength of rock are portable and easy to use, so they can be practically used in the field. Also, these tests require less or almost no sample preparation. The point load test exhibits strong linear correlations with the compressive strength of the coal measure rocks


check whether the impact strength test can be used to estimate the compressive strength of high strength rocks.

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